Mechanistic Investigations of Reactions of the Frustrated Lewis Pairs (Triarylphosphines/B(C₆F₅)₃) with Michael Acceptors

Frustrated Lewis pair (FLP)-catalyzed reduction of Michael acceptors is a challenging reaction that proceeds with specific FLP structures. Kinetics and equilibrium of the reactions of two phosphines (Ar₃P), namely tri­(1-naphthyl)­phosphine and tri­(<i>o</i>-tolyl)­phosphine, are reported with reference electrophiles. The reason for the failure of the FLPs (Ar₃P/B­(C₆F₅)₃) to reduce activated alkenes under H₂ pressure is shown to be a hydrophosphination process that inhibits the reduction reaction. Kinetic and thermodynamic factors controlling both pathways are discussed in light of Mayr’s free linear energy relationships