JPH045714B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to compositions and methods for improving the operation of internal combustion engines, particularly by reducing their fuel consumption. In particular, this invention relates to lubricating compositions used in internal combustion engines to reduce fuel consumption, methods of using the lubricating compositions to achieve this objective, and novel and novel methods useful for achieving this objective. This invention relates to additives for lubricants. The additive has the formula R ¹ (COOR ² ) _n () where R ¹ is an aliphatic hydrocarbon group containing about 10 to about 35 carbon atoms and containing no acetylenic unsaturation. whose at least 8 carbon atoms are in linear form, each R ² independently
hydrogen or an alkenyl group containing up to about 18 carbon atoms, at least one R ² is hydrogen and at least one R ² is the alkenyl group, m is an integer from 2 to 5) A composition comprising at least one acidic ester. In view of recent oil scarcity, rising costs of petroleum products, and conservation demands for natural resources such as petroleum, fuel economy is an important factor in the design of engines and the materials used therein. The desirability of a situation in which fuel consumption is minimized is readily recognized due to the conservation requirements of natural resources and the economical nature of the situation to the users of the engine. It is therefore a primary object of the present invention to provide lubricating compositions that minimize fuel consumption when used in internal combustion engines, as well as additive concentrates useful in the preparation of such lubricating compositions. . A further object of the invention is to provide a method for reducing fuel consumption in an internal combustion engine. Another object of the present invention is to provide a composition useful as a lubricant additive for reducing gasoline consumption in automobile engines and the like. A feature of the invention is the ester of the formula described and the use of this ester in lubricants to reduce fuel consumption. In the formula, R ₁ is an aliphatic hydrocarbon group free of acetylenic unsaturation and having from about 10 to about 35 carbon atoms, preferably at least about 12 carbon atoms. In this specification, the term "aliphatic hydrocarbon group" refers to an aliphatic group having a carbon atom directly connected to the remainder of the molecule and having primarily hydrocarbon character, including the following: . (1) Aliphatic hydrocarbon group. That is, divalent to pentavalent saturated aliphatic groups corresponding to decyl, undecyl, dodecyl, tetradecyl, octadecyl, eicosyl, docosyl, triacontanyl, etc., and corresponding olefinic groups. (2) Substituted aliphatic hydrocarbon group. That is, aliphatic groups containing non-hydrocarbon substituents that do not alter the predominant hydrocarbon character of the group, such as hydroxyl, alkoxy groups (particularly lower i.e.
(alkoxy groups having up to 3 carbon atoms), carbalkoxy groups (especially lower carbalkoxy groups), nitro groups, alkylsulfoxy groups,
Some contain an alkyl sulfone group. (3) Hetero group. That is, aliphatic groups having primarily hydrocarbon character but containing atoms other than carbon in the chain or ring. Heteroatoms are well known, such as nitrogen, oxygen, and sulfur. Generally, the number of substituents or heteroatoms present per 10 carbon atoms in the aliphatic hydrocarbon group is up to about 3, preferably up to about 1. At least 8, preferably at least 10 carbon atoms in R ¹ are in linear form. Subject to this restriction, R ¹ may have a branch on any carbon atom in the chain. These branches include pendant alkyl groups such as methyl, ethyl, propyl, butyl and hexyl (including all isomers). Preferred acidic esters are those in which R ¹ is of the formula (Here, R ³ is about 8 to about 30, usually about
a substantially straight-chain alkyl or alkenyl group having from 12 to about 25 carbon atoms), and m is 2. In these compounds, each free bond in the formula is
