Cationic P(OPh)₃‐ or PPh₃‐Rhodium(I) Complex‐Catalyzed Isomerizations of 5‐Alkynals to δ‐Alkynyl Ketones, Cyclopent‐1‐enyl Ketones, and Cyclohexenones

Abstract

Abstract We have developed catalytic isomerizations of 5‐alkynals to γ‐alkynyl ketones and cyclopent‐1‐enyl ketones using [Rh{P(OPh) 3 } 2 ]BF 4 as a catalyst. Cu(OTf) 2 and AgBF 4 are also effective catalysts for the formation of γ‐alkynyl ketones. The substituents at the 4‐positions in 5‐alkynals play important roles in the selection of two different isomerization pathways. The first catalytic endo / trans hydroacylation of acyclic 5‐alkynals leading to cyclohexenones was also developed with [Rh(PPh 3 ) 2 ]BF 4 as a catalyst. Crossover deuterium‐labeling studies indicated that these isomerization reactions proceed intramolecularly.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)