JP7688348B2 - Method for producing carbamic acid ester - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act: CO2 Resource Recovery Technology for a Non-Oil World - Focusing on Chemical and Biological Processes -, pp. 165-171, CMC Publishing Co., Ltd. Publication date: July 31, 2020

The present invention relates to a method for producing carbamic acid esters.

Carbamic acid esters are useful compounds with a wide range of applications in pharmaceuticals, agricultural chemicals, various fine chemicals, and as raw materials for synthesizing these.

So far, methods for producing carbamic acid esters using carbon dioxide at normal pressure have been proposed.
For example, Non-Patent Document 1 discusses a multi-component synthesis of carbamate derivatives using ammonium carbamate synthesized in situ from the reaction of an amine as a carbonyl source with _CO2 , an equivalent amount of triphenylphosphine, and an equivalent amount of trichloroisocyanuric acid (TCCA). In Non-Patent Document 1, _CO2 gas at 1 atmosphere is used, and a non-renewable sacrificial reagent is used.
Non-Patent Document 2 reports a method for synthesizing carbamic acid esters via carbamic acid using _CO2 gas at 1 atm and 1,8-diazabicyclo[5.4.0]undec-7-ene as a catalyst. In Non-Patent Document 2, non-recyclable sacrificial reagents _PBu3 and DBAD (di-tert-butyl azodicarboxylate) are used.
Non-Patent Document 3 reports the synthesis of carbamic acid esters from amines and _CO2 gas using a non-renewable sacrificial reagent such as _KO2 / _Et4NBr .
Non-Patent Document 4 reports the synthesis of carbamic acid esters by the Mitsunobu reaction using CO ₂ gas at 1 atmosphere and sacrificial reagents Ph ₃ P and DEAD (diethyl azodicarboxylate) via carbamic acid.

S. S. E. Ghodsinia, B. Akhlaghinia, Phosphorus Sulfur Silicon Relat. Elem., 2016, vol.191, p.1-7 Scott L. Peterson, Sabrina M. Stucka, Christopher J. Dinsmore, Org. Lett. 2010, 12, p.1340-1343 Krishna Nand Singh, Synth. Commun. 2007, 37, p.2651-2654 Devdutt Chaturvedi, Nisha Mishra, Virendra Mishra, Monats. Chem. 2007, 138, p.57-60

The above-mentioned methods for preparing carbamates all require sacrificial reagents or alkyl halides which are poorly environmentally friendly.
An object of the present invention is to provide a method for producing a carbamic acid ester using a carbamate salt as a raw material without using a sacrificial reagent or an alkyl halide.

（上記式中、Ｒ^１はそれぞれ独立して水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し、Ｒ^２、Ｒ^３はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し、Ｒ^２０は無置換もしくは置換基を有する２価の炭化水素基を表す。）
［２］ 前記カルバミン酸塩が式（Ｂ－１）で表されるカルバミン酸塩であり、前記金属アルコキシドが式（Ｃ－１）で表される金属アルコキシドであり、前記式（ａ－１）で表される構造を有するカルバミン酸エステルが式（Ａ－１）で表されるカルバミン酸エステルである、［１］に記載のカルバミン酸エステルの製造方法。

（上記式中、Ｒ^１１は水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^２１、Ｒ^３１はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^４１はそれぞれ独立して無置換もしくは置換基を有する炭化水素配位子、－ＯＲ^３１とは異なる、無置換もしくは置換基を有するアルコキシ配位子、アミド配位子、ハライド配位子を表し；Ｍは金属原子または半金属原子を表し；ｎはＭの酸化数を表し、（ｎ－ｍ）は１～６の整数であり；ｍは０以上、（ｎ－１)以下の整数を表し、Ｑはｑ価の対カチオンを表し；ｑは１又は２である。）
［３］ 前記Ｑが、アンモニウムカチオン、アミジニウムカチオン、グアニジニウムカチオン、ホスホニウムカチオン、ホスファゼニウムカチオン、カルボカチオン、アルカリ金属カチオン、アルカリ土類金属カチオンからなる群より選択されるカチオンである、［２］に記載のカルバミン酸エステルの製造方法。
［４］ 前記カルバミン酸塩が式（Ｂ－２）で表されるカルバミン酸塩であり、前記金属アルコキシドが式（Ｃ－２）で表される金属アルコキシドであり、前記式（ａ－２）で表される構造を有するカルバミン酸エステルが式（Ａ－２）で表されるカルバミン酸エステルである、［１］に記載のカルバミン酸エステルの製造方法。

（上記式中、Ｒ^１２はそれぞれ独立して水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^２２は無置換もしくは置換基を有する２価の炭化水素基を表し；Ｒ^３２はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^４２はそれぞれ独立して無置換もしくは置換基を有する炭化水素配位子、－ＯＲ^３２とは異なる、無置換もしくは置換基を有するアルコキシ配位子、アミド配位子、又はハライド配位子を表し；Ｍ’は金属原子または半金属原子を表し、ｎ’はＭ’の酸化数を表し、（
ｎ’－ｍ’）は１～６の整数であり；ｍ’は０以上、（ｎ’－１)以下の整数を表し；Ｑ’は（２／ｑ’）価の対カチオンを表し；ｑ’は１又は２である。）
［５］ 前記Ｑ’が、アンモニウムカチオン、アミジニウムカチオン、グアニジニウムカチオン、ホスホニウムカチオン、ホスファゼニウムカチオン、カルボカチオン、アルカリ金属カチオン、アルカリ土類金属カチオンからなる群より選択されるカチオンである、［４］に記載のカルバミン酸エステルの製造方法。
［６］ 前記金属アルコキシドが、チタン化合物、ケイ素化合物からなる群より選択される少なくとも1種である、［１］～［５］のいずれかに記載のカルバミン酸エステルの製造方法。
［７］ 前記金属アルコキシドがアルコキシシランであり、前記反応工程が触媒の存在下で行われる、［１］～［６］のいずれかに記載のカルバミン酸エステルの製造方法。
［８］ 前記触媒が、有機塩基カルボン酸塩、アルカリ金属塩、亜鉛化合物、チタン（IV）化合物、及びジルコニウム（IV）化合物からなる群より選択される少なくとも1種である、［７］に記載のカルバミン酸エステルの製造方法。
［９］前記反応工程が非プロトン性溶媒の存在下で行われる、［１］～［８］のいずれかに記載のカルバミン酸エステルの製造方法。
［１０］溶媒中で、塩基存在下、アミノ基含有有機化合物を、二酸化炭素含有混合ガスと接触させることにより、前記カルバミン酸塩を生成するカルバミン酸塩生成工程を更に含み、前記二酸化炭素含有混合ガス中の二酸化炭素の体積が０．０１％以上である、［１］～［９］のいずれかに記載のカルバミン酸エステルの製造方法。 Means for Solving the Problems The present inventors have conducted extensive research to solve the above problems, and as a result have found that a carbamic acid ester can be produced from a carbamate and a metal alkoxide, thereby completing the present invention.
The present invention provides the following specific embodiments.
[1] A compound represented by the following formula (a-1) or (a-2) is produced from a carbamate and a metal alkoxide:
A method for producing a carbamic acid ester, comprising producing a carbamic acid ester having a structure represented by the following formula:

(In the above formula, ^R1 's each independently represent a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, R2 ^'s and R3 ^'s each independently represent an unsubstituted or substituted monovalent hydrocarbon group, and ^R20 's represents an unsubstituted or substituted divalent hydrocarbon group.)
[2] The method for producing a carbamic acid ester according to [1], wherein the carbamic acid salt is a carbamic acid salt represented by formula (B-1), the metal alkoxide is a metal alkoxide represented by formula (C-1), and the carbamic acid ester having a structure represented by formula (a-1) is a carbamic acid ester represented by formula (A-1).

(In the above formula, R ¹¹ represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²¹ and R ³¹ each independently represent an unsubstituted or substituted monovalent hydrocarbon group; R ⁴¹ each independently represent an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³¹ ; M represents a metal atom or a metalloid atom; n represents the oxidation number of M, (n-m) is an integer of 1 to 6; m represents an integer of 0 or more and (n-1) or less, Q represents a q-valent counter cation; and q is 1 or 2.)
[3] The method for producing a carbamic acid ester according to [2], wherein Q is a cation selected from the group consisting of an ammonium cation, an amidinium cation, a guanidinium cation, a phosphonium cation, a phosphazenium cation, a carbocation, an alkali metal cation, and an alkaline earth metal cation.
[4] The method for producing a carbamic acid ester according to [1], wherein the carbamic acid salt is a carbamic acid salt represented by formula (B-2), the metal alkoxide is a metal alkoxide represented by formula (C-2), and the carbamic acid ester having a structure represented by formula (a-2) is a carbamic acid ester represented by formula (A-2).

