Kinetics of the 1,5‐dipolar cyclization of 2‐Vinylpyridinium ylides to Indolizines. Determination of rate parameters by laser flash photolysis

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°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 inodolizine. 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. © 1994 John Wiley & Sons, Inc.