Cycloaddition of nitrones to free and coordinated (E)-cinnamonitrile: effect of metal coordination and microwave irradiation on the selectivity of the reaction

[2 + 3] Cycloaddition of N-methyl-C-phenylnitrone to transition metal coordinated (E)-cinnamonitrile occurs exclusively at the nitrile C (identical)N bond, leading to (delta)4-1,2,4-oxadiazoline complexes, from which the heterocyclic ligand can be released and isolated in high yield. In contrast, the reaction of the nitrone with free cinnamonitrile involves the C=C bond only, yielding a diastereomeric mixture of isoxazolidine-4-carbonitriles. Microwave irradiation enhances the reaction rates of both transformations considerably, without changing their regioselectivity with respect to the thermal reactions. The two nitrile ligands in complexes of the type [MCl2 (cinnamonitrile)2](M = Pt or Pd) are significantly different in reactivity. Thus, short-time microwave irradiation allows for the selective synthesis of the mono-cycloaddition product [PtCl2(cinnamonitrile)(oxadiazoline)], even in the presence of an excess of nitrone. Using longer irratiation times, this complex can be further transformed into the bis-cycloaddition product [PtCl2(oxadiazoline)2]. The latter compound is also produced when thermal heating is applied, however, the formation of the mono-cycloaddition product fails to be selective under thermal conditions.