(In the above formula, R ¹² each independently represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²² each independently represents an unsubstituted or substituted divalent hydrocarbon group; R ³² each independently represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴² each independently represents an unsubstituted or substituted hydrocarbon ligand, an unsubstituted or substituted alkoxy ligand, an amide ligand, or a halide ligand that is different from -OR ³² ; M' represents a metal atom or a metalloid atom, n' represents the oxidation number of M', (
n'-m') is an integer of 1 to 6; m' is an integer of 0 or more and (n'-1) or less; Q' is a (2/q')-valent counter cation; and q' is 1 or 2.
[5] The method for producing a carbamic acid ester according to [4], wherein Q' is a cation selected from the group consisting of an ammonium cation, an amidinium cation, a guanidinium cation, a phosphonium cation, a phosphazenium cation, a carbocation, an alkali metal cation, and an alkaline earth metal cation.
[6] The method for producing a carbamic acid ester according to any one of [1] to [5], wherein the metal alkoxide is at least one selected from the group consisting of titanium compounds and silicon compounds.
[7] The method for producing a carbamic acid ester according to any one of [1] to [6], wherein the metal alkoxide is an alkoxysilane, and the reaction step is carried out in the presence of a catalyst.
[8] The method for producing a carbamic acid ester according to [7], wherein the catalyst is at least one selected from the group consisting of an organic base carboxylate, an alkali metal salt, a zinc compound, a titanium (IV) compound, and a zirconium (IV) compound.
[9] The method for producing a carbamic acid ester according to any one of [1] to [8], wherein the reaction step is carried out in the presence of an aprotic solvent.
[10] The method for producing a carbamic acid ester according to any one of [1] to [9], further comprising a carbamic acid salt production step of producing the carbamic acid salt by contacting an amino group-containing organic compound with a carbon dioxide-containing mixed gas in a solvent in the presence of a base, wherein a volume of carbon dioxide in the carbon dioxide-containing mixed gas is 0.01% or more.

The present invention provides a method for producing carbamic acid esters using carbamate salts as raw materials without using sacrificial reagents or alkyl halides.

Specific examples will be given to explain the details of the present invention, but the invention is not limited to the following content and can be modified as appropriate without departing from the spirit of the invention.

（上記式中、Ｒ^１はそれぞれ独立して水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し、Ｒ^２、Ｒ^３はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し、Ｒ^２０は無置換もしくは置換基を有する２価の炭化水素基を表す。） <1. Method for producing carbamic acid ester>
A method for producing a carbamic acid ester according to one embodiment of the present invention is characterized by including a reaction step (hereinafter sometimes abbreviated as "reaction step") of producing a carbamic acid ester having a structure represented by the following formula (a-1) or (a-2) from a carbamic acid salt and a metal alkoxide.

(In the above formula, ^R1 's each independently represent a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, R2 ^'s and R3 ^'s each independently represent an unsubstituted or substituted monovalent hydrocarbon group, and ^R20 's represents an unsubstituted or substituted divalent hydrocarbon group.)

本発明は、犠牲試薬も環境調和性に乏しいハロゲン化アルキルも用いずに、カルバミン酸エステルを製造することができる。また、後述するように、本発明は低濃度の二酸化炭素混合ガスを原料としてカルバミン酸塩を製造し、カルバミン酸エステルを製造することも可能であり、排気ガス等に含まれる低濃度二酸化炭素の有効活用を可能とする。さらに、本発明はアルコールで再生可能な金属アルコキシドを用いるため、実質的には低濃度二酸化炭素、アミン、アルコールのみが消費される環境調和性に優れたカルバミン酸エステルの製造を実現できる。以下、本発明のカルバミン酸エステルの製造方法について詳細に説明する。 An example of a reaction for producing a carbamic acid ester having a structure represented by formula (a-1) or (a-2) from a carbamate and a metal alkoxide is the reaction between N-benzylammonium benzylcarbamate and titanium tetramethoxide shown below. The reaction mechanism in this case is presumed to be as follows.

The present invention can produce carbamic acid esters without using sacrificial reagents or alkyl halides that are poor in environmental compatibility. As described below, the present invention can also produce carbamic acid salts using low-concentration carbon dioxide mixed gas as a raw material, and produce carbamic acid esters, making it possible to effectively utilize low-concentration carbon dioxide contained in exhaust gases and the like. Furthermore, since the present invention uses a metal alkoxide that can be regenerated with alcohol, it is possible to realize the production of carbamic acid esters that are excellent in environmental compatibility, in which essentially only low-concentration carbon dioxide, amines, and alcohol are consumed. The method for producing carbamic acid esters of the present invention will be described in detail below.

<1-1. Carbamic acid ester having a structure represented by formula (a-1) or (a-2)>
According to the production method according to one embodiment of the present invention, a carbamic acid ester having a structure represented by the formula (a-1) or (a-2) can be obtained.
(R ¹ )
Each R ¹ independently represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group.
In this specification, the term "hydrocarbon group" is not limited to a linear saturated hydrocarbon group, but may have a carbon-carbon unsaturated bond, a branched structure, or a cyclic structure.
The number of carbon atoms in R ¹ is not particularly limited, but is usually 1 or more, and is usually 30 or less, preferably 24 or less, and more preferably 20 or less.
Examples of the unsubstituted hydrocarbon group represented by ^R1 include an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a neopentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, an n-undecyl group, an n-dodecyl group, an n-tridecyl group, an n-tetradecyl group, an n-pentadecyl group, an n-hexadecyl group, an n-heptadecyl group, an n-octadecyl group, an n-nonadecyl group, an n-octyl group, an n-pentyl group, an n-hexadecyl group, an n-heptadecyl group, an n-octadecyl group, an n-nonadecy group, an n-octyl ... cycloalkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups; aromatic hydrocarbon groups such as phenyl, 1-naphthyl, 2-naphthyl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl, 4-phenanthryl, 9-phenanthryl, 1-anthryl, 2-anthryl, 9-anthryl, 1-pyrenyl, 2-pyrenyl, 4-pyrenyl, 1-triphenylenyl, and 2-triphenylenyl groups; and the like.
When the hydrocarbon group represented by ^R1 has a substituent, examples of the substituent include a deuterium atom; an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, or a tert-butyl group; a cycloalkyl group having 3 to 4 carbon atoms, such as a cyclopropyl group, or a cyclobutyl group; an aromatic hydrocarbon group having 6 to 10 carbon atoms, such as a phenyl group, a 1-naphthyl group, or a 2-naphthyl group; an oxygen-containing heterocyclic group, such as a furanyl group, a sulfur-containing heterocyclic group, such as a thienyl group, or a nitrogen-containing heterocyclic group, such as a pyrrolyl group or a pyridyl group; a hydroxyl group; and an alkoxy group. Therefore, when the hydrocarbon group represented by ^R1 has a substituent, preferred examples of ^R1 include aralkyl groups such as a benzyl group, a phenethyl group, a 1-naphthylmethyl group, and a 2-naphthylmethyl group; a cycloalkylalkyl group such as a cyclohexylmethyl group; a hydrocarbon group having an oxygen-containing heterocycle such as a furfuryl group; a hydrocarbon group having a sulfur-containing heterocycle such as a thienylmethyl group; and a hydrocarbon group having a nitrogen-containing heterocycle such as a pyridylmethyl group, with a benzyl group being particularly preferred.
When the hydrocarbon group has a substituent, the number of carbon atoms means the total number of carbon atoms of the substituent and the number of carbon atoms of the hydrocarbon group.
R ¹ is preferably hydrogen from the viewpoint of the usefulness of the carbamic acid ester compound.

