Synthesis of optically active α-hydroxy carbonyl compounds by the catalytic, enantioselective oxidation of silyl enol ethers and ketene acetals with (salen)manganese(III) complexes
Synthesis of optically active α-hydroxy carbonyl compounds by the catalytic, enantioselective oxidation of silyl enol ethers and ketene acetals with (salen)manganese(III) complexes
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DOI：
10.1021/ja9726668
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发表时间：
1998-02-04
影响因子：
15
通讯作者：
Saha-Möller, CR
中科院分区：
文献类型：
--
作者：
Adam, W;Fell, RT;Saha-Möller, CR
一系列具有α-和/或β-苯基以及不同空间位阻的所有取代基的甲硅烷基烯醇醚和烯酮缩醛1a-h已被手性（salen）Mn-III配合物3进行了对映选择性催化氧化。最高的转化率和最好的对映选择性已获得漂白剂，而不是碘酰苯作为氧源的活性氧代金属物种。关于基板结构，ee值高达89%已实现与短的和无支链的烷基取代基的烯醇醚在甲硅烷氧基位置。虽然β-苯基有利于对映体控制，但甲硅烷氧基官能团的苯基取代基α导致较低的ee值，而二苯基取代的衍生物Id显示出最低的立体选择性，事实上，β-与α-苯基取代基不仅在大小上而且在意义上表现出差异，（相对绝对产品配置）的立体选择性可以用作有价值的机理探针，以评估作为类型的函数的底物和催化剂中的空间和电子效应。研究结果表明，基质与金属络合物之间的空间相互作用是影响金属络合物立体选择性的主要因素。实际上，当采用较大的衍生物时，即使对于远程甲硅烷氧基也实现了显著增加的对映选择性。相比之下，主要是电子效应在（salen）Mn-III催化剂3中起作用，因为水杨醛配体的5，5 '位上的供电子基团在该催化氧化中提供更高的ee值。倾斜的侧上的方法（轨迹B）的衬底上的氧代-金属催化剂是有利的，金属氧杂环丁烷机制充分占观察到的对映选择性。因此，通过用（salen）Mn-III络合物催化、对映选择性氧化甲硅烷基烯醇醚1，制备光学活性的α-羟基羰基产物2的合成有价值的方法是可行的。
A ser of silyl enol ethers and ketene acetals 1a-h with alpha- and/or beta-phenyl as well as all;yl substituents of different steric bull; has been submitted to the enantioselective catalytic oxidation by chiral (salen)Mn-III complexes 3. Highest conversions and best enantioselectivities have been obtained with bleach rather than iodosobenzene as oxygen source for the active oxo-metal species. With regard to substrate structure, ee values up to 89% have been achieved for enol ethers with short and unbranched alkyl substituents at the siloxy position. While beta-phenyl groups are beneficial for enantiofacial control, phenyl substituents a to the siloxy functionality result in lower ee values, while the diphenyl-substituted derivative Id displays the lowest stereoselectivity, The fact that beta- versus alpha-phenyl substituents exhibit not only differences in the magnitude but also in the sense (opposite absolute product configuration) of the stereoselectivity may be utilized as a valuable mechanistic probe to assess steric and electronic effects in the substrate and the catalyst as a function of the type and pattern of substitution Our results display that steric interactions between the substrate and the ore-metal complex are mainly responsible for the observed stereochemical preferences. Indeed, significantly increased enantioselectivities are achieved even for the remote siloxy group when bulkier derivatives are:employed. In contrast, primarily electronic effects operate in the (salen)Mn-III catalyst 3 since electron-donating groups in the 5,5' positions of the salicylaldehyde ligand afford higher ee values in this catalytic oxidation. The skewed side-on approach (trajectory b) of the substrate onto the oxo-metal catalyst is favored, the metallaoxetane mechanism adequately accounts for the observed enantioselectivities. Herewith a synthetically-valuable method for the preparation of optically active alpha-hydroxy carbonyl products 2 has been made available through the catalytic, enantioselective oxidation of the silyl enol ethers 1 by (salen)Mn-III complexes.