N-heterocyclic carbenes as versatile nucleophilic catalysts for transesterification/acylation reactions

[reaction: see text] Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification between esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with vinyl acetate in convenient reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with numerous alcohols in the presence of alkyl-substituted NHC to form efficiently the corresponding esters in very short reaction times.     

Transesterification/Acylation of secondary alcohols mediated by N-heterocyclic carbene catalysts

N-Heterocyclic carbenes (NHC) are efficient catalysts for transesterification/acylation reactions involving secondary alcohols. The catalytic transformations are carried out employing low catalyst loadings in convenient reaction times at room temperature.     

咪唑(啉)-2-羧酸盐作为潜在的/可热活化的有机小分子催化剂催化转酯反应(英文)

采用咪唑(啉)-2-羧酸盐(ImCO2)作为催化剂前驱体,通过温度控制脱除ImCO2的2-位羧酸根,再原位释放出高活性的N-杂环卡宾(NHCs),进而催化转酯反应.研究了NHCs在苯甲酸乙酯与一元醇、二元醇及氨基醇反应体系中的催化活性.结果表明,在该体系中不需要选用活化酯为原料,无需加入分子筛或加大醇酯比提高产率,仅用少量催化剂(0.5mol%),在较短时间内(3h),即可使反应转化率达到95%.     

Transesteeification of malonic esters

Conclusions Hither esters of malonic acid have been prepared in good yield by transesterification of lower esters with appropriate primary or secondary alcohols in the presence of alkaline or acid catalysts.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 892–895 (1968).     

Synthesis of 11C labelled methyl esters: transesterification of enol esters versus BF3 catalysed esterification—a comparative study

C-11 labelled methyl esters have been synthesized via the transesterification of enol esters in the presence of C-11 methanol and 1,3 dichlorodibutylstannoxane as catalyst. This method leaves functional groups intact and allows access to a wider variety of C-11 labelled methyl esters compared to the BF₃ catalysed ester formation, which uses carboxylic acids and C-11 methanol as starting materials.     

Transesterification of esters of adipic acid

Conclusions The corresponding higher esters were produced in good yields by transesterification of lower esters of adipic acid by alcoholysis with alcohols in the presence of an alkaline or acid catalyst. Acid catalysts (p-toluenesulfonyl chloride) are more effective than alkaline catalysts (KOH).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2439–2443, November, 1971.     

Ligand-assisted rate acceleration in lanthanum(III) isopropoxide catalyzed transesterification of carboxylic esters

The transesterification of an equimolar mixture of carboxylic esters and primary (1°), secondary (2°), and tertiary (3°) alcohols in hydrocarbon solvents was promoted with high efficiency by a lanthanum(III) complex, which was prepared in situ from lanthanum(III) isopropoxide (1 mol %) and 2-(2-methoxyethoxy)ethanol (2 mol %). The present La(III) catalyst was highly effective for the chemoselective transesterification in the presence of competitive 1°- and 2°-amines. Remarkably, esters were obtained in good to excellent yields as colorless materials without an inconvenient workup procedure.     

Unexpected migration and oxidative cyclization of substituted 2-acetophenone triflates under basic conditions: synthetic and mechanistic insights

Oxidative ring closure of alkyl-substituted 2-hydroxyacetophenone trifluoromethanesulfonate esters (triflates) occurs upon exposure to base in anaerobic DMF at 20-90 degrees C. Alkyl substitution is required for ring closure. A migrated enol triflate product forms at lower temperature in high yield via migration of the trifluoromethanesulfonate in the unsubstituted and monoalkyl-substituted cases. The alkyl-substituted enol triflates also enter into the benzofuran-3-one ring-forming process under thermal cyclization conditions. Potential mechanistic pathways are evaluated.     