( ^R2 , ^R3 )
R ² and R ³ each independently represent an unsubstituted or substituted monovalent hydrocarbon group.
The number of carbon atoms in ^R2 is not particularly limited, but is usually 1 or more, and is usually 30 or less, preferably 24 or less, and more preferably 20 or less.
The number of carbon atoms in ^R3 is not particularly limited, but is usually 1 or more, and is usually 30 or less, preferably 24 or less, and more preferably 20 or less.
Examples of the unsubstituted hydrocarbon group represented by R ² and R ³ include an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a neopentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, an n-undecyl group, an n-dodecyl group, an n-tridecyl group, an n-tetradecyl group, an n-pentadecyl group, an n-hexadecyl group, an n-heptadecyl group, an n-octadecyl group, an n-nonadecyl group, cycloalkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups; aromatic hydrocarbon groups such as phenyl, 1-naphthyl, 2-naphthyl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl, 4-phenanthryl, 9-phenanthryl, 1-anthryl, 2-anthryl, 9-anthryl, 1-pyrenyl, 2-pyrenyl, 4-pyrenyl, 1-triphenylenyl, and 2-triphenylenyl groups; and the like.
When the hydrocarbon group represented by R ² or R ³ has a substituent, examples of the substituent include a deuterium atom; an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, or a tert-butyl group; a cycloalkyl group having 3 to 4 carbon atoms, such as a cyclopropyl group, or a cyclobutyl group; an aromatic hydrocarbon group having 6 to 10 carbon atoms, such as a phenyl group, a 1-naphthyl group, or a 2-naphthyl group; an oxygen-containing heterocyclic group, such as a furanyl group, a sulfur-containing heterocyclic group, such as a thienyl group, or a nitrogen-containing heterocyclic group, such as a pyrrolyl group or a pyridyl group, and other heterocyclic groups. Therefore, when the hydrocarbon group represented by R ² and R ³ has a substituent, preferred examples of R ² and R ³ include aralkyl groups such as benzyl, phenethyl, 1-naphthylmethyl, and 2-naphthylmethyl; cycloalkylalkyl groups such as cyclohexylmethyl; hydrocarbon groups having an oxygen-containing heterocycle such as furfuryl; hydrocarbon groups having a sulfur-containing heterocycle such as thienylmethyl; and hydrocarbon groups having a nitrogen-containing heterocycle such as pyridylmethyl, with benzyl being particularly preferred.
In addition, when the hydrocarbon group is a branched alkyl group, the number of carbon atoms in the main chain is counted as the number of carbon atoms in the hydrocarbon group. In addition, when the hydrocarbon group has a substituent, the number of carbon atoms in the substituent is not included in the number of carbon atoms in the hydrocarbon group.
From the viewpoint of availability of raw materials, ^R2 is preferably an alkyl group such as an n-hexyl group; a cycloalkyl group such as a cyclohexyl group; an aromatic hydrocarbon group such as a phenyl group; or an aralkyl group such as a benzyl group; more preferably a benzyl group, an n-hexyl group, a cyclohexyl group, or a phenyl group, and even more preferably a benzyl group or a phenyl group.
From the viewpoint of availability of raw materials, ^R3 is preferably an alkyl group, more preferably a methyl group, an ethyl group, an n-propyl group, or an n-butyl group.

( ^R20 )
R ²⁰ represents an unsubstituted or substituted divalent hydrocarbon group.
Examples of the divalent hydrocarbon group include a methylene group; an ethylene group; a linear, branched or cyclic alkylene group having 3 or more carbon atoms; and an arylene group having 6 or more carbon atoms.
The number of carbon atoms in the divalent hydrocarbon group is not particularly limited, but is preferably 2 or more, and is preferably 10 or less, and more preferably 7 or less. The divalent hydrocarbon group may have an unsaturated bond. When the divalent hydrocarbon group has a substituent, the number of carbon atoms in the divalent hydrocarbon group means the number of carbon atoms including the number of carbon atoms in the substituent. Examples of the substituent include those exemplified in the explanation of item (R ¹ ).
Specific examples of R ²⁰ include chain hydrocarbon groups such as a methylene group, an ethylene group, a tetramethylethylene group, an n-propylene group (a trimethylene group), a 1-methylpropylene group, a 1,1-dimethylpropylene group, a 2-methylpropylene group, a 1,2-dimethylpropylene group, a 2,2-dimethylpropylene group, a 1,1,2-trimethylpropylene group, a 1,1,3-trimethylpropylene group, an n-butylene group (a tetramethylene group), a 2-methyl-1,4-butylene group, a 3-methyl-1,4-butylene group, a 2,2-dimethyl-1,4-butylene group, a 2,3-dimethyl-1,4-butylene group, a 2,2,3-trimethyl-1,4-butylene group, an n-pentylene group (a pentamethylene group), and an n-hexanylene group (a hexamethylene group); Examples of divalent groups include alicyclic hydrocarbon groups such as a cyclohexylene group; a 1,4-phenylene group, a 1,2-phenylene group, or a 1,3-phenylene group obtained by removing two hydrogen atoms from a benzene ring; aromatic hydrocarbon groups such as a dimethylphenylene group (xylyl group) obtained by removing two hydrogen atoms from the benzene ring of xylene, a methylphenylene group (tolylene group) obtained by removing two hydrogen atoms from the benzene ring of toluene, or a naphthalene group obtained by removing two hydrogen atoms from naphthalene; and a 1,4-phenylenebis(methylene) group, a 1,4-phenylenebis(ethylene) group, a group obtained by removing one hydrogen atom from each of the two benzene rings of biphenyl, and a group obtained by removing one hydrogen atom from each of the two benzene rings of diphenylmethane.

Examples of the carbamic acid ester represented by formula (a-1) or (a-2) include the following compounds.

<1-2. Carbamate>
The carbamate used in this embodiment may be any carbamate capable of producing a carbamate ester represented by formula (a-1) or (a-2). That is, it may be a carbamate containing a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group R ¹ and an unsubstituted or substituted monovalent hydrocarbon group R ² , or a carbamate containing an unsubstituted or substituted monovalent hydrocarbon group R ¹ and an unsubstituted or substituted divalent hydrocarbon group R ^20. Preferred examples include carbamates represented by formula (B-1) or formula (B-2) described below.

The carbamate may be commercially available or synthesized. When synthesized, the method of production is not particularly limited, but for example, an amino group-containing organic compound having one or more primary or secondary amino groups, more specifically, an aliphatic monoamine such as benzylamine or hexylamine, or an aliphatic diamine such as ethylenediamine, may be reacted with carbon dioxide. In addition, pure carbon dioxide gas may be used as the carbon dioxide used in the reaction, but a mixed gas containing carbon dioxide at a partial pressure of 1 atmosphere or less, for example, a mixed gas having a carbon dioxide content of 0.01% or more by volume, may also be used.
In one embodiment of the present invention, it is preferable to further include a carbamate production step in which an amino group-containing organic compound is contacted with a carbon dioxide-containing mixed gas in a solvent to produce a carbamate. The volume of carbon dioxide in the carbon dioxide-containing mixed gas is usually 0.01% or more, preferably 1% or more, more preferably 10% or more, even more preferably 15% or more, particularly preferably 20% or more, and preferably 50% or less. The solvent used in the carbamate production step is not particularly limited, but a hydrocarbon solvent such as hexane, benzene, or toluene can be preferably used. The reaction time may be appropriately adjusted according to the carbon dioxide partial pressure in the carbon dioxide-containing mixed gas and the reaction scale. For example, when the volume of carbon dioxide in the carbon dioxide-containing mixed gas is 15%, a carbamate can be synthesized with a yield of 80% or more by reacting 1 mmol of an amino group-containing organic compound as a raw material for 5 to 10 minutes. In addition, when 40 mmol of an amino group-containing organic compound is used, a yield of 90 minutes or more can be achieved with a reaction time of about 180 minutes. The produced carbamate can be easily isolated by filtration.

The present inventors have confirmed that a carbamate can be produced in a high yield, for example, 99% or more, by the reaction shown in the following scheme as a carbamate production step. In addition, a carbamate was obtained in a high isolated yield of 93% even in a synthesis using a mixed gas of low partial pressure carbon dioxide.

Previously, methods for producing carbamic acid esters using carbon dioxide as a raw material have been reported, but all of these methods require a sacrificial reagent or an alkyl halide that is poor in environmental compatibility. On the other hand, the production method of the present embodiment does not require a sacrificial reagent or an alkyl halide, and can also use a mixed gas containing low partial pressure carbon dioxide. For example, exhaust gas from a thermal power plant usually contains about 15% carbon dioxide. The method of producing carbamic acid salts and carbamic acid esters using such a mixed gas containing low concentration carbon dioxide is a reaction that allows the effective use of low concentration carbon dioxide, is effective in reducing greenhouse gas emissions, and is an environmentally friendly production method.

