1,3-Dipolar addition of nitrile ylide to olefins using the laser flash photolysis of 3-(biphenyl-4-yl)-2H-azirine

Abstract

The 1,3-dipolar addition of a nitrile ylide (NY) to various olefins (using the laser flash photolysis of 3-(biphenyl-4-yl)-2H-azirine (AZ) to generate NY) was studied as function of the solvent polarity and temperature. A transient absorption band of NY was observed at 410 nm and the decay was enhanced on addition of a dipolarophile. The bimolecular rate constant of 1,3-dipolar addition ( k obs) increased with increasing ionization potential (IP) of the dipolarophile, and the k obs value 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 value at a dielectric constant ϵ of 3–7. Arrhenius plots of k obs for DF gave negative ectivation energies in c-Hex and MeCN. These results suggest that the reaction proceeds via a complex of DF and NY. The activation energies for adduct formation and dissociation from the complex were dependent on the solvent polarity. The solvent effect is discussed in terms of the stabilization energies on solvation of NY, the complex and the adduct.