Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope[3+2] Cycloaddition of Cyclopropanes and Alkenes

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents indiverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to successwas 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 transitionmetal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simpleoperation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without anactivating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for thefirst time.Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to sixsubstituents at allfive ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction wasalso successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged orspirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysisfeaturing a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates thefirst approach to use tetraalkoxydiboron(4)compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organicreactions