Fast identification of mycobacterium species by GC analysis with trimethylsulfonium hydroxide (TMSH) for transesterification

Trimethylsulfonium hydroxide (TMSH) reproducibly converts fatty acids bound in, e.g., biomolecules such as phospholipids and/or glycerides, into the corresponding fatty acid methyl esters (FAMEs). The transesterification can be performed at room temperature in a fast single step reaction. Surprisingly, secondary alcohols and mycolic acid cleavage products (MACPs) are also released from mycobacteria under these conditions. The complex reaction mixtures containing FAMEs, MACPs, and secondary alcohols can easily be separated by high resolution temperature-programmed capillary GC. Different species of mycobacteria give rise to characteristic chromatographic patterns and the amount of lipids from a single colony of mycobacteria is sufficient for reliable identification of the bacteria. The profiles of the chromatograms match well those obtained from other sample preparation techniques. The TMSH method of identification of mycobacteria from the patterns of the gas chromatograms is faster and more sensitive than conventional methods, which also involve transesterification. The identification of mycobacterial species by microbiological culture techniques is difficult to perform and requires several weeks.     

Combined epimerisation and acylation: Meerwein-Ponndorf-Verley-Oppenauer catalysts in action

A practical racemisation-epimerisation method for chiral secondary alcohols has been developed. Meerwein-Ponndorf-Verley-Oppenauer catalysts such as neodymium(III) isopropoxide are able to racemise these alcohols with retention of other stereocentres in the molecule. This is particularly useful for the recycling of the undesired products of kinetic resolutions of alcohols. By combination of such a racemisation with an acylation using isopropenyl or ethoxyvinyl esters as acyl donors, a fast straightforward recycling of starting materials may be achieved. The combined epimerisation and acylation process is demonstrated for the steroid estradiol methyl ether.     

12-Tungstophosphoric acid supported on inorganic oxides as heterogeneous and reusable catalysts for the selective preparation of alkoxymethyl ethers and their deprotections under different reaction conditions

A wide variety of primary and secondary alcohols were efficiently converted to their corresponding methoxymethyl (MOM) and ethoxymethyl (EOM) ethers in the presence of catalytic amounts of supported H₃PW₁₂O₄₀ on silica gel and zirconia at room temperature and under microwave irradiation at solvent-free conditions, whereas, phenols and tertiary alcohols remained intact under the same reaction conditions. Deprotection of these ethers to their parent alcohols was also achieved using these heterogeneous catalysts in ethanol, as a green solvent, under reflux conditions and microwave irradiation. Selective deprotection of primary and secondary MOM- and EOM-ethers in the presence of phenolic and tertiary ones, methyl and benzyl ethers, esters and trimethylsilyl ethers was achieved by these reagent systems. The present methodology offers several advantages such as short reaction times, high yields, simple procedure, heterogeneous reaction conditions, selectivity, non-toxicity and reusability of the catalysts.     

Asymmetric hydrogenation of di and trisubstituted enol phosphinates with N,P-ligated iridium complexes

The iridium-catalyzed asymmetric hydrogenation of various di- and trisubstituted enol phosphinates has been studied. Excellent enantioselectivities (up to >99% ee) and full conversion were observed for a range of substrates with both aromatic and aliphatic side chains. Enol phosphinates are structural analogues of enol acetates, and the hydrogenated alkyl phosphinate products can easily be transformed into the corresponding alcohols with conservation of stereochemistry. We have also hydrogenated, in excellent ee, several purely alkyl-substituted enol phosphinates, producing chiral alcohols that are difficult to obtain highly enantioselectively from ketone hydrogenations.     

Significant Heterogeneous Carbonate Salt Catalyzed Acetylation of Alcohols via a Transesterification Process with Carbonate Salt‐activated Alcohol 1H NMR Evidence

Heterogeneous carbonate salt catalyzed acetylation of alcohols via a transesterification process has been developed. Various esters are furnished up to 97% yield. Established procedure is simple and air‐tolerant with readily available reagents. Ethyl acetate and isobutyl acetate are used as not only acetylating agents, but also reaction solvents in transesterification. Aliphatic linear alcohols, allylic alcohols and benzyl alcohols show high reactivities in the presence of 1 or 5 mol% Cs₂CO₃ at 125°C. Cesium carbonate can be recycled by pumping liquid phase out of reactor after reaction. During four cycle runs for reaction of 2‐phenylethanol and ethyl acetate, high yields of phenethyl acetate are provided (>60% yield). Based on experiments and ¹H NMR investigation, bifunctional catalysis is proposed, alcohol activated by carbonate ion is confirmed, and higher activity of catalytic amount than stoichiometric cesium carbonate is interpreted.     

