Palladium/tin catalyzed alkoxycarbonylation of naturally occurring bicyclic monoterpenes

The alkoxycarbonylation of camphene and beta-pinene has been studied. The following systems have been used as catalysts: PdCl2(PPh3)(2)/SnCl2/PPh3 and PdCl2(diphosphine)/SnCl2/PPh3 whose diphosphines were 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane and 1,4-bis(diphenylphosphino)butane. It was observed that several concurrent transformations of monoterpenes occur in the reaction solutions. To find the optimum conditions for alkoxycarbonylation the effect of the reaction Variables on the product distribution has been investigated. The reaction of beta-pinene exclusively yields the products of the Lewis acid catalyzed skeletal rearrangement accompanied by a nucleophilic addition of chloride and methoxy groups. No products of carbon monoxide incorporation were observed. Camphene was converted to the corresponding linear ester composed of approximately equal amounts of exo and endo isomers with a selectivity of 90% and virtually 100% regioselectivity (linear/branched esters) using a PdCl2(PPh3)(2)/SnCl2/PPh3 system as a catalyst. Methyl bornyl ether was the only major by-product. SnCl2 and PdCl2(PPh3)(2) exhibited a strong synergetic effect on the camphene alkoxycarbonylation. PdCl2(PPh3)(2) alone showed a very low catalytic activity promoting the predominant formation of the thermodynamically more stable exo isomer (the exo/endo ratio is approximately 2:1). (C) 1998 Elsevier Science B.V. All rights reserved.