Tandem palladium-catalyzed cyclocarbonylation of isolimonene: A mechanistic investigation and theoretical calculations on the fully diastereoselective step

Abstract

Cyclocarbonylation of isolimonene catalyzed by complexes of the type [HPd(SnCl3)L2] provides two cyclopentanone isomers (2a and 2b) containing two new stereogenic centers with a good diastereomeric excess (up to 69% with L2 = dppf). These results show that chiral phosphane ligands are not necessary to ensure asymmetric induction. The diastereomeric excess is due, in fact, to the substrate itself, assisted by the steric hindrance of the diphosphanes. A full characterization of the two isomers by 1H, 13C and DPFGSE NOE NMR spectroscopy gives all the signal assignations and all the positions of the substituents of interest on the chiral carbon atoms. In order to have a better understanding of the catalytic process, we also present a density functional study of the crucial intermediate species 4 involved in the proposed catalytic cycle. Our calculations show that there is no coordination of the C=C bond in an exo mode. Conversely, we have found a pentacoordinate species with a trigonal bipyramidal geometry in which the C=C bond is coordinated in an endo mode. Selected bond lengths and bond orders are reported. The calculated net charge distribution supports the cyclization process, which proceeds through a C-C(O) and a C-Pd coupling. A β-hydride elimination reaction of 4 provides the two 2a and 2b isomers. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.