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.     

Further studies on the utility of sodium hypochlorite in organic synthesis. Selective oxidation of diols and direct conversion of aldehydes to esters

Sodium hypochlorite in acetic acid solution selectively oxidizes secondary alcohols to ketones in the presence of primary alcohols and converts aldehydes to methyl esters in the added presence of methanol.     

Soybean biodiesel methyl esters,free glycerin and acid number quantification by 1H nuclear magnetic resonance spectroscopy

Production of alternative fuels, such as biodiesel, from transesterification of vegetable oil driven by heterogeneous catalysts is a promising alternative to fossil diesel. However, achieving a successful substitution for a new renewable fuel depends on several quality parameters. ¹H NMR spectroscopy was used to determine the amount of methyl esters, free glycerin and acid number in the transesterification of soybean oil with methanol in the presence of hydrotalcite‐type catalyst to produce biodiesel. Reaction parameters, such as temperature and time, were used to evaluate soybean oil methyl esters rate conversion. Temperatures of 100 to 180 °C and times of 20 to 240 min were tested on a 1 : 12 molar ratio soybean oil/methanol reaction. At 180 °C/240 min conditions, a rate of 94.5 wt% of methyl esters was obtained, where free glycerin and free fatty acids were not detected. Copyright © 2012 John Wiley & Sons, Ltd.     

三异丙氧钐催化下β-酮酸烯丙酯的合成

在甲苯中，以Sm(i-PrO)~3(10%)为催化剂，β-酮酸乙酯与烯丙醇进行酯交换反应得到中等产率的β-酮酸烯丙酯。     

酯交换制生物柴油的CaO固体碱催化剂

用不同的前驱物合成了三种CaO催化剂, 并以X射线衍射(XRD)、扫描电子显微镜(SEM) 、程序升温脱附(TPD)等方法加以表征. 这些CaO被用作大豆油(SBO)经酯交换制取脂肪酸甲酯(FAME), 即生物柴油的催化剂, 由方解石制备的氧化钙(Cal(N))表现了最好的SBO酯交换活性. 检测发现CaO的酯交换活性与它们的碱性强度密切相关, 当暴露于CO2气氛下, 显著降低了CaO的酯交换催化活性(Raman光谱测试显示当置CaO于常温空气中, 其表面形成的CaCO3和Ca(OH)2将阻止CaO继续参与SBO的酯交换反应). CO2的毒化颇受制于CaO前驱体种类, Cal(N)比来自文石的CaO(即Ara(N))有更好的抗CO2毒化能力; 这些受损的CaO催化活性可部分复原. 提出了CaO催化剂受CO2毒化及其再生的机理, 同时讨论了SBO酯交换活性相到底是CaO固体表面, 拟或溶解了的CaO的问题.     

Unnatural Amino Acids. 2. Simple Method of Obtaining Esters of Aziridine-2-carboxylic Acids by a Transesterification Reaction

A series of N-unsubstituted esters of aziridine-2-carboxylic acid has been obtained by transesterification in basic medium using primary, secondary, and tertiary alcohols. Methods of transesterification using various bases (K₂CO₃, ROLi, t-BuOK) have been compared. Transesterification with lithium alcoholates also affords the possibility of obtaining esters of N-substituted aziridine-2-carboxylic acids. Transesterification of chiral esters proceeds with retention of the configuration of the chiral center.     

Catalytic synthesis of biodiesel from high free fatty acid-containing feedstocks

Recyclable and reusable heterogeneous diarylammonium catalysts are highly effective in catalyzing the esterification of the free fatty acid (FFA) present in greases to methyl esters to reduce the FFA content from 12-40 wt% to 0.5-1 wt%; the resulting ester-glyceride mixture (pretreated grease) could then be readily converted to methyl esters by base-catalyzed transesterification.