[3+2] cycloadditions and protonation by alcohols of photochemically generated nitrile ylides from 2H-azirines. Formation and reactivities of azaallenium cations

Albrecht E, Mattay J, Steenken S. [3+2] cycloadditions and protonation by alcohols of photochemically generated nitrile ylides from 2H-azirines. Formation and reactivities of azaallenium cations. Journal of the American Chemical Society. 1997;119(48):11605-11610.On photolysis of 2H-phenylazirines id acetonitrile or alcohol solution with 248 nm laser light, phenylnitrile ylides are formed by heterolytic cleavage of the C-C bond of the azirines. The absorption spectra of the ylides are characterized by two strong bands, at ca. 240 and 280 nm, and a weak band, at 380 nm. Electron-deficient olefins react with the nitrile ylides by 1,3-dipolar cycloaddition to yield 5-membered N-heterocycles (rate constants between 4 x 10(5) and 7 x 10(9) M-1 s(-1), the Hammett-p value for the para-substituent on the phenyl ring is -0.9). In alcohols as solvents, the nitrile ylides are protonated to yield azaallenium cations, whose spectroscopic properties and reactivities with nucleophiles are described. The protonation rates of the ylides in alcohols increase with the acidity of the alcohol. On the basis of the large kinetic isotope effect for protonation of ylide by alcohol (k(H)/k(D) = 5.5), the transition state for the ylide protonation is concluded to be linear