Hövelmann Claas H, Muñiz Kilian
Kekulé-Institut für Organische Chemie und Biochemie der Universität Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
Chemistry. 2005 Jun 20;11(13):3951-8. doi: 10.1002/chem.200500095.
A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of olefins is described that yields cyclic boronic esters from olefins in a straight-forward manner. This process represents the first real product alteration in asymmetric dihydroxylation, since all previous protocols lead to free diols exclusively. A protocol based on the Sharpless AD conditions (for enantioselective oxidation of prochiral olefins) was developed that gives cyclic boronic esters with excellent enantiomeric excesses (ee's). Some of the ee's are higher than those reported for conventional AD. The unprecedented role of phenyl boronic acid on the course of the AD reaction was investigated in detail. PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of olefin oxidation intact. The main role of the boronic acids-apart from protecting the diol products against potential overoxidation-relies on removing the diol entity in an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols. Thus, a mechanistically new cleavage for enantioselective dihydroxylation reactions is introduced within the present work.
本文描述了一种铱催化的烯烃不对称二羟基化反应（AD）的变体，该反应能以直接的方式从烯烃生成环状硼酸酯。这一过程代表了不对称二羟基化反应中首次真正的产物变化，因为此前所有的方法仅得到游离二醇。基于夏普莱斯AD条件（用于前手性烯烃的对映选择性氧化）开发的一种方法，能得到对映体过量（ee值）优异的环状硼酸酯。一些ee值高于传统AD反应报道的数值。详细研究了苯基硼酸在AD反应过程中前所未有的作用。PhB(OH)₂ 不干扰手性配体，使烯烃氧化的对映选择性步骤保持完整。硼酸的主要作用——除了保护二醇产物防止潜在的过度氧化外——在于通过亲电裂解去除二醇实体，这与AD方法中传统的水解裂解形成对比。因此，本文引入了一种对映选择性二羟基化反应在机理上全新的裂解方式。
