Chloroperoxidase-catalyzed asymmetric oxidations: substrate specificity and mechanistic study.
作者:Aleksey Zaks、David R. Dodds
DOI:10.1021/ja00147a001
日期:1995.10
The substrate specificity of chloroperoxidase from Caldaromyces fumago in a number of halide-independent reactions was investigated and the ability of this enzyme to perform benzylic hydroxylations with high enantioselectivity is revealed. The substrate repertoire of chloroperoxidase is expanded and the enantioselectivity data for synthetically useful epoxidations are reported. The enzyme epoxidizes straight chain aliphatic and cyclic cis-olefins in a highly stereoselective manner favoring small unsubstituted substrates in which the double bond is not more than two carbon atoms from the terminal. The epoxidation of short-chain prochiral terminal dienes proceeds with high diastereoselectivity and moderate enantioselectivity, yielding monoepoxides exclusively. Unsubstituted straight-chain terminal olefins seven carbons or longer are epoxidized poorly. Aliphatic and aromatic alcohols are efficiently oxidized to aldehydes and acids. The utilization of radical probe substrate trans-2-phenyl-1-methylcyclopropane revealed that the mechanism of chloroperoxidase-catalyzed hydroxylation is incompatible with the existence of a discrete radical intermediate and most likely proceeds via transfer of the oxygen atom from the high valent iron oxo intermediate directly to the substrate.
Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical Investigation of the Mechanism
210 h−1 for the epoxidation of cyclohexene catalyzed by I with H2O2. The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO42− ligand in I played an important role in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and spectroscopic results showed that the present
具有SeO 4 2-配体(TBA)2 [SeO 4 WO(O 2)2 } 2 ](I ; TBA = [(n -C 4 H 9)4 N] +)的双核过氧钨酸盐可以起作用作为有效的均相催化剂,以H 2 O 2为唯一氧化剂,可选择性氧化各种有机物质,例如烯烃,醇和胺。I与H 2催化环己烯环氧化的周转频率(TOF)高达210 h -1O 2。催化剂易于回收并在保持催化性能的情况下重复使用。SEO的4 2-配体在我在控制peroxotungstates,其中显著影响它们的电氧气转移反应的路易斯酸性中发挥了重要的作用。几个动力学和光谱结果表明,本催化环氧化包含以下两个步骤:(i)形成的随后的过氧物种[缝的米ö Ñ ] ø - (II ;米= 1和2)通过反应我与的烯烃和(ii)再生我通过反应II具有H 2 O 2。在稳态周转条件下,化合物I是优势种。催化环氧化的反应速率显示出对烯烃和I的浓度的一阶依赖性,以及对H