Carbene-Catalyzed Enantioselective Petasis-Like Alkenylation

The N-heterocyclic carbene (NHC)-catalyzed enantioselective Petasis-like alkenylation of o-hydroxycinnamaldehydes or hydroxyl-tethered α,β-unsaturated aldehydes with styryl, dienyl, or trienyl boronic acids is disclosed. This method involves the addition of π-system-containing boronic acids to NHC-bounded α,β-unsaturated acyl azoliums and allows access to divergent assembly of β-alkenyl substituted dihydrocoumarin and γ- and δ-lactones. DFT calculations suggest that an unprecedented zwitterionic intermediate and 1,4- or 1,5- migration of alkenyl groups play a crucial role in the reaction. More in-depth studies of orbital and noncovalent interaction analysis provide more detailed explanations for pathways and stereoselectivity control.