Process for the production of esters

Abstract

A process for making methyl esters in high yields. The process comprises contacting aliphatic or aromatic aldehydes and methanol with a homogeneous dimeric ruthenium catalyst, to catalyze the dehydrogenative coupling between aliphatic or aromatic aldehydes and methanol. The reaction is highly selective (<99.9%) toward the formation of methyl esters over homoesters and alcohols and operates at temperatures of less than 100 C. for 2-8 hours.

Claims

1. A process for preparing esters comprising: a) combining an aldehyde having the formula R.sub.1CO with an alcohol having the formula R.sub.2OH and a dimeric ruthenium catalyst to form a first mixture; b) heating the first mixture to form an ester having the formula R.sub.1COR.sub.2O and H.sub.2; wherein R.sub.1 is a C.sub.4 to C.sub.8 aliphatic, alicyclic or aromatic group and wherein R.sub.2 is a C.sub.1 to C.sub.4 group, and wherein the weight percent of the ester after step b) is greater than 97 wt. %.

2. The process of claim 1 wherein said catalyst is 1-Hydroxytetraphenyl-cyclopentadienyl(tetraphenyl-2,4-cyclopentadien-1-one)-mu-hydrotetracarbonyldiruthenium(II).

3. The process of claim 1 wherein step b) is conducted at a temperature of less than 100 C. for about 2 hours to about 8 hours.

4. The process of claim 1 wherein the molar ratio of R.sub.1CO to R.sub.2OH in said first mixture is from 1:1 to 1:5.

5. The process of claim 1 wherein the molar amount of catalyst in said first mixture is from about 0.25 mole percent to about 1 mole percent.

6. The process of claim 1 wherein said first mixture further comprises a non-polar aromatic solvent.

7. The process of claim 6 wherein said non-polar aromatic solvent is toluene, p-xylene, mesitylene or a mixture thereof.

8. The process of claim 1 wherein step b) is conducted at a pressure of from atmospheric pressure to 2 ATM.

9. The process of claim 1 wherein R.sub.2OH is selected from the group consisting of methanol, ethanol, propanol, isopropanol, butanol, isobutanol and ethylene glycol.

10. The process of claim 1 wherein R.sub.2OH is methanol.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) As used herein the following terms have the meaning as described below:

(2) The term TEG-2EH refers to triethylene glycol di-ethyl hexanoate.

(3) The term 2EH refers to 2-ethylhexaldehyde.

(4) The term 2EHOMe refers to methyl 2-ethylhexanoate.

(5) The term 2EHOH refers to 2-ethylhexyl alcohol.

(6) The term 2EHOEH refers to (2-ethylhexyl)2-ethylhexanoate.

(7) The term 2EHenal refers to 2-ethylhexenal.

(8) The term iHBu refers to isobutyraldehyde.

(9) The term nHBu refers to n-butyraldehyde.

(10) The term DMT refers to dimethyl terephthalate.

(11) As used herein, the indefinite articles a and an mean one or more, unless the context clearly suggests otherwise. Similarly, the singular form of nouns includes their plural form, and vice versa, unless the context clearly suggests otherwise.

(12) While attempts have been made to be precise, the numerical values and ranges described herein should be considered to be approximations (even when not qualified by the term about). These values and ranges may vary from their stated numbers depending upon the desired properties sought to be obtained by the present invention as well as the variations resulting from the standard deviation found in the measuring techniques. Moreover, the ranges described herein are intended and specifically contemplated to include all sub-ranges and values within the stated ranges. For example, a range of 0 to 100 is intended to describe and include all values within the range including sub-ranges such as 0.1-99.9, 60 to 90 and 70 to 80.

(13) It has been surprisingly discovered that an efficient method to form a variety of methyl esters via direct coupling of oxo aldehydes with MeOH in the presence of Shvo's catalyst.

(14) ##STR00001##

Shvo's Catalyst

(15) Shvo's catalyst (1-Hydroxytetraphenyl-cyclopentadienyl(tetraphenyl-2,4-cyclopentadien-1-one)-mu-hydrotetracarbonyldiruthenium(II)) is available commercially from Millipore Sigma.

