1,2-Migratory ring expansion of a BN-naphthalene

Abstract

Organoboranes hold an undeniable value in modern organic chemistry and show ever-growing potential in polymer design and functionalization. Azaborines, in particular, are still being explored as a promising class of organoboron compounds. We investigated the reactivity of a 1,2-azaborine towards Matteson homologation, a cornerstone of organoborane transformations. The azaborine was shown to be reactive towards nucleophiles. Addition of n-BuLi led to exchange of the R group, while addition of chloromethyllithium formed the desired tetracoordinate intermediate. However, rather than homologation, the complex readily underwent ring expansion via migration of the adjacent C(sp²), followed by deborylation. Computational analysis revealed a stable geometry of the azaboronate in which the migrating carbon is antiperiplanar to the leaving group, and which we propose as being responsible for the observed reactivity. This finding suggests a unique role for azaborines in molecular transformations which may be unachievable with other organoboron frameworks.