A redox-active diborane platform performs C(sp3)-H activation and nucleophilic substitution reactions

Abstract

Organoboranes are among the most versatile and widely used reagents in synthetic chemistry. A significant further expansion of their application spectrum would be achievable if boron-containing reactive intermediates capable of inserting into C-H bonds or performing nucleophilic substitution reactions were readily available. However, current progress in the field is still hampered by a lack of universal design concepts and mechanistic understanding. Herein we report that the doubly arylene-bridged diborane(6) 1H2 and its BB-bonded formal deprotonation product Li2[1] can activate the particularly inert C(sp3)-H bonds of added H3CLi and H3CCl, respectively. The first case involves the attack of [H3C]- on a Lewis-acidic boron center, whereas the second case follows a polarity-inverted pathway with nucleophilic attack of the BB double bond on H3CCl. Mechanistic details were elucidated by means of deuterium-labeled reagents, a radical clock, α,ω-dihaloalkane substrates, the experimental identification of key intermediates, and quantum-chemical calculations. It turned out that both systems, H3CLi/1H2 and H3CCl/Li2[1], ultimately funnel into the same reaction pathway, which likely proceeds past a borylene-type intermediate and requires the cooperative interaction of both boron atoms.