<1-3. Metal alkoxide>
The metal alkoxide used in this embodiment may be any metal alkoxide capable of producing a carbamic acid ester represented by formula (a-1) or (a-2). In other words, any metal alkoxide containing an alkoxide group ^-OR3 may be used. Preferred examples include metal alkoxides represented by formula (C-1) or (C-2) described below. The metal alkoxide may be a commercially available product or may be synthesized.
In addition, the metal alkoxide is a reactant that can be regenerated with alcohol. After carrying out the reaction step, the residue can be recovered from the reaction product and regenerated with alcohol. The regenerated metal alkoxide may be reused in the reaction step. Therefore, as one embodiment of the present invention, a method for producing a carbamate ester that is environmentally friendly can be provided, in which a carbamate is produced using low-concentration carbon dioxide, and essentially only low-concentration carbon dioxide, amine, and alcohol are consumed.

上記式中、Ｒ^１１は水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^２１、Ｒ^３１はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^４１はそれぞれ独立して無置換もしくは置換基を有する炭化水素配位子、－ＯＲ^３１とは異なる、無置換もしくは置換基を有するアルコキシ配位子、アミド配位子、又はハライド配位子を表し；Ｍは金属原子または半金属原子を表し；ｎはＭの酸化数を表し、（ｎ－ｍ）は１～６の整数であり；ｍは０以上（ｎ－１)以下の整数を表し；Ｑはｑ価の対カチオンを表し；ｑは１又は２である。 <1-4. Method for producing carbamic acid ester represented by formula (A-1)>
In one embodiment of the present invention, it is preferable that the carbamate is a carbamate represented by formula (B-1), the metal alkoxide is a metal alkoxide represented by formula (C-1), and the carbamate ester having a structure represented by formula (a-1) is a carbamate ester represented by formula (A-1).

In the above formula, R ¹¹ represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²¹ and R ³¹ each independently represent an unsubstituted or substituted monovalent hydrocarbon group; R ⁴¹ each independently represent an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³¹ ; M represents a metal atom or a semimetal atom; n represents the oxidation number of M, and (n-m) is an integer of 1 to 6; m represents an integer of 0 or more and (n-1) or less; Q represents a q-valent counter cation; and q is 1 or 2.

<1-4-1. Carbamic acid ester represented by formula (A-1)>
In formula (A-1), R ¹¹ represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, and R ²¹ and R ³¹ each independently represent an unsubstituted or substituted monovalent hydrocarbon group.
For details of R ¹¹ , the above description of R ¹ applies.
For details of R ²¹ , the above description of R ² applies.
For details of R ³¹ , the above description of R ³ applies.

<1-4-2. Carbamate represented by formula (B-1)>
In formula (B-1), R ¹¹ represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, R ²¹ and R ³¹ each independently represent an unsubstituted or substituted monovalent hydrocarbon group, and Q represents a counter cation.
R ¹¹ in formula (B-1) corresponds to R ¹¹ in formula (A-1). R ²¹ in formula (B-1) corresponds to R ²¹ in formula (A-1). R ³¹ in formula (B-1) corresponds to R ³¹ in formula (A-1). Therefore, the above explanation of R ¹ applies to details of R ^11. The above explanation of R ² applies to details of R ^21. The above explanation of R ³ applies to details of R ³¹ .

(Q)
Q represents a counter cation in the carbamate, and is not particularly limited as long as it is a monovalent or divalent cation. Examples of the counter cation include an ammonium cation, an amidinium cation, a guanidinium cation, a phosphonium cation, a phosphazenium cation, a carbocation, a sulfonium cation, an iodonium cation, an alkali metal cation, and an alkaline earth metal cation.
Examples of the ammonium cation include primary ammonium cations such as n-butylammonium cation; secondary ammonium cations such as diethylammonium cation; tertiary ammonium cations such as triethylammonium cation; and quaternary ammonium cations such as tetramethylammonium cation, phenyltrimethylammonium cation, and tetrabutylammonium cation.
Examples of the amidinium cation include formamidinium cation, acetamidinium cation, 1,5-diazabicyclo[4.3.0]non-5-enium cation, and 1,8-diazabicyclo[5.4.0]undec-7-enium cation, which are protonated forms of formamidine, acetamidine, 1,5-diazabicyclo[4.3.0]non-5-ene, and 1,8-diazabicyclo[5.4.0]undec-7-ene, respectively, and derivatives thereof having one or more substituents. Examples of the substituents include hydrocarbon groups such as alkyl groups, cycloalkyl groups, and aromatic hydrocarbon groups exemplified in the explanation of item (R ¹ ).
Examples of the guanidinium cation include 1,1,3,3-tetramethylguanidinium cation, 2-tert-butyl-1,1,3,3-tetramethylguanidinium cation, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, which are protonated, respectively, and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, as well as derivatives thereof having one or more substituents. Examples of the substituent include hydrocarbon groups such as alkyl groups, cycloalkyl groups, and aromatic hydrocarbon groups exemplified in the explanation of item (R ¹ ).
Examples of the phosphonium cation include tertiary phosphonium cations such as triphenylphosphonium cation and tri-tert-butylphosphonium cation; and quaternary phosphonium cations such as tetraphenylphosphonium cation, tetra-p-tolylphosphonium cation, triphenylbenzylphosphonium cation, triphenylbutyl cation, tetraethylphosphonium cation, and tetrabutylphosphonium cation.
Phosphazenium cations include tert-butylimino-tris(dimethylamino)phosphorane, tert-butylimino-tri(pyrrolidino)phosphorane, 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine, 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ ⁵ ,4λ ⁵ -catenadi(phosphazene), and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylideneamino]-2λ ⁵ ,4λ ^{5 .} Examples of such cations include tert-butylimino-tris(dimethylamino)phosphoranium cation, tert-butylimino-tri(pyrrolidino)phosphoranium cation, 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorinium cation, 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ ⁵ ,4λ ⁵ -catenadi(phosphazene), and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylideneamino]-2λ ⁵ ,4λ ⁵ -catenadi(phosphazene)onium cation, each of which is protonated. Examples of the substituent include hydrocarbon groups such as alkyl groups, cycloalkyl groups, and aromatic hydrocarbon groups exemplified in the description of item (R ¹ ).
Examples of the carbocation include monovalent carbocations such as the triphenylmethyl cation, the tropylium cation, and the azulenium cation.
Examples of the sulfonium cation include triphenylsulfonium cation, 4-(phenylthio)phenyldiphenylsulfonium cation, bis[4-(diphenylsulfonio)phenyl]sulfide, and 4-hydroxyphenylmethylbenzylsulfonium cation.
Examples of the iodonium cation include a diphenyliodonium cation, a di-p-tolyliodonium cation, and a 4-isopropylphenyl(p-tolyl)iodonium cation.
Examples of the alkali metal cation include a lithium cation, a sodium cation, and a potassium cation.
Alkaline earth metal cations include magnesium cations and calcium cations.
From the viewpoint of ease of synthesis, preferred are ammonium cations, amidinium cations, guanidinium cations, phosphonium cations, phosphazenium cations, carbocations, alkali metal cations, and alkaline earth metal cations, and more preferred are ammonium cations and amidinium cations. Among these, preferred are primary ammonium cations, tertiary ammonium cations, and 1,8-diazabicyclo[5.4.0]undec-7-enium cations.
(q)
q is 1 or 2. q represents the number of carbamate anions and has the same value as the valence of Q. Therefore, when Q is a monovalent cation, q is 1, and when Q is a divalent cation, q is 2.

Specific examples of the carbamate represented by formula (B-1) include methylammonium N-methylcarbamate, ethylammonium N-ethylcarbamate, propylammonium N-propylcarbamate, butylammonium N-butylcarbamate, pentylammonium N-pentylcarbamate, hexylammonium N-hexylcarbamate, benzylammonium N-benzylcarbamate, phenylammonium N-phenylcarbamate, compounds represented by the following formulas, and amidinium salts, guanidinium salts, phosphonium salts, phosphazenium salts, carbocation salts, alkali metal salts, or alkaline earth metal salts in which the counter cation is an amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, alkali metal cation, or alkaline earth metal cation other than 1,8-diazabicyclo[5.4.0]undec-7-enium cation, but the examples are not limited thereto.