Connected to COOR ² . That is, the preferred acid ester is an acid ester of substituted succinic acid. Any carbon atom of R ³ may be attached to the succinic acid moiety, either directly or through a methylene group. The term "substantially straight chain" refers to groups containing up to about 2 methyl groups as substituents. Such substituted succinic acids and derivatives thereof (e.g., anhydrides, acid halides, esters) are combined with maleic or fumaric acids or derivatives thereof, preferably maleic anhydride, containing from about 8 to about 30 carbon atoms. It can be conveniently prepared by reaction with one or more olefins containing and having the necessary linear properties. Many such olefins are available in pure form or in the form of commercial mixtures. The olefin linkages in the olefins may be terminal or intermediate, and thus may be present as vinyl groups, vinylidene groups or intermediate olefin groups. Many such compounds contain primarily vinyl or vinylidene groups and can be represented by the formulas _CH2 =CHR and _CH2 = _CR2 , respectively, where each R is independently an alkyl group. Reaction of the olefin or olefin mixture with maleic anhydride or a similar acid or acid derivative typically creates new carbon-carbon-polymer bonds between the maleic acid or similar moiety and the olefin as well as in the olefin. This is an ``ene reaction'' that causes a double bond transition. The conditions for this ene reaction are well known to those skilled in the art and need not be discussed in detail here. The alkenyl group R ² contains up to about 18, usually up to about 12 carbon atoms. Suitable groups of this type are allyl, butenyl, hexenyl, octenyl, decenyl, dodecenyl and octadecenyl (including all isomers). Particular preference is given to lower alkenyl radicals, especially those having 2 to 4 carbon atoms. m is 2 to 5, preferably 2. As mentioned above, since the esters of this invention are acidic esters, at least one R ² is hydrogen and at least one R ² is alkenyl. Particularly preferred esters are the substituted succinic esters mentioned above in which one R ² is hydrogen and the other R ² is an alkenyl group containing 1 to 4 carbon atoms, usually an allyl group. The acidic esters of this invention can be prepared in conventional manner, for example by reacting at least one suitable polycarboxylic acid or derivative thereof with at least one unsaturated alcohol. In this case, the number of equivalents of alcohol is at least one less than the number of equivalents of acid or its derivative. Although substituted succinic anhydrides are the preferred acid derivatives, substituted succinic acids or acid halides may also be used. In the latter case, it is necessary to hydrolyze the remaining acid halide groups to free acid groups. Details of methods for making acid esters are known to those skilled in the art and need not be discussed in further detail here. Examples of producing useful acid esters are described below. All "parts" and "%" are by weight unless otherwise indicated, and phenolphthalein was used as an indicator in determining all acid values. Example 1 500 parts of dodecenylsuccinic anhydride (3.74 equivalents)
and 108 parts (1.87 equivalents) of allyl alcohol was heated at 110° C. for 10 hours with stirring. further adding 2.5 parts and 6.5 parts of allyl alcohol;
Heating was continued for 4 hours. The reaction mixture was then vacuum stripped. The residue was the desired acidic allyl ester with an acid value of 173.6. Example 2 To a reactor containing 2254 parts (23 moles) of maleic anhydride was added 5700 parts (25 moles) of a commercial mixture of _C15 - _C18 predominantly linear alpha-olefins. The mixture was heated at 209-214°C under nitrogen for about 15 hours. Add 1500 parts of textile benzyl and continue stirring for 1 hour, then filter the reaction mixture through a filter aid,
Volatiles were removed by vacuum stripping. The residue was filtered again. The filtrate was the desired substituted succinic anhydride. 485 grams (2.94 equivalents) of this substituted succinic anhydride
and 85.9 grams (1.47 equivalents) of allyl alcohol was heated at 110° C. for 9.5 hours with stirring. 150 ml of toluene were added and water was removed from the mixture by azeotropic distillation. The toluene was then removed under vacuum and 10.6 grams of allyl alcohol and 10.1
Added more grams. This mixture was vacuum stripped. The residue was the desired acidic allyl ester with an acid value of 154. Example 3 A portion of the commercial α-olefin mixture of Example 2 was heated at 180-195°C in the presence of an acidic ion exchange resin.
Isomerization was achieved by heating for 10 hours. This isomerization mixture was reacted with maleic anhydride and then the resulting substituted succinic anhydride was reacted with allyl alcohol. These reaction steps were the same as in Example 2 except that azeotropic distillation in the presence of toluene was omitted.
