Spectroscopic, kinetic, and structural properties of rotamers of the 2-vinylpyridinium ylide of phenylchlorocarbene

The analysis of the time-resolved UV-vis absorption spectra of the 2-vinylpyridinium ylide of phenylchlorocarbene, measured by laser-flash photolysis, indicates the existence of two rotamers of this species. The absorption spectrum, rise time, and decay time of each rotamer were determined by a global analysis method. The kinetic analysis of the results indicates an interconversion between the rotamers and a large difference between their rates of cyclization. The observed rotamerism involves a rotation of the vinyl pyridine with respect to the carbene, the two rotamers being differentiated by the fact that the vinyl group faces either the chlorine or the phenyl groups of the carbene. Calculations of the absorption spectra and enthalpies of formation of numerous possible structures indicate that the 100-nm difference between the wavelengths of maximum absorption is well explained by a change in the tilt angle between the phenyl and the yfide planes from 35-40degrees for the "red-absorbing" ylide to 55-60degrees for the other. However, the calculated enthalpies of formation of the various structures being nearly the same, these calculations do not allow us to assign one specific structure to one or the other of the observed species.