<1-4-3. Metal alkoxide represented by formula (C-1)>
In formula (C-1), R ³¹ represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴¹ each independently represents an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³¹ ; M represents a metal atom or a metalloid atom; n represents the oxidation number of M, (n-m) is an integer of 1 to 6; and m represents an integer of 0 or more and (n-1) or less.
(M)
M represents a metal atom or a metalloid atom.
The types of metal atoms and metalloid atoms are not particularly limited, and examples thereof include metal atoms and metalloid atoms selected from the group consisting of silicon, titanium, zirconium, germanium, indium, tin, tantalum, zinc, and tungsten. Among these, silicon and titanium are preferred from the viewpoint of availability.
( ^R31 )
R ³¹ represents an unsubstituted or substituted monovalent hydrocarbon group.
R ³¹ in formula (C-1) corresponds to R ³¹ in formula (A-1). Therefore, the details of R ³¹ are the same as those of R ³ described above.
( ^R41 )
Each R ⁴¹ independently represents an unsubstituted or substituted hydrocarbon ligand, an unsubstituted or substituted alkoxy ligand, an amide ligand, or a halide ligand which is different from —OR ³¹ .
Examples of the unsubstituted hydrocarbon ligand include alkyl ligands such as a methyl ligand and an ethyl ligand, cycloalkyl ligands such as a cyclohexyl ligand, aryl ligands such as a phenyl ligand, a naphthyl ligand, a cyclopentadienyl ligand, a cyclohexadienyl ligand, a cyclooctadienyl ligand, a cyclooctatetraenyl ligand, and a norbornadienyl ligand, a benzyl ligand, a methylcyclopentadienyl ligand, and a methylcyclohexadienyl ligand,
Examples of the substituent that the hydrocarbon ligand may have include aralkyl ligands such as methylcyclooctadienyl ligand, methylcyclooctatetraenyl ligand, etc. Examples of the substituent that the hydrocarbon ligand may have include a hydroxy group, an ester group (-COOR), an amide group (-CONRR'), a halogen atom, an alkylthio group (-SR), an amino group (-NRR'), a carboxy group, a nitro group, a sulfonic acid group (-SO ₃ H), an oxygen-containing heterocyclic group such as a furanyl group, a sulfur-containing heterocyclic group such as a thienyl group, and a nitrogen-containing heterocyclic group such as a pyridyl group.
Examples of unsubstituted alkoxy ligands different from -OR ³¹ include methoxy ligands, ethoxy ligands, propoxy ligands, butoxy ligands, pentoxy ligands, dodecyloxy ligands, phenoxy ligands, etc. Examples of the substituent that the alkoxy ligand may have include a hydroxy group, an ester group (-COOR), an amide group (-CONRR'), a halogen atom, an alkylthio group (-SR), an amino group (-NRR'), a carboxy group, a nitro group, a sulfonic acid group (-SO ₃ H), an oxygen-containing heterocyclic group such as a furanyl group, a sulfur-containing heterocyclic group such as a thienyl group, and a nitrogen-containing heterocyclic group such as a pyridyl group.
Examples of the amide ligand include an unsubstituted amide ligand (NH ₂ ), a methyl amide ligand (NHMe), a dimethyl amide ligand (NMe ₂ ), a diethyl amide ligand (NEt ₂ ), a di-n-propyl amide ligand (NPr ₂ ), an isopropyl amide ligand, a di-n-butyl amide ligand, and a di-t-butyl amide ligand.
Examples of the halide ligand include a fluorine ligand, a chlorine ligand, a bromine ligand, and an iodine ligand. When m is 2 to 5, a plurality of R ⁴¹ may be the same or different, but from the viewpoint of availability, it is preferable that all R ⁴¹ are the same.
(n)
n represents the oxidation number of M, and (n−m) is an integer of 1 to 6.
(m)
m represents an integer of 0 to (n-1), i.e., an integer from 0 to 5.

Specific examples of the metal alkoxide represented by formula (C-1) include:
Tetramethoxysilane, tetraethoxysilane, tetra(n-propoxy)silane, tetra(isopropoxy)silane, tetra(n-butoxy)silane, tetra(2-butoxy)silane, tetra(t-butoxy)silane, trimethoxy(isopropoxy)silane, trimethoxy(n-butoxy)silane, trimethoxy(2-butoxy)silane, trimethoxy(t-butoxy)silane, trimethoxysilane, triethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, methyltrippropoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrippropoxysilane, propyltrimethoxysilane, propyltrimethoxysilane, Alkoxysilanes such as riethoxysilane, propyltrimethoxysilane, cyclohexyltrimethoxysilane, cyclohexyltriethoxysilane, cyclohexyltrippropoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltrippropoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltrippropoxysilane, dimethoxydimethylsilane, diethoxydimethylsilane, dimethoxydiethylsilane, diethoxydiethylsilane, dimethoxymethylvinylsilane, dimethoxydiphenylsilane, dimethoxymethylphenylsilane, and N-(2-aminoethyl)3-aminopropyltrimethoxysilane;
Tetramethoxytitanium (titanium tetramethoxide), tetraethoxytitanium, tetraallyloxytitanium, tetra-n-propoxytitanium, tetraisopropoxytitanium (titanium tetra-iso-propoxide), tetra-n-butoxytitanium (titanium tetra-n-butoxide), tetraisobutoxytitanium, tetra-sec-butoxytitanium, tetra-t-butoxytitanium (titanium tetra-t-butoxide), tetra-n-pentyloxytitanium, tetracyclopentyloxytitanium, tetrahexyloxytitanium, tetracyclohexyloxytitanium, tetrabenzyloxytitanium, tetraoctyloxytitanium, tetrakis(2-ethylhexyloxy)titanium, tetradecyloxytitanium, tetradodecyloxytitanium, tetrastearyloxytitanium, tetrakis(8-hydroxyoctyloxy)titanium, diisopropoxybis(2-ethyl-1,3-hexanediolato)titanium, bis(2-ethylhexyloxy)bis(2-ethyl-1, 3-hexanediolato)titanium, tetrakis(2-chloroethoxy)titanium, tetrakis(2-bromoethoxy)titanium, tetrakis(2-methoxyethoxy)titanium, tetrakis(2-ethoxyethoxy)titanium, butoxytrimethoxytitanium, dibutoxydimethoxytitanium, butoxytriethoxytitanium, dibutoxydiethoxytitanium, butoxytriisopropoxytitanium, dibutoxydiisopropoxytitanium, tetraphenoxytitanium, tetrakis(o-chlorophenoxy)titanium, tetrakis(m-nitrophenoxy)titanium, tetrakis(p-methylphenoxy)titanium, tetrakis(trimethylsilyloxy)titanium, triethoxytitanium, trimethoxytitanium, triisopropoxytitanium, tributoxytitanium, methyldimethoxytitanium, ethyltriethoxytitanium, methyltriisopropoxytitanium, tetradimethylaminotitanium, dimethyltitanium diacetylacetonate, ethyltitanium triacetylacetonate and other alkoxytitaniums;
Tetramethoxyzirconium, tetraethoxyzirconium, tetraallyloxyzirconium, tetra-n-propoxyzirconium, tetraisopropoxyzirconium, tetra-n-butoxyzirconium, tetraisobutoxyzirconium, tetra-sec-butoxyzirconium, tetra-t-butoxyzirconium, tetra-n-pentyloxyzirconium, tetracyclopentyloxyzirconium, tetrahexyloxyzirconium, tetracyclohexyloxyzirconium, tetrabenzyloxyzirconium, tetraoctyloxyzirconium, tetrakis(2-ethylhexyloxy)zirconium, tetradecyloxyzirconium, tetradodecyloxyzirconium, tetrastearyloxyzirconium, tetrakis(8-hydroxyoctyloxy)zirconium, diisopropoxybis Alkoxy zirconium such as (2-ethyl-1,3-hexanediolato)zirconium, bis(2-ethylhexyloxy)bis(2-ethyl-1,3-hexanediolato)zirconium, tetrakis(2-chloroethoxy)zirconium, tetrakis(2-bromoethoxy)zirconium, tetrakis(2-methoxyethoxy)zirconium, tetrakis(2-ethoxyethoxy)zirconium, butoxytrimethoxyzirconium, dibutoxydimethoxyzirconium, butoxytriethoxyzirconium, dibutoxydiethoxyzirconium, butoxytriisopropoxyzirconium, dibutoxydiisopropoxyzirconium, tetraphenoxyzirconium, tetrakis(o-chlorophenoxy)zirconium, tetrakis(m-nitrophenoxy)zirconium, and tetrakis(p-methylphenoxy)zirconium;
Alkoxygermaniums such as tetraethoxygermanium, tetrapropoxygermanium, tetraisopropoxygermanium, tetra(n-butoxy)germanium, tetra(2-butoxy)germanium, and tetra(t-butoxy)germanium;
Alkoxyindium such as tetra(n-butoxy)indium, tetra(2-butoxy)indium, tetra(t-butoxy)indium, trimethoxyindium, triethoxyindium, tri(n-propoxy)indium, triisopropoxyindium, tri(n-butoxy)indium, triisobutoxyindium, tri(t-butoxy)indium, and tri(s-butoxy)indium;
Alkoxytins such as dibutyldimethoxytin, dibutyldiethoxytin, and dibutyldipropoxytin;
Alkoxytantalum, such as tetramethoxytantalum, tetraethoxytantalum, tetra(n-propoxy)tantalum, tetraisopropoxytantalum, tetra(n-butoxy)tantalum, tetra(2-butoxy)tantalum, tetra(t-butoxy)tantalum, and pentaethoxytantalum;
Alkoxy zinc such as isopropoxy zinc, tetra(n-butoxy) zinc, tetra(2-butoxy) zinc, and tetra(t-butoxy) zinc;
Alkoxytungsten such as pentamethoxytungsten, pentaethoxytungsten, pentaisopropoxytungsten, tungsten(V) pentabutoxytungsten, triisobutoxytungsten, and tri(t-butoxy)tungsten;
Examples include:
The metal alkoxide represented by formula (C-1) is preferably at least one selected from the group consisting of titanium compounds and silicon compounds, from the viewpoint of ease of availability, more preferably titanium tetramethoxide, titanium tetra-iso-propoxide, titanium tetra-n-butoxide, titanium tetra-t-butoxide, tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, and tetra-n-butoxysilane, and even more preferably titanium tetramethoxide, titanium tetra-iso-propoxide, titanium tetra-n-butoxide, titanium tetra-t-butoxide, and tetramethoxysilane.
In this embodiment, a metal alkoxide compound may be added to the reaction system, or a metal alkoxide may be generated in the reaction system and used. For example, a method of reacting zirconium tetrachloride (ZrCl ₄ ) with sodium ethoxide to generate tetraethoxyzirconium may be used. Another method of reacting zirconocene dichloride (Cp ₂ ZrCl ₂ ) with sodium ethoxide to generate zirconocene diethoxide may be used. Another method of reacting dibutyl dichlorotin (Bu ₂ SnCl ₂ ) with sodium ethoxide to generate dibutyl diethoxytin may be used.

