Boron-targeted H-atom transfer drives disproportionation of 2,1-benzazaborolyl radical anions beyond reversible Gomberg-Krause dimerization

Abstract

This contribution provides a detailed mechanistic insight into a remarkably complex reaction of neutral 1-Ph-2-tert-butyl-1H-1,2-benzazaborole (1) with elemental potassium in THF, yielding the known 10 pi-aromatic 1H-2,1-benzazaborolyl (2,1-Bab-) potassium salt (2), an isoelectronic species with indenyl potassium, along with a racemic mixture of a new potassium hydridoborate complex (3), formed together with 2 in ca. 1 : 1 molar ratio. Unlike derivatives of isoelectronic 1H-indene, which undergo various self-protonation processes under analogous conditions, for 1, we propose a completely different and rather complex, non-linear tandem mechanism. Key species are four enantiomeric pairs of short-lived, dimeric, diamagnetic intermediates, denoted alpha, beta, omega 1 and omega 2, possessing structures related to Gomberg's dimer and Krause's adduct. Based on an exhaustive multinuclear NMR analysis of the reaction mixture, including experiments with C3-methylene deuterium-labelled starting compound 1-dn (n is an element of {0-2}) and the observation of a primary AKIE (kH/kD >> 1), as well as a detailed analysis of full scan mass spectra and observed ions in ultrahigh-resolution LDI-MS data of the resulting mixture of isotopologues of the final product 3-dn (n is an element of {0-4}), comprising multiple isotopomers, in combination with probabilistic calculations, electrochemical studies, and DFT calculations, we were able to reconstruct the full sequence of reaction steps and pathways. The presented results support the involvement of various H-atom transfer (HAT) processes within pairs of radical-anionic species K+1(center dot)-, proceeding through different transition states, and, most notably, reveal an extremely rare boron-targeted HAT. These findings provide a basis for developing synthetic routes to unconventional boron hydrides.