Quaternizing Diboranes(4): Highly Divergent Outcomes and an Inorganic Wagner–Meerwein Rearrangement

Apart from a few compounds under heavy use in organic chemistry, diboranes are relatively exotic and poorly understood. Recently, interest in these molecules has intensified with the advent of so-called “sp2–sp3” diboranes which exhibit useful reactivity toward organic substrates. In our hands, addition of Lewis bases to dihalodiorganyl diboranes(4) has previously shown some very surprising reactivity, including a substituent exchange between the boron atoms, and diboranes in which halide atoms bridge the B–B bond. Herein we have expanded the range of diborane(4) and Lewis base reaction partners, in the process uncovering three new reactivity patterns as well as some cases where mixtures are obtained. Trends are established for the preferential formation of certain products which rationalize the results based on electronic and steric effects. The substituent exchange, clearly based on an inorganic version of the well-known Wagner–Meerwein rearrangement, was also found to be an equilibrium reaction with the halide-bridged Lewis adducts on the other side.

除了少数在有机化学中广泛应用的化合物外，二硼烷（diboranes）属于相对罕见且研究尚不充分的物种。近年来，随着所谓的"sp²–sp³"型二硼烷的问世，这类分子的研究热度显著攀升——这类二硼烷对有机底物展现出极具应用价值的反应活性。此前我们的研究表明，向二卤二有机基二硼烷(4)中加入路易斯碱（Lewis base）时，可观测到诸多意想不到的反应行为，包括硼原子间的取代基交换，以及卤原子桥连B-B键的二硼烷物种。本文中，我们拓展了二硼烷(4)与路易斯碱的反应底物范围，并在此过程中发现了三种全新的反应模式，同时也观测到部分混合产物的生成案例。我们确立了特定产物的优先生成规律，并基于电子效应与位阻效应对实验结果进行了合理化阐释。该取代基交换反应本质上是经典瓦格纳-迈尔外因重排（Wagner–Meerwein rearrangement）的无机变体，同时我们还发现该反应属于可逆平衡反应，其平衡另一侧为卤原子桥连的路易斯加合物（Lewis adduct）。