The photochemistry of α-substituted 2-vinylstilbenes

Abstract

Irradiation of a solution of 2-vinylstilbene (1) under anaerobic conditions gives 9-phenyltricyclo-[6.1.1.0^2,7]deca-2,4,6-triene. Derivatives of (1), substituted at the α-position of the stilbene double bond, give 2-substituted 3-phenyltricyclo[4.4.0.0^2,4]deca-1(10),6,8-trienes, with the exception of the α-methoxy-derivative which is photostable. α-Substituted derivatives of (1) containing a CO₂H, CO₂CH₃, or CF₃ group yield only the endo-3-phenyl subsituted product. Derivatives of (1) having an α-CN, Cl, or CH₃ substituent yield mixtures of endo- and exo-3-phenyl derivatives. It is shown that only the size of the α-substituents is responsible for the stereoselectivity of the reaction: derivatives containing small substituents give cycloaddition products from both the trans- and cis-isomer.