Bifunctional Phosphonium Salt Directed Enantioselective Formal [4 + 1] Annulation of Hydroxyl-Substituted para-Quinone Methides with α-Halogenated Ketones.

A highly diastereo- and enantioselective [4 + 1] cycloaddition of para-quinone methides to α-halogenated ketones was realized by bifunctional phosphonium salt catalysis, furnishing functionalized 2,3-dihydrobenzofurans in high yields and excellent stereoselectivities (>20:1 dr and up to >99.9% ee). Mechanistic observations suggested that the reaction underwent a cascade intermolecular substitution/intramolecular 1,6-addition process. DFT calculations revealed the presence of multiple H-bonding interactions between the catalyst and the enolate intermediate in the stereodetermining transition states.