It was the same. In this way, the desired acidic allyl ester was obtained. Its acid value was 147. Example 4 The process of Example 3 was repeated using a commercially available mixture of _C15 to _C20 predominantly linear α-olefins. acid value
136.6 of product was obtained. Example 5 A mixture of C ₁₆ to _{C 18} predominately linear α-olefins is isomerized substantially according to the steps of Example 3;
This isomer was reacted with maleic anhydride. The obtained substituted succinic anhydride was converted to an acidic allyl ester. Its acid value was 118. Example 6 Substantially following the steps of Example 2, a commercially available mixture of _C18 - _C24 olefins consisting primarily of vinyl and vinylidene group-containing alpha-olefins was reacted with maleic anhydride. The obtained substituted succinic anhydride was reacted with allyl alcohol. The product was the desired acidic allyl ester with an acid number of 111. Examples 7-8 Substituted succinic anhydride according to the steps of Examples 4 and 5, respectively, wherein the substituents are
The corresponding acidic allyl esters were obtained from commercial mixtures of _C24 - _C28 alpha-olefins and those derived from their isomerized derivatives. The acid values of the products were 79.7 and 87.6, respectively. As previously stated, the ester-containing compositions of this invention are useful as lubricant additives, primarily for reducing fuel consumption. The compositions of this invention can be used in a variety of lubricants based on a variety of oils of lubricating viscosity, including natural and synthetic lubricating oils as well as mixtures thereof. These lubricants include crankcase lubricants for spark ignition and compression ignition internal combustion engines such as automobile and truck engines, two-stroke engines, piston aircraft engines, marine and railroad diesel engines, and the like. The compositions of this invention may also be used in gas engines, stationary power engines, turbines, and the like. Automotive transmission fluids, transmission shaft lubricants, gear lubricants, metalworking lubricants, pressure fluids and other lubricating oil and grease compositions, etc., also benefit from the compositions of this invention. Natural oils include liquid petroleum, paraffinic, naphthenic, or mixed paraffin-naphthenic solvent-treated or acid-treated mineral lubricating oils. Oils with lubricating viscosities derived from coal or shale are also useful base oils. Synthetic lubricating oils include polymerized and interpolymerized olefins [e.g., polybutylene, polypropylene, propylene-isobutylene copolymer, chlorinated polybutylene, poly(1-hexene), poly(1-octene), poly(1-decene)]. , alkylbenzenes [e.g. dodecylbenzene, tetradecylbenzene, dinonylbenzene, di(2-ethylhexyl)benzene], polyphenyl (e.g. biphenyl, terphenyl, alkylated polyphenyl),
Included are hydrocarbon oils and halo-substituted hydrocarbon oils such as alkylated diphenyl ethers and alkylated diphenyl sulfides and their derivatives, analogs and congeners. Polymers and interpolymers of alkylene oxide, as well as derivatives thereof whose terminal hydroxyl groups have been modified by esterification, etherification, etc., are also known synthetic lubricating oils. Examples of these are:
Polyoxyalkylene polymers obtained by polymerization of ethylene oxide or propylene oxide, alkyl and aryl ethers of these polyoxyalkylene polymers (for example, methylpolyisopropylene glycol ether with an average molecular weight of 1000,
Diphenyl ether of polyethylene glycol with a molecular weight of 500-1000, diethyl ether of polypropylene glycol with a molecular weight of 1000-1500, etc.) or mono- and polycarboxylic acid esters such as acetate ester, mixed _C3 - _C8 fatty acid ester, or tetraethylene glycol. C ₁₃ oxo acid diester, etc. Other suitable synthetic lubricating oils include dicarboxylic acids (e.g. phthalic acid, succinic acid, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, malonic acid) and various alcohols (e.g. butyl alcohol, Hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacin, diisooctyl azelaate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, It is a complex ester obtained by reacting residual dieicosyl sebacic acid and 1 mole of sebacic acid with 2 moles of tetraethylene glycol and 2 moles of 2-ethylhexanoic acid. Esters useful as synthetic oils include those made from C ₅ -C ₁₂ monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythrite, dipentaerythrite, tripentaerythrite, etc. Also included. Silicone-based oils such as polyalkyl-, polyaryl-, polyalkoxy- or polyaryloxy-siloxane oils and silicate oils are also useful synthetic lubricants (e.g., tetraethylsilicate, tetraisopropylsilicate, tetra(2-ethylhexyl)). silicate, tetra(4-methyl-2-ethylhexyl) silicate,
tetra(p-tert-butylphenyl)silicate, hexa(4-methyl-2-pentoxy)disiloxane, poly(methyl)siloxane, poly(methylphenyl)siloxane, etc.). Other synthetic lubricating oils include liquid esters of phosphorous acids (tricresyl phosphate, trioctyl phosphate, diethyl ester of decanephosphonic acid, etc.), polymerized tetrahydrofurans, and the like. Unrefined, refined and rerefined oils of the above types, natural or synthetic (and even mixtures of two or more of these), may be used in the lubricant compositions of this invention. Unrefined oils are those obtained directly from natural or synthetic sources and have not undergone any refining treatment. For example, sierre oil obtained directly from a retorting operation, petroleum oil obtained directly from distillation, or ester oil obtained directly from an esterification process and used without further treatment are unrefined oils. Refined oils are similar to unrefined oils except that they have been treated with one or more refining steps to improve one or more properties. Many such purification methods are known to those skilled in the art. For example, distillation, solvent extraction, acid or base extraction, filtration, percolation, etc.