上記式中、Ｒ^１２はそれぞれ独立して水素原子または無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^２２は無置換もしくは置換基を有する２価の炭化水素基を表し；Ｒ^３２はそれぞれ独立して無置換もしくは置換基を有する１価の炭化水素基を表し；Ｒ^４２はそれぞれ独立して無置換もしくは置換基を有する炭化水素配位子、－ＯＲ^３２とは異なる、無置換もしくは置換基を有するアルコキシ配位子、アミド配位子、又はハライド配位子を表し；Ｍ’は金属原子または半金属原子を表し；ｎ’はＭ’の酸化数を表し、（ｎ’－ｍ’）は１～６の整数であり；ｍ’は０以上、（ｎ’－１)以下の整数を表し；Ｑ’は（２／ｑ’）価の対カチオンを表し、ｑ’は１又は２である。 <1-5. Method for producing carbamic acid ester represented by formula (A-2)>
In addition, as another embodiment of the present invention, it is preferable that the carbamate is a carbamate represented by formula (B-2), the metal alkoxide is a metal alkoxide represented by formula (C-2), and the carbamate ester having a structure represented by formula (a-2) is a carbamate ester represented by formula (A-2).

In the above formula, R ¹² each independently represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²² each independently represents an unsubstituted or substituted divalent hydrocarbon group; R ³² each independently represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴² each independently represents an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand that is different from -OR ³² ; M' represents a metal atom or a semimetal atom; n' represents the oxidation number of M', and (n'-m') is an integer of 1 to 6; m' represents an integer of 0 or more and (n'-1) or less; Q' represents a (2/q')-valent counter cation, and q' is 1 or 2.

<1-5-1. Carbamic acid ester represented by formula (A-2)>
In formula (A-2), R ¹² each independently represent a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, R ²² each independently represent an unsubstituted or substituted divalent hydrocarbon group, and R ³² each independently represent an unsubstituted or substituted monovalent hydrocarbon group.
For details of ^R12 , the above description of ^R1 applies.
For details of R ²² , the above description of R ²⁰ applies.
For details of R ³² , the above description of R ³ applies.

<1-5-2. Carbamate represented by formula (B-2)>
In formula (B-2), R ¹² each independently represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group, R ²² represents an unsubstituted or substituted divalent hydrocarbon group, and Q′ represents a counter cation.
R ¹² in formula (B-2) corresponds to R ¹² in formula (A-2). R ²² in formula (B-2) corresponds to R ²² in formula (A-2). Therefore, the details of R ¹² are the same as those described above for R ^1. The details of R ²² are the same as those described above for R ² .

(Q')
Q' represents a counter cation in the carbamate, and is not particularly limited as long as it is a monovalent or divalent cation, and examples thereof include ammonium cation, amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, sulfonium cation, iodonium cation, alkali metal cation, alkaline earth metal cation, etc. Specific examples of these cations include those mentioned above in the section on (Q).
From the viewpoint of ease of synthesis, preferred are ammonium cation, amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, alkali metal cation, and alkaline earth metal cation, and more preferred are ammonium cation, amidinium cation, guanidinium cation, and phosphazenium cation. Among these, preferred are tert-butylimino-tri(pyrrolidino)phosphoranium cation and 1,8-diazabicyclo[5.4.0]undec-7-enium cation.
(q′)
q′ is 1 or 2. (2/q′) represents the number of counter cations Q, that is, when Q is a monovalent cation, q′ is 2, and when Q is a divalent cation, q′ is 1.

式（Ｂ－２）で表されるカルバミン酸塩の具体的な例として、下記式で表される化合物及びこれの対カチオンがアミジニウムカチオン、グアニジニウムカチオン、ホスホニウムカチオン、ｔｅｒｔ－ブチルイミノ－トリ（ピロリジノ）ホスホラニウムカチオン以外のホスファゼニウムカチオン、カルボカチオン、アルカリ金属カチオン、又はアルカリ土類金属カチオンである、アミジニウム塩、グアニジニウム塩、ホスホニウム塩、ホスファゼニウム塩、カルボカチオン塩、アルカリ金属塩、又はアルカリ土類金属塩なども挙げられるが、これらに限定するものではない。

Specific examples of the carbamate represented by formula (B-2) include compounds represented by the following formulas and amidinium salts, guanidinium salts, phosphonium salts, phosphazenium salts, carbocation salts, alkali metal salts, or alkaline earth metal salts in which the counter cation is an amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, alkali metal cation, or alkaline earth metal cation other than 1,8-diazabicyclo[5.4.0]undec-7-enium cation, and the like.

Specific examples of the carbamate represented by formula (B-2) include, but are not limited to, compounds represented by the following formula and amidinium salts, guanidinium salts, phosphonium salts, phosphazenium salts, carbocation salts, alkali metal salts, or alkaline earth metal salts in which the counter cation is a phosphazenium cation other than an amidinium cation, guanidinium cation, phosphonium cation, or tert-butylimino-tri(pyrrolidino)phosphoranium cation, a carbocation, an alkali metal cation, or an alkaline earth metal cation.

<1-5-3. Metal alkoxide represented by formula (C-2)>
In formula (C-2), R ³² represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴² each independently represents an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³² ; M' represents a metal atom or a metalloid atom, n' represents the oxidation number of M', (n'-m') is an integer of 1 to 6; m' represents an integer of 0 or more and (n'-1) or less (M').
M' represents a metal atom or a metalloid atom.
The types of metal atoms and metalloid atoms are not particularly limited, and examples thereof include metal atoms and metalloid atoms selected from the group consisting of silicon, titanium, zirconium, germanium, indium, tin, tantalum, zinc, and tungsten. Among these, silicon and titanium are preferred from the viewpoint of availability.
( ^R32 )
R ³² in formula (C-2) corresponds to R ³² in formula (A-2). Therefore, the details of R ³² are the same as those of R ³ described above.
( ^R42 )
Each R ⁴² independently represents an unsubstituted or substituted hydrocarbon ligand, an unsubstituted or substituted alkoxy ligand, an amide ligand, or a halide ligand which is different from —OR ³² .
Specific examples of the unsubstituted or substituted hydrocarbon ligand, the unsubstituted or substituted alkoxy ligand, the amide ligand, and the halide ligand that are different from -OR ³² include the ligands exemplified for R ^41. When m is 2 to 5, multiple R ⁴² may be the same or different, but from the viewpoint of ease of availability, it is preferable that all R ⁴² are the same.
(n′)
n' represents the oxidation number of M', and (n'-m') is an integer of 1 to 6.
(m')
m′ represents an integer of 0 to (n′−1), i.e., an integer of 0 to 5.
Specific examples of the metal alkoxide represented by formula (C-2) include the metal alkoxides exemplified by formula (C-1). In terms of availability, the metal alkoxide is preferably at least one selected from the group consisting of titanium compounds and silicon compounds, and more preferably titanium tetramethoxide, titanium tetra-iso-propoxide, titanium tetra-n-butoxide, titanium tetra-t-butoxide, tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, and tetra-n-butoxysilane.
More preferred are titanium tetramethoxide, titanium tetra-iso-propoxide, titanium tetra-n-butoxide, titanium tetra-t-butoxide, and tetramethoxysilane.

