[3-AZIRINES - FORMATION AND REACTIVITIES OF AZAALLENIUM CATIONS(2]CYCLOADDITION AND PROTONATION BY ALCOHOLS OF PHOTOCHEMICALLY GENERATED NITRILE YLIDES FROM 2H)

On photolysis of 2H-phenylazirines id acetonitrile or alcohol solution with 248 nm laser light, phenylnitrile ylides are formed by heterolyt ic 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 yi eld azaallenium cations, whose spectroscopic properties and reactiviti es with nucleophiles are described. The protonation rates of the ylide s in alcohols increase with the acidity of the alcohol. On the basis o f 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.