DFT Study on the Mechanism of 4,4′-Bipyridine-Catalyzed Nitrobenzene Reduction by Diboron(4) Compounds

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 N,N′-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 (B2nep2), which features the formation of arylnitrene
intermediates. However, the mechanism of N,N′-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 N-boryl 4,4′-bipyridyl radical was found
to be a crucial intermediate in both the formation of N,N′-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（diboron(4) compounds）是有机合成中用于硼化反应、双硼化反应及还原反应的实用试剂。研究发现多种吡啶衍生物可活化二硼(4)化合物，且已阐明多种不同的反应机理。2015年，研究人员发现4,4'-联吡啶可活化二硼(4)化合物，生成N,N'-二硼基-4,4'-联吡啶亚胺；而近期研究表明，该转化在4,4'-联吡啶催化双(新戊二醇二氧基)二硼 (B2nep2) 还原硝基芳烃的反应中发挥关键作用，该反应以芳基氮宾中间体的形成为核心特征。然而，N,N'-二硼基-4,4'-联吡啶亚胺的生成机理，以及其在硝基芳烃转化为芳基氮宾过程中所起到的作用，目前仍未明确。本研究通过密度泛函理论（DFT）计算，探究了该引人关注的转化过程的可能路径，并鉴定出多种关键中间体。研究发现，N-硼基-4,4'-联吡啶自由基是生成N,N'-二硼基-4,4'-联吡啶亚胺以及还原硝基芳烃过程中的关键中间体。在亚硝基苯的生成及其还原过程中，我们鉴定出一类包含三个阶段的单步反应，涵盖一次解离过程与两次迁移步骤。研究表明，芳基氮宾的生成过程发生在三重态势能面上，而体系间窜跃是实现芳基氮宾生成过程适宜活化能垒的关键步骤。本研究期望能够加深对该反应本质的理解，从而为合理设计吡啶类及联吡啶类催化剂提供理论指导。