Rerefined oil is obtained by applying a method similar to that used to obtain refined oil to already used refined oil. Such rerefined oils, also known as reclaimed oils, are often further processed by methods to remove spent additives and oil breakdown products. The present invention is a method for reducing fuel consumption in an internal combustion engine, the engine being operated during its operation using a large amount of lubricating oil and a small amount of the formula R ¹ (COOR ⁴ ) _n () (where R ¹ and m is the same as the formula, each R ⁴ is
independently hydrogen or at least one ester having at least one R ⁴ alkyl or alkenyl group containing up to about 12 carbon atoms. It also relates to a product characterized in that it is lubricated with. Esters of the formula include, in addition to esters of the formula, similar neutral esters and alkanol esters. The neutral ester is present in at least 2 equivalents (usually 2 equivalents) per equivalent of acid or acid derivative.
The acidic alkenyl esters can be prepared in a similar manner to the above-mentioned acidic alkenyl esters using more than an equivalent amount of alcohol. Suitable esters (acidic and neutral) include methanol, ethanol, probanol, butanol, hexanol, octanol, decanol,
It can also be prepared from alkanols such as dodecanol and octadecanol (including all isomers). Preferred alkanols are those having 1 to 4 carbon atoms, especially n-propanol. However, the esters used in the process of this invention are usually esters of the formula R ¹
is shown by the formula. Examples of producing alkyl esters used in the present invention will be described below. Example 9 A substituted succinic anhydride substantially identical to Example 6 was similarly reacted with an equal amount of n-propyl alcohol. The product was the desired acidic n-propyl ester with an acid number of 126. Generally, the lubricating compositions of this invention contain a sufficient amount of the ester-containing compositions of this invention to provide fuel consumption reducing properties. Typically, this amount is approximately
0.05 to about 5.0% by weight, preferably about 0.1 to about 2.0% by weight. This invention also contemplates the use of other additives in addition to the ester-containing compositions of this invention. Other additives of this type are, for example, ash-forming or ashless detergents and dispersants, corrosion inhibitors and antioxidants, pour point depressants, extreme pressure agents, color stabilizers,
and antifoaming agents. Examples of ash-forming detergents include sulfonic acids,
A carboxylic acid or an olefin polymer (e.g., polyisobutene with a molecular weight of 1000) is treated with a phosphating agent such as phosphorus trichloride, phosphorus heptasulfide, phosphorus pentasulfide, phosphorus trichloride and sulfur, white phosphorus and sulfur halide, or phosphorodithioyl chloride. oil-soluble neutral and basic salts of organic phosphorus-containing acids characterized by having at least one direct carbon-phosphorous bond, such as those obtained by treatment with alkali metals or alkaline earth metals; be. The most commonly used salts are those of sodium, potassium, lithium, calcium, magnesium, strontium and barium. "Basic salt" means a metal salt in which the metal is present in a stoichiometrically larger amount than the organic acid radical.
A commonly used method for preparing this basic salt is to neutralize a mineral oil solution of the acid with a stoichiometric excess of the metal, e.g. metal oxide, hydroxide, carbonate, bicarbonate or sulfide. This method involves heating the product at a temperature of 50°C or higher and filtering the resulting product.
It is known to use "accelerators" in the neutralization step to assist in introducing large excesses of metal.