<1-6. Reaction process>
The reaction step may be, for example, by putting the raw material carbamate and metal alkoxide into a sealed reaction vessel and reacting them at 200°C for 48 hours. Alternatively, for example, an amine is put into a sealed reaction vessel, and a carbon dioxide-containing gas is blown in to generate a carbamate. Titanium alkoxide, which is a metal alkoxide, is added thereto, and the mixture is heated to react. The reaction conditions are described below.

(Reaction Solvent)
In the reaction step, a reaction solvent may or may not be used, but it is preferable to use a reaction solvent. The type of reaction solvent is not particularly limited, but ethers such as diethyl ether, tetrahydrofuran, 1,4-dioxane, etc.; nitriles such as acetonitrile, propionitrile, etc.; and aprotic polar solvents such as dimethylacetamide, N,N-dimethylformamide, N,N'-dimethylpropyleneurea, 1,3-dimethyl-2-imidazolidinone, N-methylpyrrolidone, etc. are preferable. As the reaction solvent, an aprotic solvent is more preferable, and ethers and aprotic polar solvents are further preferable, and 1,4-dioxane, which is an aprotic ether, and N-methylpyrrolidone, which is an aprotic polar solvent, are particularly preferable. When the above reaction solvent is used, a carbamic acid ester can be produced more efficiently. In addition, when the difference between the boiling point of the solvent and the boiling point of the product is large, purification can be easily performed, and the solvent can also be reused. One type of reaction solvent may be used, or two or more types may be used.
The amount of the reaction solvent used is not particularly limited and can be appropriately selected depending on the desired carbamic acid ester.

(catalyst)
In one embodiment of the present invention, the reaction step may be carried out in the presence of a catalyst. In particular, when an alkoxysilane is used as the metal alkoxide, the reaction step is preferably carried out in the presence of a catalyst. Examples of the catalyst include organic base carboxylates, alkali metal salts, zinc compounds, titanium (IV) compounds, and zirconium (IV) compounds.
Examples of the organic base carboxylate include acetate of 1,8-diazabicyclo[5.4.0]undec-7-ene, acetate of 1,5-diazabicyclo[4.3.0]non-5-ene, and nitrate of 1,1,3,3-tetramethylguanidine.
Examples of the alkali metal salt include alkali metal acetates such as lithium acetate, sodium acetate, potassium acetate, rubidium acetate, and cesium acetate; alkali metal carbonates such as lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide; and alkali metal alkoxides such as lithium alkoxide, sodium alkoxide, potassium alkoxide, rubidium alkoxide, and cesium alkoxide.
Examples of the zinc compound include zinc halides such as zinc chloride and zinc bromide; zinc sulfate; zinc sulfonates such as zinc p-toluenesulfonate and zinc trifluoromethanesulfonate; and zinc carboxylate derivatives such as zinc formate, zinc acetate, zinc propionate, zinc octanoate, zinc salicylate, zinc pivalate, zinc acrylate, p-chlorobenzoic acid, zinc phenolate, zinc chloroacetate, zinc acetylacetonate, zinc oxalate, zinc trifluoroacetate, and zinc (II)-1,10-phenanthroline complex.
Examples of titanium(IV) compounds include titanium(IV) oxysulfate n-hydrate, TiCl ₄ , TiBr ₄ , Ti(OC ₂ H ₅ )Cl ₃ , Ti(OC ₅ H ₇ )Cl ₃ , and Ti(O-iso-C ₄ H ₉ )Cl ₃ .
Zirconium (IV) compounds include zirconium (IV) chloride and zirconium (IV) oxychloride octahydrate.
The catalyst may be prepared from two or more compounds. For example, zinc acetate and 1,10-phenanthroline may be added to a reaction vessel to form a zinc(II)-1,10-phenanthroline (phen) complex, which may be used as the catalyst.
Among these, zinc compounds such as zinc acetate, zinc pivalate, zinc(II)-1,10-phenanthroline (phen) complex, zinc p-toluenesulfonate, and zinc trifluoromethanesulfonate are preferred, and zinc acetate and zinc(II)-1,10-phenanthroline (phen) complex are more preferred.
The amount of catalyst used (charged amount) in the reaction step is not particularly limited and should be appropriately selected depending on the target carbamic acid ester, but is preferably 10 mol % or more and 70 mol % or less, more preferably 50 mol % or less, based on the amount of the carbamic acid salt. In addition, one type of catalyst may be used, or two or more types may be used.

(Reaction temperature)
The temperature of the reaction step (sometimes referred to as "reaction temperature") is not particularly limited and may be appropriately adjusted depending on the reaction scale, etc. It is usually 140°C or higher, preferably 150°C or higher, and usually 250°C or lower, preferably 210°C or lower.

(Reaction Time)
The reaction time is not particularly limited and may be appropriately adjusted depending on the reaction temperature, reaction scale, etc. It is usually 30 minutes or more, preferably 1 hour or more, and usually 48 hours or less, preferably 24 hours or less, more preferably 20 hours or less.

(Reaction atmosphere)
The atmosphere in the reaction step may be an air atmosphere or an inert gas atmosphere such as nitrogen, argon, etc. The reaction step may be carried out under either pressurized or reduced pressure conditions, and the pressure is usually 0.01 atm or more, preferably 0.05 atm or more, more preferably 0.1 atm or more, and usually 10 atm or less, preferably 5 atm or less, more preferably 2 atm or less.

(Reaction vessel)
The reaction vessel is not particularly limited and should be appropriately selected depending on whether the process is a continuous process or a batch process. In one embodiment of the present invention, the process may be a continuous process or a batch process. In the case of a batch process, the reaction vessel is preferably a sealed reaction vessel (sealed reaction vessel), and more preferably a sealed reaction vessel that has a volume equal to that of a mixture of a carbamate, a metal alkoxide, and optionally a reaction solvent and a catalyst.

(Other processes)
The method for producing a carbamic acid ester according to the present embodiment may include any other step in addition to the reaction step. Examples of the optional step include the carbamic acid salt production step described in the (Carbamic Acid Salt) section, the step of regenerating a metal alkoxide with an alcohol, and a purification step for increasing the purity of the carbamic acid ester. In the purification step, a purification method commonly used in the field of organic synthesis, such as filtration, adsorption, column chromatography, and distillation, can be used.

The present invention will be explained in more detail below with reference to examples, but these can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited to the specific examples shown below.

<Example 1-1>
Benzylammonium N-benzylcarbamate (2.0 mmol), titanium tetramethoxide (2.0 mmol), and 1,4-dioxane (5 mL) were added to a 5 mL sealed reaction vessel and reacted at 200° C. for 48 hours. The yield of N-benzylcarbamic acid alkyl ester was 8%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

<Example 1-2>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-1, except that the reaction temperature was changed as shown in Table 1. The yield is shown in Table 1.

<Example 1-3>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-2, except that the reaction time was changed as shown in Table 1. The yield is shown in Table 1.

<Example 1-4>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-1, except that the amounts of metal alkoxide and solvent were changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-5>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 1-4, except that the reaction temperature was changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-6>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 1-5, except that the reaction temperature was changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-7>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-1, except that the amounts of metal alkoxide and solvent were changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-8>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 1-7, except that the reaction temperature was changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-9>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-7, except that the reaction time and reaction temperature were changed as shown in Table 1. The yield is shown in Table 1.

<Example 1-10>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 1-7, except that the reaction temperature was changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-11>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 1-1, except that the amounts of metal alkoxide and solvent were changed as shown in Table 1. The yields are shown in Table 1.

