Tunable Construction of Multisubstituted 1,3-Dienes and Allenes via a 1,4-Palladium Migration/Carbene Insertion Cascade.

Efficient palladium-catalyzed vinylic C-H alkenylation and allenylation of gem-disubstituted ethylenes with N-tosylhydrazones of aryl alkyl and diaryl ketones were achieved to access trisubstituted 1,3-dienes and tetrasubstituted allenes, respectively. An aryl to vinyl 1,4-palladium migration/carbene insertion/β-hydride elimination sequence proceeded to switch the chemo- and regioselectivities to give structurally diverse products. Use of 2-FC6H4OH additive enables enhancement of the reaction efficiency through accelerating the key 1,4-palladium migration process.