(16) We prepared an initial reaction with 2-ethylhexaldehyde (2EH) and methanol (MeOH) as model substrates (Table 1). In the absence of a catalyst, reacting 2-ethylhexaldehyde with excess of methanol (5 equivalents) converted the aldehyde only into a dimethyl acetal in MeOH at 90 C. No ester formation was observed under catalyst-free condition. Next, several homogeneous catalysts were screened in order to facilitate the formation of corresponding methyl 2-ethylhexanoate (2EHOMe) and these results are summarized in Table 1. Simple metal precursor complexes such as [Cp*RhCl.sub.2].sub.2, [Cp*IrCl.sub.2].sub.2, Ru.sub.3(CO).sub.12 (Cp*=1,2,3,4,5-pentamethylcyclopentadiene) failed to produce any 2-ethylhexyl alcohol (entries 1-3) and only trace amounts of 2-ethylhexyl alcohol (2EHOH) were produced in these reactions. Well-defined ruthenium and iridium-based homogeneous dehydrogenation catalysts, supported by pincer-type ligands, also showed negligible reactivity and selectivity toward forming methyl 2-ethylhexanoate. For example, Milstein's (PNN)Ru(H)(CO)Cl catalyst 14 (available commercially from Sigma Aldrich) in the presence of KOH afforded methyl 2-ethylhexanoate and 2-ethylhexylalcohol with a relative ratio of 1:1.2 and the overall conversion of 2EH reached to 22% after 6 hours at 90 C. (entry 4). The homoester of 2EH aldehyde, (2-ethylhexyl)2-ethylhexanoate (2EHOEH), was also formed in a considerable amount (11%) during this reaction. In addition to Milstein's catalyst, Takasago's Ru-MACHO15 catalyst (available commercially from Millipore Sigma) afforded 2-ethylhexylalcohol and (2-ethylhexyl)2-ethylhexanoate as major products and methyl 2-ethylhexanoate was only produced in small quantities (entry 5).

(17) ##STR00002##

(18) TABLE-US-00001 TABLE 1 Catalyst Screening Studies for 2EH-MeOH Coupling Reaction. Yield of Yield of Yield of Additives 2EHOMe 2EHOH 2EHOEH Entry Catalyst (mol %) (mol %) (%) (%) (%) 1.sup.a [Cp*RhCl.sub.2].sub.2 (0.5) NaOAc(2) 0 3.1 0 2.sup.a [Cp*IrCl.sub.2].sub.2 (0.5) NaOAc(2) 0 2.2 0 3.sup.a Ru.sub.3(CO).sub.12 (0.33) 0 6.9 0 4 NaOMe(2) 2.3 13.2 11.6 5 NaOMe(2) 1.1 15.3 13.9

(19) A dimeric ruthenium catalyst, 1-Hydroxytetraphenyl-cyclopentadienyl(tetraphenyl-2,4-cyclopentadien-1-one)-mu-hydrotetracarbonyldiruthenium(II) (C.sub.62H.sub.41O.sub.6Ru.sub.2) commonly known as the Shvo's catalyst, showed excellent reactivity and selectivity toward the formation of methyl esters from corresponding aldehydes via coupling with MeOH. When one equivalent of 2EH aldehyde was treated with five equivalents of MeOH at 90 C. in the presence of 0.5 mol % of Shvo's catalyst, almost exclusive (>99.5 wt %) formation of methyl 2-ethylhexanoate (2EHOMe) was observed after 5 hours (Table 2, entry 1). No starting aldehyde remained after the reaction suggesting 100% conversion within 5 hours. A trace amount of 2EHOEH homoester was also produced as a byproduct in this reaction. Analysis of the crude reaction mixture by gas chromatography (GC) did not detect the formation of 2EHOH, a commonly occurring side-product in such reactions. The catalytic reaction could also be carried out in non-polar aromatic solvents such as toluene, p-xylene or mesitylene without affecting the reactivity and selectivity. Analysis of the volatile components by 1H NMR spectroscopy clearly showed the formation of H.sub.2 gas ( 4.46, singlet, benzene-d6) from the coupling reaction.

(20) Table 2 summarizes the generality of this coupling method to form methyl esters. Isobutyraldehyde reacted smoothly with methanol to afford methyl isobutyrate in a quantitative yield within four hours (entry 2). A trace amount of isobutyl isobutyrate (IBIB) was also detected by GC.

