Qi Jian-Qing, Jiao Lei
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China.
J Org Chem. 2020 Nov 6;85(21):13877-13885. doi: 10.1021/acs.joc.0c01963. Epub 2020 Oct 28.
Diboron(4) compounds serve as useful reagents for borylation, diboration, and reduction in organic synthesis. A variety of pyridine derivatives have been found capable of activating diboron(4) compounds, and different reaction mechanisms have been identified. 4,4'-Bipyridine was found to activate diboron(4) to form ,'-diboryl-4,4'-bipyridinylidene in 2015, and very recently, it has been found that this transformation is crucial in the 4,4'-bipyridine-catalyzed reduction of nitroarenes by bis(neopentylglycolato)diboron (Bnep), which features the formation of arylnitrene intermediates. However, the mechanism of ,'-diboryl-4,4'-bipyridinylidene formation, as well as its role in the transformation of nitroarene to arylnitrene, remains unknown. In this work, we investigated the possible pathways of this intriguing transformation and discovered several important intermediates through density functional theory (DFT) calculations. An -boryl 4,4'-bipyridyl radical was found to be a crucial intermediate in both the formation of ,'-diboryl-4,4'-bipyridinylidene and the reduction of nitroarene. A type of single-step reaction with three stages, including a dissociation and two migration steps, was identified in the generation of nitrosobenzene and its reduction. Arylnitrene formation was found to occur on a triplet potential energy surface, and an intersystem crossing was found to be important for achieving a reasonable activation energy barrier for nitrene formation. We anticipate our work to provide deeper insights into the nature of this reaction that could facilitate further rational design of pyridine- and bipyridine-based catalysts.
二硼（4）化合物是有机合成中用于硼化、双硼化和还原反应的有用试剂。已发现多种吡啶衍生物能够活化二硼（4）化合物，并确定了不同的反应机理。2015年发现4,4'-联吡啶能活化二硼（4）形成α,α'-二硼基-4,4'-联吡啶叉，最近还发现这种转化在4,4'-联吡啶催化双（新戊二醇）二硼（Bnep）还原硝基芳烃中起关键作用，该反应以芳基氮烯中间体的形成为特征。然而，α,α'-二硼基-4,4'-联吡啶叉的形成机理及其在硝基芳烃向芳基氮烯转化中的作用仍不清楚。在这项工作中，我们通过密度泛函理论（DFT）计算研究了这种有趣转化的可能途径，并发现了几种重要的中间体。发现α-硼基4,4'-联吡啶自由基是α,α'-二硼基-4,4'-联吡啶叉形成以及硝基芳烃还原过程中的关键中间体。在亚硝基苯的生成及其还原过程中确定了一种包含解离和两个迁移步骤的三步单步反应类型。发现芳基氮烯的形成发生在三重态势能面上，并且发现系间窜越对于实现合理的氮烯形成活化能垒很重要。我们期望我们的工作能为该反应的本质提供更深入的见解，从而有助于进一步合理设计基于吡啶和联吡啶的催化剂。
