Kinetics of the 1,5-Dipolar Cyclization of 2-Vinylpyridinium Ylides to Indolizines. Determination of Rate Parameters by Laser Flash Photolysis

Article abstract of DOI:10.1002/kin.550261205

Nanosecond laser flash photolysis (λ = 355 nm) of an aqueous solution of 3-chloro-3-p-chlorophenyldiazirine in isooctane produces a transient absorption at 310 nm due to the formation of the carbene. In the presence of 2-vinylpyridine, a second transient with a broad absorption band peaking at 520 nm grows in. This absorption is attributed to 2-vinylpyridinium ylide. The ylide decays with a lifetime equal to 33 μs at 25 deg C independent of the concentration of 2-vinylpyridine. As the ylide decays, there is a concomitant growth of an absorption at 330 nm, attributed to the formation of indolizine. The activation parameters for the 1,5-dipolar cyclization of the ylide to indolizine were determined; E_a = 12.1 kcal mol^-1 and log A = 13.4.