1,3-DIPOLAR ADDITION OF NITRILE YLIDE TO OLEFINS USING THE LASER FLASH-PHOTOLYSIS OF 3-(BIPHENYL-4-YL)-2H-AZIRINE

The 1,3-dipolar addition of a nitrile ylide (NY) to various olefins (u sing the laser hash photolysis of 3-(biphenyl-4-yl)-2H-azirine (AZ) to generate NY) was studied as a function of the solvent polarity and te mperature, A transient absorption band of NY was observed at 410 nm an d the decay was enhanced on addition of a dipolarophile. The bimolecul ar rate constant of 1,3-dipolar addition (k(obs)) increased with incre asing ionization potential (IP) of the dipolarophile, and the k(obs) v alue in cyclohexane (c-Hex) was found to be 1.2-1.5 times larger than that in acetonitrile (MeCN). The k(obs) value of dimethyl fumarate (DF ) was dependent on the solvent polarity, providing a minimum k(obs) va lue at a dielectric constant epsilon of 3-7. Arrhenius plots of k(obs) for DF gave negative activation energies in c-Hex and MeCN. These res ults suggest that the reaction proceeds via a complex of DF and NY, Th e activation energies for adduct formation and dissociation from the c omplex were dependent on the solvent polarity. The solvent effect is d iscussed in terms of the stabilization energies on solvation of NY, th e complex and the adduct.