The crossed Rauhut-Currier reaction couples matched pairs of Michael acceptors under nucleophilic catalysis. The mechanism is often assumed to be analogous to the Morita-Baylis-Hillman reaction. However, this does not explain why most Rauhut-Currier reactions employ tertiary phosphines as catalysts, despite both tertiary phosphines and amines being effective in Morita-Baylis-Hillman reactions. The archetypal crossed Rauhut-Currier reaction between acrylate and fumarate esters, catalyzed by Cy3P, has been investigated by in situ 1H, 19F, and 31P NMR spectroscopy, 2H / 13C labelling and KIEs, and computation. The mechanism that is elucidated explains why phosphines are efficient Rauhut-Currier catalysts and amines are not, and how the crossed selectivity is achieved through differential b-substitution in the two Michael acceptors. Reversible addition of the bulky but nucleophilic phosphine to the less hindered acrylate generates an enolate that is selectively trapped by the more electrophilic Michael acceptor, the fumarate. The catalytic cycle is completed by intramolecular β®g' proton transfer to generate a phosphonium ylide, tautomerization, and elimination of the phosphine. The intermediacy of the ylide bypasses a high barrier a®g' pathway and results in the productdetermining step for the crossed Rauhut-Currier product being the enolate addition to the Michael acceptor, not the proton transfer
