Palladium-catalyzed allene synthesis enabled by β-hydrogen elimination from sp2-carbon

Abstract

The rational design based on a deep understanding of the present reaction mechanism is an important, viable approach to discover new organic transformations. β-Hydrogen elimination from palladium complexes is an fundamental reaction in palladium catalysis. Normally, the eliminated β-hydrogen has to be attached to a sp3-carbon. We envision that the hydrogen elimination from sp2-carbon is possible by using delicately planed reaction systems, which may offer a new strategy for the preparation of allenes. With this consideration in mind, the palladium-catalyzed cross-coupling of 2,2-diarylvinyl bromides with diazo compounds was realized with a β-vinylic hydrogen elimination from allylic palladium intermediate as the key step. Both aryl diazo carbonyl compounds and N-tosylhydrazones are competent carbene precursors in this reaction. The reaction mechanism was explored by control experiments, KIE studies and DFT calculations