Active esters base catalyzed transesterification

The alternative method for making activated esters is base-catalyzed transesterification. Fmoc-amino acids are esterified in excellent yields by reaction with pentafluorophenyl trifluoroacetate at 40°C in the presence of pyridine (Figure 7.13). A mixed anhydride is formed initially, and the anhydride is then attacked by the pentafluorophenoxy anion that is generated by the pyridine. Succinimido, chlorophe-nyl, and nitrophenyl esters were made by this method when it was introduced decades ago. A unique variant of this approach is the use of mixed carbonates that contain an isopropenyl group [Cf C CfyO-COjR]. These react with hydroxy compounds in the presence of triethylamine or 4-dimethylaminopyridine (see Section 4.19) to give the esters and acetone.30 35... 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

In the case of base-catalyzed reactions the substrate comes into contact with either HO or any other highly electron-rich catalyst (e.g., alcoholates, strongly basic amines, metal alkyls). Again, the substrate is activated, typically via the intermediate formation of carbanion species. A technically important example of base catalysis is the transesterification of natural oils to fatty acid methyl esters (FAME, better known as biodiesel ), a process typically catalyzed by methanolate salts. 

In a previous patent, Bayense et al. reported the use of ETS-10 and ETS-4 for the transesterification of TGs with alcohols.In their studies, both batch and fixed bed reactors were used to conduct the reaction. Under batch conditions, reactions were carried out at 220°C, 21 bar, and a catalyst loading of 0.23 wt% based on total autoclave content. Experiments with soybean oil and methanol, at a methanol/oil ratio of 4.2, resulted in 69.0% total oil conversion with an ester yield of 52.6% for ETS-10 and 96.9% conversion with an ester yield of 85.7% for ETS-4. When tallow was used under the same conditions, conversion and ester yield were 30.6% and 19.1% for ETS-10 and 44.1% and 29.6% for ETS-4, respectively. The slightly better activity shown by ETS-4 might imply that only basic sites at the pore opening of these zeolites were catalyzing the reaction. The effective radius of the TG molecule is such that it can not enter through the small pore openings of ETS-4 (diameter of 3.7 A) or even ETS-10... 

We conclude that a commercial immobilized lipase from C. antarctica (Novozym 435) was stable in SCC02 for all experimental conditions investigated. Based on the results obtained here and comparison of them with the results obtained by other investigators, it can be concluded that the magnitude of pressure, temperature, decompression rate, and exposure time needed to inactivate the enzyme strongly depends on the nature and the source of enzyme and, primarily, whether the enzyme is in its native or immobilized form. For the purpose of using this enzyme to catalyze the transesterification reaction of vegetable oils in order to produce esters, the results obtained herein are relevant, because the immobilized lipase can be used with low activity loss at typical conditions of temperature and pressure employed in many biotransformations of raw materials. 

The different properties of ILs, with regard to their polarity, hydrophobicity, and solvent miscibility behavior through combination with different anions, are the reason for the different biocatalyst activities. Good to excellent activity of CALB was observed with a decrease in polarity and hydrophobicity and a viscosity increase of the ILs. In [bmim][PF6] a conversion of (R)-l-phenylethanol into the ester of 48.9% and an ee of 95.6% were achieved after 5h and 100% of (R)-l-phenylethanol was converted into the enantiopure (R)-l-phenylethyl acetate after a 1-day reaction. Immobilized CALB exhibited excellent stability, activity, and selectivity towards the (R)-enantiomer of 1-phenylethanol in [bmim][PF6]. In some research bis(trifluoromethylsulfonyl)imide-based ILs have been regarded as very suitable media for biocatalysis [39, 46, 50]. On the contrary, in the present work, lower suitability of the same IL was demonstrated. Since immobilized CALB catalyzed both hydrolytic and transesterification reactions, its enantioselectivity for long reaction times was lower. 

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