1,3-Dipolar cycloaddition of nitrone-type dipoles to uncomplexed and metal-bound substrates bearing the CN triple bond

Metal-free and metal-mediated routes for 1,3-dipolar cycloaddition of nitrone-type dipoles to substrates bearing the C N triple bond are compared and the reactions are categorized. In 1,3-dipolar cycloaddition of nitrones to RCN species, the outcome of the reaction is determined by a degree of activation of the dipolarophile. This activation could be greatly enhanced either by introduction of electron-withdrawing substituents into a nitrite molecule or coordination of RCN to a metal center, or by both methods. The ligation makes favorable the 1,3-dipolar cycloaddition of nitrones to a wide range of RCN substrates, even those with strong electron-donor substituents. This cycloaddition reaction forms the basis for the general method for synthesis of so far poorly studied 2,3-dihydro-1,2,4-oxadiazoles, 2,3-dihydro-1,2,4-oxadiazole-based heterocycles, and 2,3-dihydro-1,2,4-oxadiazole ligand systems. The most efficient activators explored so far are Pt-IV and Pt-II centers that fortunately combine such properties as the kinetic inertness of their complexes in substitution reactions and the softness of platinum in terms of the HSAB principle. The coordination of isocyanides to a metal center also greatly activates RNC species toward cycloaddition of nitrones. The reaction of nitrones with palladium(II)-bound isocyanides allows the generation of rather stable cyclic aminocarbenes. These species are relevant to Pd complexes with N-heterocyclic carbenes that are widely employed in catalysis of various organic transformations. (C) 2011 Elsevier B.V. All rights reserved.