Copper-Catalyzed Vinylogous Aerobic Oxidation of Unsaturated Compounds with Air

A mild and operationally simple copper-catalyzed
vinylogous aerobic
oxidation of β,γ- and α,β-unsaturated esters
is described. This method features good yields, broad substrate scope,
excellent chemo- and regioselectivity, and good functional group tolerance.
This method is additionally capable of oxidizing β,γ-
and α,β-unsaturated aldehydes, ketones, amides, nitriles,
and sulfones. Furthermore, the present catalytic system is suitable
for bisvinylogous and trisvinylogous oxidation. Tetramethylguanidine
(TMG) was found to be crucial in its role as a base, but we also speculate
that it serves as a ligand to copper­(II) triflate to produce the active
copper­(II) catalyst. Mechanistic experiments conducted suggest a plausible
reaction pathway via an allylcopper­(II) species. Finally, the breadth
of scope and power of this methodology are demonstrated through its
application to complex natural product substrates.