<Example 1-12>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 1-11, except that the reaction temperature was changed as shown in Table 1. The yields are shown in Table 1.

Examples 1-1 to 1-12 demonstrate that carbamic acid alkyl esters can be produced using various titanium compounds as metal alkoxides.

<Example 2-1>
Benzylammonium N-benzylcarbamate (2.0 mmol), tetramethoxysilane (2.0 mmol), zinc acetate (0.2 mmol), and 1,4-dioxane (4.5 mL) were added to a 5 mL sealed reaction vessel and reacted at 200° C. for 1 hour. The yield of N-benzylcarbamic acid methyl ester was 18%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

<Example 2-2>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 2-1, except that the reaction temperature was changed as shown in Table 2. The yields are shown in Table 2.

<Example 2-3>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 2-2, except that the reaction time was changed as shown in Table 2. The yield is shown in Table 2.

<Example 2-4>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 2-3, except that the reaction temperature was changed as shown in Table 2. The yields are shown in Table 2.

<Example 2-5>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 2-3, except that the catalyst was changed as shown in Table 2. The yield is shown in Table 2. In Table 2, phen represents 1,10-phenanthroline.

It was shown that carbamic acid alkyl esters can be produced using silicon compounds as metal alkoxides.

<Example 3-1>
Benzylamine (2.0 mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (2.0 mmol), and 1,4-dioxane (4.5 mL) were added to a 5 mL sealed reaction vessel, and a CO ₂ /N ₂ mixed gas (v:v=15:85) was bubbled through for 15 minutes. Titanium tetramethoxide (2.0 mmol) was then added, and the reaction was carried out at 200° C. for 1 hour. The yield of N-benzylcarbamic acid methyl ester was 50%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

<Example 3-2>
N-benzylcarbamic acid alkyl esters were obtained in the same manner as in Example 3-1, except that the reaction temperature was changed as shown in Table 3. The yields are shown in Table 3.

<Example 3-3>
N-benzylcarbamic acid alkyl ester was obtained in the same manner as in Example 3-1, except that the volume ratio of the CO ₂ /N ₂ mixed gas was changed as shown in Table 3. The yield is shown in Table 3.

<Example 4-1>
A mixture containing a dicarbamate was prepared by passing a carbon dioxide/nitrogen mixed gas (1 atm, v:v=15:85) through a mixture of hexamethylenediamine (116 mg, 1.0 mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (457 g, 3.0 mmol), and N-methylpyrrolidone (1.0 mL) at a flow rate of 0.1 L/min for 20 minutes. The mixture was placed in a 5 mL sealed reaction vessel, to which titanium tetrabutoxide (680 mg, 2.0 mmol) and N-methylpyrrolidone (3.6 mL) were added, and the mixture was reacted at 180° C. for 3 hours. The yield of N,N'-hexamethylenebis(dibutylcarbamate) was 60%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

<Example 4-2>
A mixture of aniline (186 mg, 2.0 mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (1.52 g, 10.0 mmol), and N-methylpyrrolidone (1.5 ml) was aerated with carbon dioxide/nitrogen mixed gas (1 atm, v:v=15:85) at a flow rate of 0.1 L/min for 20 minutes to prepare a carbamate-containing mixed solution, which was placed in a 5 mL sealed reaction vessel, to which titanium tetrabutoxide (680 mg, 2.0 mmol) and N-methylpyrrolidone (1.6 ml) were added, and the mixture was reacted at 180° C. for 3 hours. The yield of butyl-N-phenylcarbamate was 40%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

<Example 4-3>
A mixture containing a dicarbamate was prepared by passing a carbon dioxide/nitrogen mixed gas (1 atm, v:v=15:85) through a mixture of hexamethylenediamine (116 mg, 1.0 mmol), tert-butylimino-tri(pyrrolidino)phosphorane (1250 mg, 4.0 mmol), and N-methylpyrrolidone (2.0 ml) at a flow rate of 0.1 L/min for 20 minutes. The mixture was placed in a 5 mL sealed reaction vessel, to which titanium tetrabutoxide (680 mg, 2.0 mmol) and N-methylpyrrolidone (3.6 ml) were added, and the mixture was allowed to react at 180° C. for 3 hours. The yield of N,N'-hexamethylenebis(dibutylcarbamate) was 52%. The yield was determined by ¹ H NMR using mesitylene (50 mg) as an internal standard.

It was shown that carbamic acid alkyl esters can be produced by adding titanium alkoxide, a metal alkoxide, to a reaction system in which carbamic acid salts are generated using amine and carbon dioxide as raw materials, and then reacting the metal alkoxide. Moreover, it was shown that carbamic acid alkyl esters can be produced using low-concentration carbon dioxide gas, with the volume of carbon dioxide in the carbon dioxide-containing mixed gas being 15%.

According to the present invention, carbamic acid esters can be produced using carbamic acid salts and metal alkoxides as raw materials. The present invention also makes it possible to produce carbamic acid esters by producing carbamic acid salts using low-concentration carbon dioxide mixed gas as a raw material, which is a reaction that enables the effective use of low-concentration carbon dioxide contained in exhaust gases, etc. Furthermore, since the metal alkoxide used in the present invention can be recovered after the reaction and then regenerated using alcohol, this is an environmentally friendly reaction in which essentially only low-concentration carbon dioxide, amine, and alcohol are consumed.

Claims (8)

The method includes a reaction step of producing a carbamic acid ester represented by the following formula (A-1) or (A-2) from a carbamate and a metal alkoxide,
The carbamate is a carbamate represented by formula (B-1) or (B-2),
The metal alkoxide is at least one selected from the group consisting of alkoxytitanium and alkoxysilane,
When the metal alkoxide is the alkoxysilane, the reacting step is carried out in the presence of a catalyst;
The method for producing a carbamic acid ester , wherein the catalyst is a zinc compound .

(In the above formula, R ¹¹ represents a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²¹ and R ³¹ each independently represent an unsubstituted or substituted monovalent hydrocarbon group; R ¹² each independently represent a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group; R ²² represents an unsubstituted or substituted divalent hydrocarbon group; R ³² each independently represent an unsubstituted or substituted monovalent hydrocarbon group; Q represents a q-valent counter cation; q is 1 or 2; Q' represents a (2/q')-valent counter cation; q' is 1 or 2.) The method for producing a carbamic acid ester according to claim 1, wherein the carbamic acid salt is a carbamic acid salt represented by the formula (B-1), the metal alkoxide is a metal alkoxide represented by the following formula (C-1), and the carbamic acid ester is a carbamic acid ester represented by the formula (A-1):

(In the above formula , R ³¹ each independently represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴¹ each independently represents an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³¹ ; M represents silicon or titanium ; n is 4; (n-m) is an integer of 1 to 4 ; and m is an integer of 0 to 3. ) The method for producing a carbamic acid ester according to claim 1 or 2, wherein Q is a cation selected from the group consisting of ammonium cation, amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, alkali metal cation, and alkaline earth metal cation. The method for producing a carbamic acid ester according to claim 1, wherein the carbamic acid salt is a carbamic acid salt represented by the formula (B-2), the metal alkoxide is a metal alkoxide represented by the following formula (C-2), and the carbamic acid ester is a carbamic acid ester represented by the formula (A-2):

(In the above formula , R ³² each independently represents an unsubstituted or substituted monovalent hydrocarbon group; R ⁴² each independently represents an unsubstituted or substituted hydrocarbon ligand, or an unsubstituted or substituted alkoxy ligand, amide ligand, or halide ligand different from -OR ³² ; M' represents silicon or titanium , n' is 4; (n'-m') is an integer of 1 to 4 ; and m' is an integer of 0 to 3. ) The method for producing a carbamic acid ester according to claim 1 or 4, wherein Q' is a cation selected from the group consisting of ammonium cation, amidinium cation, guanidinium cation, phosphonium cation, phosphazenium cation, carbocation, alkali metal cation, and alkaline earth metal cation. The method for producing a carbamic acid ester according to any one of claims 1 to 5 , wherein the metal alkoxide is an alkoxysilane. The method for producing a carbamic acid ester according to any one of claims 1 to 6 , wherein the reaction step is carried out in the presence of an aprotic solvent. The method for producing a carbamic acid ester according to any one of claims 1 to 7, further comprising a carbamic acid salt production step of producing the carbamic acid salt by contacting an amino group-containing organic compound with a carbon dioxide-containing mixed gas in a solvent in the presence of a base , wherein a volume of carbon dioxide in the carbon dioxide-containing mixed gas is 0.01% or more.