Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene |
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Authors: | Márcio José da Silva Rosemeire Brondi Alves Elena V Gusevskaya |
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Institution: | a Departamento de Química ICEx, Universidade Federal de Minas Gerais, Quimica, Belo Horizonte 31270-901, MG, Brazil b Centre for NMR, Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK |
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Abstract: | 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 2H, resulting in 90% camphene-d1 (ca. 100% stereoselectivity) and 10% camphene-d2 at 75% conversion of camphene (6 h, 25 °C). Neither π-allyl nor π-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 1H and 2H 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)-O2 and Pd(OAc)2-benzoquinone systems. Similar mechanisms for the deuteriation and oxidative coupling of camphene are proposed, which involve the formation of σ-vinyl palladium hydride intermediates. No deuteriation neither oxidative coupling of limonene, myrcene and β-pinene were observed under the same conditions. |
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Keywords: | Palladium Camphene Deuteriation Oxidative coupling |
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