Mechanistic studies of the rhodium-catalysed cyclization of α,ω-alkynoci acids to alkylidene lactones. Crystals structures of two iridium model catalytic intermediates

A mechanism for the Rh-catalysed cyclization of alkynoic acids to alkylidene lactones which accounts for the formation of Z-isomers only, is presneted with the structures of Ir cis-hydrido-carboxylate and cis-hydrido-σ-vinyl model intermediates.