Examples of compounds useful as accelerators include phenolics such as phenols, naphthols, alkylphenols, thiophenols, sulfurized alkylphenols, and condensation products of formaldehyde and phenolic compounds; methanol, 2
- alcohols such as propanol, octyl alcohol, cellosolve, carbitol, ethylene glycol, stearyl alcohol and cyclohexyl alcohol; and aniline, phenylene diamine, phenothiazine, phenyl-β-
Amines such as naphthylamine and dodecylamine. A particularly effective method for making basic salts is to mix the acid with an excess of a basic alkaline earth metal neutralizer and at least one alcoholic promoter, and to bring the mixture to an elevated temperature. The process consists of carbonation, for example at 60-200°C. Ashless detergents and dispersants are so called despite the fact that, depending on their structure, they burn to produce non-volatile substances such as boron oxide and phosphorus pentoxide. However, it usually does not contain metals and therefore does not produce metal-containing ash upon combustion. Many types thereof are known, all of which are suitable for the lubricants of this invention. Examples are listed below. (1) at least about 34, preferably at least about
Reaction products of carboxylic acids (or their derivatives) having 54 carbon atoms with nitrogen-containing compounds such as amines, organic hydroxyl compounds such as phenols and alcohols, and/or basic inorganic substances. Examples of this "carboxylic acid dispersant" are described in US Pat. Nos. 3,272,746, 3,381,022 and 4,234,435. (2) Reaction products of relatively high molecular weight aliphatic or cycloaliphatic halides and amines, preferably polyalkylene polyamines. These can be referred to as "amine-based dispersants" and are described, for example, in the following US patents: Nos. 3275554, 3454555, 3438757, 3565804(3) Alkylphenols in which the alkyl group contains at least about 30 carbon atoms and aldehydes (especially formaldehyde) and amines (especially polyalkylene polyamines) reaction product. This can be characterized as a "Mannitsuchi dispersant" and is described in U.S. Pat.
No. 3413347 and No. 3980569. (4) The above carboxylic acid-based, amine-based or Mannitz-based dispersants may be mixed with urea, thiourea, carbon disulfide, aldehyde, ketone, carboxylic acid, hydrocarbon-substituted succinic anhydride, nitrile, epoxide, boron compound, phosphorus compound, etc. The product obtained by post-treatment with the reagent. Materials of this type are described in numerous US patents. (5) Oil-soluble monomers such as decyl methacrylate, vinyl decyl ether, and high molecular weight olefins, and monomers with polar groups such as aminoalkyl acrylates, acrylamide, and poly(oxyethylene)-substituted acrylates. interpolymers of. This is characterized as a "polymer-based dispersant" and is described in the following US patents: No. 3329658 No. 3666730 No. 3449250 No. 3687849 No. 3519565 No. 3702300 Examples of extreme pressure agents and corrosion inhibitors and antioxidants include chlorinated aliphatic hydrocarbons such as chlorinated waxes, Benzyl sulfide, bis(chlorobenzyl) disulfide, dibutyl tetrasulfide, sulfurized methyl ester of oleic acid, sulfurized alkylphenol,
Organic sulfides and polysulfides such as dipentene sulfide and terpene sulfide, phosphosulfide hydrocarbons such as the reaction products of phosphorus sulfide with turpentine or methyl oleate, dibutyl phosphite, diheptyl phosphite, phosphorous acid dicyclohexyl, pentylphenyl phosphite, dipentylphenyl phosphite,
Tridecyl phosphite, distearyl phosphite, dimethylnaphthyl phosphite, oleyl phosphite 4-
Pentyl phenyl, polypropylene (molecular weight 500) substituted phenyl phosphite, diisobutyl phosphite substituted phenyl, and other phosphorus-containing esters based on dihydrocarbon or trihydrocarbon phosphite, zinc dioctyl dithiocarbamate, and barium heptyl dithiocarbamate. Thiocarbamate metal salts such as phenyl, zinc dicyclohexylphosphorodithioate, zinc dioctylphosphorodithioate, barium di(heptylphenyl)phosphorodithioate, cadmium dinonylphosphorodithioate,
and Group metal salts of phosphorodithioic acids, such as the zinc salts of phosphorodithioic acids obtained by the reaction of phosphorus pentasulfide with an equimolar mixture of isopropyl alcohol and n-hexyl alcohol. The ester-containing compositions of this invention can be added directly to lubricants. Preferably, however, it is diluted to form a concentrate with a substantially inert, normally liquid, organic diluent such as mineral oil, naphtha, benzene, toluene or xylene.
