Organometallic gold(III) and gold(I) complexes as catalysts for the 1,3-dipolar cycloaddition to nitrones: synthesis of novel gold–nitrone derivatives

Gold(III) and gold(I) anionic salts mediate the 1,3-dipolar cycloaddition of N-benzyl-C(2-pyridyl)nitrone (2-PyBN) (1) and methyl acrylate (2) (gold 5–10 mol% with respect to the nitrone) decreasing the reaction time and favouring the formation of the exo (cis) isomer. The best catalyst found was Na[AuCl4] (7) able to perform the addition reaction in 56 h (instead of the 96 h required for the control experiment) and giving an endo/exo relation between isomers of 44/56 (as opposed to 73/27, blank reaction). The catalytic activity of several organometallic gold complexes with the radicals pentafluorophenyl (C6F5) or mesityl (2,4,6-(CH3)3C6H2) has been also investigated. In some cases the activity is very similar to that obtained with inorganic salts. With the aim of identifying possible metallic intermediates in the cycloaddition reaction, novel gold(III) and gold(I) nitrone derivatives such as [Au(C6F5)Cl2(2-PyBN)] (21), [Au(C6F5)2Cl(2-PyBN)] (22) and [Au(C6F5)(2-PyBN)] (23) have been prepared and characterized. The reaction between [AuCl3(tht)] and 2-PyBN unexpectedly affords the ionic compound [2-PyBN–H][AuCl4] (5) which also displays catalytic activity and moderate regioselectivity and whose crystal structure has been confirmed by X-ray studies.