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

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 a 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 thoroughly designed reaction systems, which may offer a new strategy for the preparation of allenes. Here, we describe a palladium-catalyzed cross-coupling of 2,2-diarylvinyl bromides and diazo compounds, where a β-vinylic hydrogen elimination from allylic palladium intermediate is proposed to be the key step. Both aryl diazo carbonyl compounds and N-tosylhydrazones are competent carbene precursors in this reaction. The reaction mechanism is explored by control experiments, KIE studies and DFT calculations. β-Hydrogen elimination is a fundamental reaction in palladium catalysis, however, the eliminated β-hydrogen is usually attached to a sp3-carbon. Here, the authors report a palladium-catalyzed cross-coupling reaction involving a β-vinylic hydrogen elimination from an allylic palladium intermediate.