ORGN 12

Nitrile oxides are generally represented in two resonance forms (structures A and B in Scheme 1). However, a nitrosocarbene structure (C of Scheme 1) is also possible. Usually nitrile oxides act as 1,3-dipoles in (3+2) cycloadditions with dipolarophiles. Density functional theory calculations at the (U)B3LYP/6-31G* level show that nitrile oxides can also act as nitrosocarbenes, as demonstrated by reactions 1 and 2 in Scheme 2. Nitrile oxide dimerization to furoxan (reaction 1) is found to be stepwise involving the dinitrosoalkene intermediates that are really bis-iminoxy diradicals. The rate-determining step of the dimerization processes corresponds to C-C bond formation with activation energies of about 11 (for alkylnitrile oxide) and 14 kcal/mol (for arylnitrile oxide). The intramolecular 1,3-dipolar ene reaction between nitrile oxide and alkene (reaction 2, Ishikawa, T.; Urano, J.; Ikeda, S.; Kobayashi, Y.; Saito, S.  Angew. Chem. Int. Ed. 2002, 41, 1586) is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate.