Auraoxetanes as Plausible Intermediates in the Oxidation of Alkenes by Oxygen- Transfer Reaction from Gold(III) Oxo Complexes.

Alkene oxidation by oxygen atom transfer reaction from late transition metal oxo complexes is, at present, an objective of particular interest. Metallaoxetanes, proposed as possible intermediates, have been recently isolated in the case of platinum(II).1 Intermediacy of metallaoxetanes in metal catalyzed alkene epoxidations has been proven in the case of silver as well.2 A series of gold(III) oxo complexes, [Au2(N,N)2(μ-O)2][PF6]2, supported by bidentate 6-substituted-2,2’- bipyridines, had been synthesized several years ago by some of us.3 Preliminary studies, indicating the feasibility of oxygen atom transfer reaction, were followed by extensive investigations on the alkene oxidation. The reaction with styrene, chosen as a model, was thoroughly studied and a gold(I) olefin complex [Au(N,N)(η2-CH2=CHPh][PF6] structurally characterised.4 Styrene oxygenated derivatives, contemporaneously formed, were identified as well. Analogous results have been successively obtained with other terminal olefins, while no reaction takes place with internal olefins. Following the hypothesis that gold(I) alkene complexes and alkene oxidation products could have been originated by a common precursor, and that the precursor was likely to be an auraoxacyclobutane, we thought to study the reaction with strained cyclic olefins, such as 2-norbornylene, nb, and 2,5-norbornadiene, nbd. These olefins, being rather reactive but lacking of active β-hydrogens, were good candidates for stabilizing a possible metallaoxetane intermediate. The reaction with both olefins proceeds slowly to give the gold(I) alkene complexes [Au2(N,N)2(μ-η4-nbd][PF6]2 and [Au(N,N)(η2-nb][PF6]2, respectively, as the main products. A second product of the reaction with nb, which depending on the substituent in 6 of the bipyridine and on the preparative conditions forms in appreciable amount, resulted to be an unprecedented auraoxetane [Au(bipyMe)(κ2-O,C-2- oxynorbornyl)][PF6].5 Plausible reaction pathways for the formation of the gold complexes and the oxygenated organic derivatives will be discussed. References [1] E. Szuromi, H. Shan, P.R. Sharp, J. Am. Chem. Soc. 2003, 125, 10522-10523; [2] S. Linic, H. Piao, K. Adib, M.A. Barteau, Angew. Chem. Int. Ed. 2004, 43, 2918-2921. [3] M.A. Cinellu, G. Minghetti, M.V. Pinna, S. Stoccoro, A. Zucca, M. Manassero, M. Sansoni, J. Chem. Soc., Dalton Trans. 1998, 1735-1741. [4] M.A. Cinellu, G. Minghetti, S. Stoccoro, A. Zucca and M. Manassero, Chem. Commun., 2004, 1618-1619 [5] M.A. Cinellu, G. Minghetti, F. Cocco, S. Stoccoro, A. Zucca, M. Manassero, submitted.