SELECTIVE PALLADIUM-CATALYZED FUNCTIONALIZATION OF LIMONENE - SYNTHETIC AND MECHANISTIC ASPECTS

The scope and limitation of palladium-catalysed functionalization of l imonene have been investigated. Various catalytic combinations were ex amined 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 conditio ns the chemoselectivity was always high as only oxidation products wer e 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 eithe r 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-p i-olefin) palladium complex, which was isolated under acetoxylation re action conditions. (C) 1998 Elsevier Science B.V. All rights reserved.