(21) The coupling reaction between 2-ethylhexenal (2EHenal), a precursor of 2EH, and methanol produced the saturated methyl 2-ethylhexanoate in 97.5 wt % yield (entry 3). The rest of the material accounts for unreacted 2EHenal and 2EHOH. This result indicates that the H.sub.2 gas produced during the first 2EHenal-methanol coupling step is used to hydrogenate the CC double bonds in-situ in the presence of Shvo's catalyst. When methyl cinnamate, an ,-unsaturated methyl ester, was treated H.sub.2 (3 atm) with 1 mole % of Shvo's catalyst in toluene, complete reduction of the CC double bond occurred. This direct coupling of 2EHenal with methanol is a highly important discovery because it eliminates the step for producing 2EH from 2EHenal by hydrogenation with a heterogeneous catalyst (Scheme 1). In addition to the aforementioned substrates aliphatic and aromatic aldehydes like n-butyraldehyde and terephthalaldehyde also afforded corresponding methyl esters in high yields and with minimal to almost no side-product formation.

(22) ##STR00005##

(23) TABLE-US-00002 TABLE 2 Ru-Catalyzed Synthesis of Methyl Esters from Aldehydes..sup.a entry substrate product time (h) wt% yield 1 5 99.5 2 4 99.8 3 0 6.5 97.8.sup.b 4 2 98.3 5 8 >99.9 .sup.aCatalytic Conditions: [catalyst] = 0.025 (M), [aldehyde] =5.0 (M), MeOH = 50 mmol, 2 mL, neat conditions, stir speed = 350 rpm, 90 C. (oil-bath temperature). Starting 2EHenal (i.e., <100% conversion) and 2EHOH were detected by GC.

(24) After the catalytic reaction was complete (90 C., 5 hours), Shvo's catalyst was precipitated by adding cold MeOH (10 mL) and isolated by filtration. A comparison between the isolated material and commercially available Shvo's catalyst by .sup.1H NMR spectroscopy revealed that the isolated material was indeed intact Shvo's catalyst.

(25) This result suggests that the catalyst remains unchanged after the first catalytic run. Encouraged by this study, we next carried out a successive-addition experiment to test if the catalytic activity of Shvo's catalyst drops or remain unaltered after several catalytic runs (i.e., longer time). The catalytic activity of Shvo's catalyst did not change after five consecutive catalytic runs indicating the catalyst remains robust under our current conditions. To test the thermal stability of Shvo's catalyst above 90 C., we heated a mesitylene-d.sub.12 solution of Shvo's catalyst at 150 C. for 16 hours and monitored the reaction by .sup.1H NMR spectroscopy to study its decomposition products. Under this condition, the catalyst did not undergo decomposition as no additional resonances appeared in the .sup.1H NMR spectrum after 16 hours. These experiments collectively suggest that Shvo's catalyst is thermally stable within the temperature range of 90-150 C. and could be reused for catalytic purposes.

(26) The temperature conducive for the TEG-2EH reaction may range, for example, from 120 to 180 C., 125 to 180 C., 130 to 180 C., 135 to 180 C., 138 to 180 C., 120 to 175 C., 120 to 170 C., 120 to 165 C., 120 to 160 C., or 138 to 160 C.

(27) The pressure at which the transesterification reaction may be carried out is not particularly limiting. For example, the pressure may range from atmospheric to 2 MPa. The reaction may be performed in an open reactor where the produced methanol may be withdrawn as the reaction proceeds. Alternatively, the reaction may be performed in a sealed reactor where the produced methanol remains in the reactor.

(28) The process according to the invention can produce TEG-2EH with yields of at least 90%, at least 95% or at least 99%. The reaction times in which these yields may be achieved include, 8 hours or less, 7 hours or less, 6 hours or less, 5 hours or less, 4 hours or less, 3 hours or less, 2 hours or less, or 1 hour or less.

(29) This invention can be further illustrated by the following examples of preferred embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLES

(30) Unless otherwise noted, all the organometallic compounds were handled under a nitrogen atmosphere using standard Schlenk and glovebox techniques. Anhydrous methanol (99.7% grade), toluene and mesitylene (98%) were purchased from Sigma Aldrich and stored with 4 molecular sieves. All aldehydes were purchased from commercial sources and freshly vacuum-distilled prior to use. Shvo's catalyst was purchased from Strem Chemicals and used without further purification. Other commercially available catalysts were purchased from respective commercial sources and used without further purification. 1H NMR spectra were recorded on Bruker Avance-500 MHz spectrometers. Chemical shift values in 1H NMR spectra were referenced internally to the residual solvent resonances ( 7.16 for benzene-d6).

