Laser flash photolysis studies of nitrogen ylides generated by the reaction of arylchlorocarbenes with substituted vinylpyridines and 1-azabuta-1,3-dienes

Laser Flash Photolysis of arylchlorodiazirines in isooctane/CH2Cl2 in the p resence of substituted vinylpyridines yields substituted vinylpyridinium yl ide (lambda = 540 nm). As the ylide decays a concomitant growth causes an a bsorption at 330 nm, attributed to the formation of substituted indolizine. The reaction experiences the intramolecular 1,5-cyclization of the ylide i ntermediate. The kinetic parameters for the ylide formation and the 1,5-cyc lization have been obtained. The activation energy for the latter process i s reduced by 3-4 kcal mol(-1) when the vinylpyridine has a phenyl ring as a substituent in beta-position of the ethylenic group. Laser Flash Photolysi s of phenylchlorodiazirine in isooctane in the presence of 1-azabuta-1,3-di ene yields azomethine ylide (lambda = 550 nm) as an intermediate. The kinet ic parameters for the ylide formations and further intramolecular 1,5-cycli zation to pyrrole have been determined. The results resemble those obtained for the 1,5-cyclization of vinylpyridinium ylide. (C) 1999 Elsevier Scienc e S.A. All rights reserved.