Palladium catalyzed transformations of monoterpenes:: stereoselective deuteriation and oxidative dimerization of camphene

Camphene undergoes a highly regio and stereoselective palladium catalyzed deuteriation in deuteriated acetic acid solutions of Pd(OAc)(2). NMR reveals that an outward oriented vinylic hydrogen is selectively exchanged for H-2, resulting in 90% camphene-d(1) (ca. 100% stereoselectivity) and 10% camphene-d(2) at 75% conversion of camphene (6 h, 25 degreesC). Neither pi-allyl nor pi-olefin palladium complexes are formed in detectable concentrations during the reaction, whereas palladium hydride (singlet at -6.86 ppm) and palladium deuteride (singlet at -6.78 ppm) intermediates have been detected by H-1 and H-2 NMR, respectively. At higher temperature, oxidative coupling of camphene readily occurs giving the (E, E)-diene, i.e., bis(3,3-dimethyl-2-norbornylidene)ethane, which formally originates by abstracting the outward oriented vinylic hydrogens and coupling the resulting fragments of two camphene molecules. The reaction is catalytic at palladium in the Pd(OAc)(2)-LiNO3(cat)-O-2 and Pd(OAc)(2)-benzoquinone systems. Similar mechanisms for the deuteriation and oxidative coupling of camphene are proposed, which involve the formation of sigma-vinyl palladium hydride intermediates. No deuteriation neither oxidative coupling of limonene, myrcene and beta-pinene were observed under the same conditions. (C) 2003 Elsevier B.V. All rights reserved.