Photochemical Generation and Structure of Vinyl Radicals

Photolysis of a series of E or Z stereoisomeric α‐R‐substituted bromostyrenes (R = CH 3 , F or CN) in methanol yields E and/or Z stereoisomeric styrenes, which stem from the corresponding vinyl radicals. The results show that the α‐Me vinyl radical is a rapidly equilibrating, bent structure, while the α‐F vinyl radical is a stable bent species, in agreement with earlier thermal results. The α‐CN vinyl radical is assigned as a rapidly inverting bent and not a linear species from the stereochemical results as a function of temperature. Stereochemical data for the α‐C(H)=O system in diethyl ether indicate a stable bent vinyl radical as product forming species. The conclusions are supported by quantum chemical computations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)