Ring-expansion reaction of cyano-substituted singlet phenyl nitrenes:: Theoretical predictions and kinetic results from laser flash photolysis and chemical trapping experiments

On the basis of the open-shell electronic structure of the lowest-singlet state of phenylnitrene, it is predicted that substitution of a radical-stabilizing cyano group at an ortho carbon should facilitate cyclization at that carbon, whereas cyano substitution at the para carbon should retard the rate of cyclization: These qualitative predictions have been tested computationally by performing (8/8)CASSCF and CASPT2/6-31G* ab initio calculations and experimentally by carrying out laser flash photolysis and chemical trapping studies. The calculations and experiments both find that, unlike the case with ortho fluoro and ortho methyl substituents, the rate of cyclization at a substituted carbon is not retarded by an ortho cyano group. In contrast, a para cyano group is found, both computationally and experimentally, to raise the barrier to cyclization of singlet phenylnitrene by >1 kcal/mol.