Highly Active and Selective Platinum(II)-Catalyzed Isomerization of Allylbenzenes: Efficient Access to (<i>E</i>)-Anethole and Other Fragrances via Unusual Agostic Intermediates

Abstract

Terminal alkene isomerization reactions can be efficiently catalyzed by PtII complexes bearing a chelating diphosphine and an alkyl or, better, aryl moiety under mild experimental conditions. In particular diphosphines, such as dppb, characterized by a large bite angle in conjunction with a pentafluorophenyl residue coordinated to Pt enable quantitative conversion of the reagent into internal alkenes within few hours at 50 °C in CHCl3 as solvent. E/Z selectivity can be as high as 98:2 for allylbenzene, and the catalytic system can be fruitfully applied to the preparation of E fragrances derived by isomerization of substituted allylbenzene derivatives. The selectivity increases during the progress of the reaction because of a subsequent catalytic step where the Z alkene coordinates to the Pt and is converted into the E isomer. NMR investigation on the catalyst showed formation of agostic Pt···H intermediate species derived by insertion of the substrate into the Pt−aryl bond followed by β-hydride elimination. Formation of such agostic species is promoted by the steric hindrance imparted by the diphosphine characterized by a large bite angle. Kinetic studies and DFT calculations on the possible agostic intermediates shed light on their structure and enable the formulation of a possible catalytic mechanism.