B-B vs. B-H Bond Activation in a (μ-Hydrido)diborane(4) Anion upon Cycloaddition with CO 2 , Isocyanates, or Carbodiimides.

The intriguing (μ-hydrido)diboranes(4) with their prominent pristine representative [B ₂ H ₅ ] ^- have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ-hydrido)diborane(4) anion [1H] ^- in cycloaddition reactions with the homologous series of heterocumulenes CO ₂ , iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO ₂ selectively activates the B-B bond of [1H] ^- , while the μ-H ligand is left untouched ([2H] ^- ). The carbodiimide iPrNCNiPr, in contrast, neglects the B-B bond and rather adds the B-bonded H ^- ion to its central C atom to generate a formamidinate bridge across the B ₂ pair ([3] ^- ). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H] ^- ) is related to [2H] ^- , the other ([5] ^- ) is an analog of [3] ^- . We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B-B vs. B-H bond activation.
(© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)