Example 1: Synthesis of Methyl 2-ethylhexanoate from 2-ethylhexaldehyde

(31) Under a nitrogen atmosphere, an oven-dried 100 mL Schlenk tube equipped with a magnetic stir-bar and a Teflon plug was charged with the Shvo's catalyst (54 mg, 50 mol, 0.5 mol %), 2-ethylhexaldehyde (2EH) (1.6 mL, 10 mmol), and anhydrous methanol (2 mL). The resulting mixture was heated at 90 C. for 5 hours using an oil-bath. The vessel was gradually allowed to come to room temperature and the volatiles (H.sub.2 and MeOH vapor) were carefully removed by opening the Teflon plug inside the hood. The liquid sample was analyzed by gas chromatography (GC) to determine the weight percent of methyl 2-ethylhexanoate (99.5 wt %).

Example 2: Synthesis of Methyl Isobutyrate from iHBu

(32) Under a nitrogen atmosphere, an oven-dried 100 mL Schlenk tube equipped with a magnetic stir-bar and a Teflon plug was charged with the Shvo's catalyst (54 mg, 50 mol, 0.5 mol %), isobutyraldehyde (iHBu) (0.92 mL, 10 mmol, 99% purity), and anhydrous methanol (2 mL). The resulting mixture was heated at 90 C. for 4 hours using an oil-bath. After that the vessel was gradually allowed to come to room temperature and the volatiles (H.sub.2 and MeOH vapor) were carefully removed by opening the Teflon plug inside the hood. The liquid sample was analyzed by GC to determine the weight percent of methyl isobutyrate (99.8 wt %).

Example 3: Synthesis of Methyl 2-ethylhexanoate from 2-EHenal

(33) Under a nitrogen atmosphere, an oven-dried 100 mL Schlenk tube equipped with a magnetic stir-bar and a Teflon plug was charged with the Shvo's catalyst (54 mg, 50 mol, 0.5 mol %), 2-ethyl-2-hexenal (2EHenal) (1.57 mL, 10 mmol), and anhydrous methanol (2 mL). The resulting mixture was heated at 90 C. for 6.5 hours using an oil-bath. After that the vessel was gradually allowed to come to room temperature and the volatiles (H.sub.2 and MeOH vapor) were carefully removed by opening the Teflon plug inside the hood. The liquid sample was analyzed by GC to determine the weight percent of methyl 2-ethylhexanoate (97.8 wt %).

Example 4: Synthesis of Methyl Butyrate from nHBu

(34) Under a nitrogen atmosphere, an oven-dried 100 mL Schlenk tube equipped with a magnetic stir-bar and a Teflon plug was charged with the Shvo's catalyst (54 mg, 50 mol, 0.5 mol %), butyraldehyde (nHBu) (0.94 mL, 10 mmol), and anhydrous methanol (2 mL). The resulting mixture was heated at 90 C. for 2 hours using an oil-bath. After that the vessel was gradually allowed to come to room temperature and the volatiles (H.sub.2 and MeOH vapor) were carefully removed by opening the Teflon plug inside the hood. The liquid sample was analyzed by GC to determine the weight percent of methyl butyrate (98.3 wt %).

Example 5: Synthesis of Dimethyl Terephthalate (DMT) from Terephthalaldehyde

(35) Under a nitrogen atmosphere, an oven-dried 100 mL Schlenk tube equipped with a magnetic stir-bar and a Teflon plug was charged with the Shvo's catalyst (108 mg, 100 mol, 1.0 mol %), terephthalaldehyde (1.35 g, 10 mmol, 99% purity), and anhydrous methanol (2 mL). The resulting mixture was heated at 90 C. for 8 hours using an oil-bath. After that the vessel was gradually allowed to come to room temperature and the volatiles (H.sub.2 and MeOH vapor) were carefully removed by opening the Teflon plug inside the hood. The liquid sample was analyzed by GC to determine the weight percent of dimethyl terephthalate (>99.9 wt %).

(36) Shvo's catalyst catalyzes the dehydrogenative coupling between aliphatic or aromatic aldehydes and MeOH to afford corresponding methyl esters in high yields. The reaction is highly selective (up to <99.9%) toward the formation of methyl esters and operates under mild conditions (<100 C., 2-8 hours). The catalytic activity and the structure of the catalyst remain unchanged after several catalytic runs.

(37) In the specification, there have been disclosed certain embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.