Transition-Metal-Like Catalysis with Tetraalkoxydibo-ron(4)/Isonicotinate Enabled an Efficient Broad-Scope [3+2] Cycloaddition of Cyclopropanes and Alkenes

Abstract

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3+2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3+2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpen-sive and stable catalysts, and simple operation, but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution pattern were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in con-cise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spiralcyclic com-pounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydibo-ron(4) compounds as catalysts and may lead to the development of new, green and efficient transition metal-like boron-catalyzed organic reactions.