Phase transfer catalyst activity evaluation

A variety of methods exists for the synthesis of optically active amino acids, including asymmetric synthesis [85-93] and classic and enzymatic resolutions [94-97], However, most of these methods are not applicable to the preparation of a,a-disubstituted amino acids due to poor stereoselectivity and lower activity at the a-carbon. Attempts to resolve the racemic 2-amino-2-ethylhexanoic acid and its ester through classic resolution failed. Several approaches for the asymmetric synthesis of the amino acid were evaluated, including alkylation of 2-aminobutyric acid using a camphor-based chiral auxiliary and chiral phase-transfer catalyst. A process based on Schollkopf s asymmetric synthesis was developed (Scheme 12) [98]. Formation of piperazinone 24 through dimerization of methyl (5 )-(+)-2-aminobutyrate (25) was followed by enolization and methylation to give (35.6S)-2,5-dimethoxy-3,6-diethyl-3.6-dihydropyrazine (26) (Scheme 12). This dihydropyrazine intermediate is unstable in air and can be oxidized by oxygen to pyrazine 27, which has been isolated as a major impurity. 

A few years ago Cahard reported a series of studies on the use of immobilized cinchona alkaloid derivatives in asymmetric reactions with phase-transfer catalysts [17[. Two types of polymer-supported ammonium salts of cinchona alkaloids (types A and B in Scheme 8.4) were prepared from PS, and their activity was evaluated. The enantioselectivity was found to depend heavily on the alkaloid immobilized, with the type B catalysts usually giving better results than the type A catalysts. By performing the reaction in toluene at -50 °C in the presence of an excess of solid cesium hydroxide and 0.1 mol equiv of catalyst 10, benzylation of the tert-butyl glycinate-derived benzophenone imine afforded the expected (S)-product in 67% yield with 94% ee, a value very close to that observed with the nonsupported catalyst. (Scheme 8.4, Equation b) Unfortunately-and again, inexplicably-the pseudoenantiomer of 10 proved to be much less stereoselective, affording the R)-product in only 23% ee. No mention of catalyst recycling was reported [18]. 

Denmark SE, Gould ND, Wolf LM (2011) A Systematic Investigation of Quaternary Ammonium Irais as Asymmetric Phase-Transfer Catalysts. Synthesis of Catalyst Libraries and Evaluation of Catalyst Activity. J Org Chem 76 4260... 

In the same context, the catalytic activity of novel calixarene-based chiral phase-transfer catalysts derived from cinchona alkaloids was evaluated by Sirit et al. by carrying out the phase-transfer benzylation of A -diphenylmethylene glycine ethyl ester with benzyl bromide. Although a good yield (89%) was... 

DeNOx - Scope of the model analysis was to evaluate on a quantitative basis the effective dependence of the intrinsic activity of the monoliths on the thermal sintering, and separate it from the contributions of inter-phase mass transfer and the effect of morphological modifications on intra-porous diffusion. When excess ammonia is present, as in the case of the experiments herein analyzed, then the Ealy-Rideal kinetic expression which is contained in the model of the SCR reactor reduces to a first order dependence on NO concentration under such conditions, an unique adaptive parameter, kc, accounts for the DeNOx intrinsic activity. Estimation of kc for the three calcined catalysts was obtained by fitting the model to each set of experiments. Input data included the operating conditions, the geometrical... 

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