Rhodium-catalyzed regio-, diastereo-, and enantioselective intermolecular [4+2] carbocyclization of 4-alkynals with electron-deficient alkenes

We established that a cationic rhodium(I)/dppf or dppb complex catalyzes a regio- and diastereos elective intermolecular [4+2] carbocyclization of 5-trimethylsilyl-4-pentynals with electron-deficient alkenes leading to cyclohexanones. We also established that a cationic rhodium(I)/(R,R)-Walphos complex catalyzes a regio- and enantioselective intermolecular [4+2] carbocyclization of 5-substituted 4-pentynals and 2-alkynylbenzaldehydes with N,N-dialkylacrylamides leading to enantio-enriched cyclohexanones and tetralones, respectively. A single olefin isomer was produced in every carbocyclization. Regioselectivities of the alkene insertion depend on the alkenes used. Mechanistic study suggested that a key intermediate in this intermolecular [4+2] carbocyclization is a five-membered acylrhodium intermediate, formed by cis addition of the rhodium hydride to the metalbound alkyne. This method serves as an attractive new route to highly functionalized cyclohexanones in view of the onestep access to 5-substituted 4-pentynals and 2-alkynylbenzaldehydes starting from readily available terminal alkynes. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.