The additive concentrate is generally from about 5 to about 50% by weight.
The composition contains the ester-containing composition of the present invention, and may also contain one or more of the other additives mentioned above. The table below lists examples of lubricating compositions of this invention. All amounts except mineral oil exclude oil used as diluent. Addition of the ester-containing compositions described above to these lubricants reduces fuel consumption when used in internal combustion engines. 【table】

Claims (1)

Claims: 1. A composition for use as a lubricant additive to reduce fuel consumption, having the formula R ¹ (COOR ⁴ ) _n , where R ¹ is about 14 to about 27 carbon atoms. an aliphatic hydrocarbon group containing and free of acetylenic unsaturation, at least 8 carbon atoms of which are in the form of a straight chain, each R ⁴ being independently:
hydrogen or an alkyl or alkenyl group containing up to about 18 carbon atoms, wherein at least one R ⁴ is hydrogen and at least one
R ⁴ is an alkyl or alkenyl group,
m is an integer from 2 to 5). 2 R ¹ is the formula (wherein R ₃ is a substantially linear alkyl or alkenyl group, and m is 2). 3. The composition of claim 2, wherein ^R4 contains up to about 12 carbon atoms. 4. The composition of claim 3, wherein one R ⁴ contains 2 to 4 carbon atoms. 5. The composition according to claim 4, wherein R ⁴ is an allyl group. The composition according to claim 4, wherein 6 R ⁴ is an n-propyl group. 7. An organic diluent that is substantially inert and normally liquid, and of the formula R ¹ (COOR ⁴ ) _n , where R ¹ contains about 14 to about 27 carbon atoms and is an acetylenic diluent. an aliphatic hydrocarbon group containing no saturation, at least 8 carbon atoms of which are in the form of a straight chain, and each R ⁴ independently represents
hydrogen or an alkyl or alkenyl group containing up to about 18 carbon atoms, wherein at least one R ⁴ is hydrogen and at least one
R ⁴ is an alkyl or alkenyl group,
1. An additive concentrate comprising from about 20 to about 90% by weight of a composition containing at least one acid ester having the formula m is an integer from 2 to 5. 8 A large amount of lubricating oil and the formula R ¹ (COOR ⁴ ) _n where R ¹ is an aliphatic hydrocarbon group containing about 14 to about 27 carbon atoms and containing no acetylenic unsaturation. and the at least 8 carbon atoms are in the linear form, and each R ⁴ is independently
hydrogen or an alkyl or alkenyl group containing up to about 18 carbon atoms, wherein at least one R ⁴ is hydrogen and at least one
R ⁴ is an alkyl or alkenyl group,
A lubricating composition comprising a small amount of a composition containing at least one acid ester (m is an integer from 2 to 5). 9. A method of reducing fuel consumption in an internal combustion engine, comprising: operating the engine during its operation using a large amount of lubricating oil and a small amount of the formula R ¹ (COOR ⁴ ) _n , where R ¹ is from about 14 to about 27 each R ⁴ independently represents
hydrogen, or an alkyl or alkenyl group containing up to about 18 carbon atoms, at least one R ₄ being the alkyl or alkenyl group, and m being an integer from 2 to 5. A lubricating composition containing an acidic ester of species. 10 R ¹ is the formula (Here, R ³ is a substantially linear alkyl group or alkenyl group, and m is 2)
The method according to claim 9, which is represented by: 11. The method of claim 10, wherein 11 R ⁴ contains up to about 12 carbon atoms. 12. The method of claim 11, wherein one R ⁴ contains 1 to 4 carbon atoms. 13. A method according to claim 12, wherein 13 R ⁴ is an alkenyl group having 2 to 4 carbon atoms. 14 Claim 1 in which R ⁴ is an allyl group
The method described in Section 3. 13. The method according to claim 12, wherein 15 R ⁴ is an alkyl group. 16. The method according to claim 15, wherein 16 R ⁴ is an n-propyl group.