Enantioselective preparation and chemoselective cross-coupling of 1,1-diboron compounds
作者:Jack Chang Hung Lee、Robert McDonald、Dennis G. Hall
DOI:10.1038/nchem.1150
日期:2011.11
The simplicity, efficiency and generality of the transition-metal-catalysed SuzukiâMiyaura cross-coupling reaction has led to its application in the preparation of a wide variety of organic compounds. Cross-coupling of alkylboron derivatives, however, remains a major challenge, in particular with regard to stereochemical control. Here, we describe the preparation and reaction of highly optically enriched 1,1-diboron compounds. A catalytic asymmetric conjugate borylation of β-boronylacrylates provided geminal diboronate products that feature two distinct boronyl units, in 99% enantiomeric excess. Chemoselective cross-coupling of one-boronyl unit, a trifluoroborate salt, occurred stereospecifically via inversion of its configuration to generate enantioenriched benzylic or allylic boronates. The difficult transmetallation in the SuzukiâMiyaura catalytic reaction cycle is believed to be facilitated by a stabilization effect from the second boronyl unit, and internal coordination by the oxygen of the proximal carboxyester. We also explored subsequent functionalization of the second boronyl unit. Stereoselective SuzukiâMiyaura cross-coupling reactions involving non-benzylic secondary alkylboronates are notoriously challenging. Here, an enantioselective synthesis of 1,1-diboronyl compounds using asymmetric conjugate borylation, followed by chemoselective mono cross-coupling with inversion at the diboron centre, is used to produce highly enantioenriched benzylic or allylic boronates, which themselves are useful reagents for a number of processes.
过渡金属催化的铃木-米亚乌拉(SuzukiâMiyaura)交叉偶联反应具有简便、高效和通用的特点,因此被广泛应用于制备各种有机化合物。然而,烷基硼衍生物的交叉偶联反应仍然是一项重大挑战,尤其是在立体化学控制方面。在此,我们介绍了高光学富集 1,1- 二硼化合物的制备和反应。δ-硼酰基丙烯酸酯的催化不对称共轭硼酰化反应提供了具有两个不同硼酰基单元的宝石级二硼酸酯产物,对映体过量率高达 99%。一个硼基单元(一种三氟硼酸盐)的化学选择性交叉偶联是通过反转其构型而立体发生的,从而生成对映体丰富的苄基或烯丙基硼酸盐。在铃木-米亚乌拉催化反应循环中,第二硼基单元的稳定作用以及近端羧基聚酯氧的内部配位作用被认为促进了这一困难的反金属化反应。我们还对第二个硼酰基单元的后续官能化进行了探索。涉及非苄基仲烷基硼酸酯的立体选择性铃木-米亚乌拉交叉偶联反应极具挑战性。在这里,利用不对称共轭硼酸化合成 1,1-二硼酸化合物的对映体选择性,然后在二硼酸中心进行化学选择性单交叉偶联反转,从而制备出高度对映体富集的苄基或烯丙基硼酸酯,这些硼酸酯本身是许多工艺的有用试剂。