C(alkenyl)–H Activation via Six-Membered Palladacycles: Catalytic 1,3-Diene Synthesis

A catalytic
method to prepare highly substituted 1,3-dienes from
two different alkenes is described using a directed, palladium­(II)-mediated
C­(alkenyl)–H activation strategy. The transformation exhibits
broad scope across three synthetically useful substrate classes masked
with suitable bidentate auxiliaries (4-pentenoic acids, allylic alcohols,
and bishomoallylic amines) and tolerates internal nonconjugated alkenes,
which have traditionally been a challenging class of substrates in
this type of chemistry. Catalytic turnover is enabled by either MnO2 as the stoichiometric oxidant or co-catalytic Co­(OAc)2 and O2 (1 atm). Experimental and computational
studies were performed to elucidate the preference for C­(alkenyl)–H
activation over other potential pathways. As part of this effort,
a structurally unique alkenylpalladium­(II) dimer was isolated and
characterized.