Selective palladium-catalysed functionalization of limonene: synthetic and mechanistic aspects

The scope and limitation of palladium-catalysed functionalization of limonene have been investigated. Various catalytic combinations were examined in order to select the most efficient system for conversion of this substrate into allylic esters, ethers or alcohols in acetic acid, methanol and water, respectively. It appears that under mild conditions the chemoselectivity was always high as only oxidation products were formed. Moreover, by a judicious choice of ligands and/or reoxidant of palladium, the reaction can be directed mainly toward the formation of functionalized compounds having their allylic double bond in either exocyclic or endocyclic position. In both cases, the trans isomer is the major product. In order to explain these results, a mechanism is proposed involving an external nucleophile attack on a bis (pi-allyl-pi-olefin) palladium complex, which was isolated under acetoxylation reaction conditions. (C) 1998 Elsevier Science B.V. All rights reserved.