Laboratorio de Química Orgánica, IQUIBA-NEA, Universidad Nacional del Nordeste, CONICET, FACENA , Av. Libertad 5460, Corrientes 3400, Argentina.
Instituto de Química Rosario (CONICET), Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario , Suipacha 531, Rosario 2000, Argentina.
J Org Chem. 2017 Jun 2;82(11):5917-5925. doi: 10.1021/acs.joc.7b01096. Epub 2017 May 22.
The mechanism of the rearrangement of oxiranyl N-methyliminodiacetyl (MIDA) boronates in dicholoromethane has been extensively investigated with density functional theory. Several reaction pathways were examined. Our results revealed that the most-favorable mechanisms for the BF-promoted rearrangement of 2-phenyl oxiranyl MIDA boronate (1) and 1-phenyl oxiranyl MIDA boronate (24) comprise two steps: ring opening of the epoxide to a carbocation intermediate followed by migration of a MIDA-boryl group (for the reaction of 1) and hydrogen (for the reaction of 24), to give the same BF-coordinated α-boryl aldehyde in both cases. The first step of the ring opening of the epoxide is the rate-determining step of these reactions. In the rearrangement step for the reaction of 1, the MIDA-boryl group migrates easily, probably because of its electron-rich sp-hybridized boron center. For 24, the most-favorable pathway involves a rare boryl-substituted carbocation. The course of these reactions is mainly controlled by electronic effects, although steric effects are also significant. The higher energy barrier calculated for the unsubstituted oxiranyl MIDA boronate (31) explains the lack of reactivity in the studied BF-promoted rearrangement.
我们采用密度泛函理论对氧杂环丁烷 N-甲基亚氨二乙酸（MIDA）硼酸酯在二氯甲烷中的重排机制进行了广泛研究。考察了几种反应途径。结果表明，BF 促进的 2-苯基氧杂环丁烷 MIDA 硼酸酯（1）和 1-苯基氧杂环丁烷 MIDA 硼酸酯（24）重排的最有利机制包括两个步骤：环氧化合物开环生成碳正离子中间体，然后 MIDA-硼基（对于 1 的反应）和氢（对于 24 的反应）迁移，在两种情况下都得到相同的 BF 配位的α-硼基醛。环氧化合物开环的第一步是这些反应的速率决定步骤。在 1 的重排步骤中，MIDA-硼基容易迁移，可能是因为其富电子的 sp 杂化硼中心。对于 24，最有利的途径涉及罕见的硼取代碳正离子。这些反应的过程主要受电子效应控制，尽管空间效应也很重要。未取代的氧杂环丁烷 MIDA 硼酸酯（31）的较高能垒解释了其在研究的 BF 促进重排中缺乏反应性。
