A duality of mechanisms for the fragmentation of substituted benzyloxychlorocarbenes

Substituted benzyloxychlorocarbenes (X-PhCH2OCCl) were generated photochemically at 25 degrees C in dichloroethane from appropriate diazirine precursors. Fragmentations of the carbenes produced were determined by laser flash photolysis. The data (X, kfrag in s-1) were: p-Me, 2.6 x 105; p-Ph, 8.3 x 104; H, 6.0 x 104; p-Cl, 5.2 x 104; m-Cl, 1.3 x 105; p-F3C, 2.1 x 106; p-O2N, 6.3 x 106. A Hammett correlation of log kfrag versus sigma+ was parabolic. The curvature was taken to imply the gradual change of the fragmentation mechanism from predominantly heterolytic for X-PhCH2CCl with electron-donating X (with developing positive charge on the benzylic carbon in the transition state) to predominantly homolytic for carbenes with electron-withdrawing X. This idea was supported by computational studies.