Photochemistry of chloro‐2‐vinylstilbenes

Short irradiation at 300 nm of α-chloro- and β-chloro-2-vinylstilbene (20 and 21) leads to a mixture of 2-vinyltolane (35) and 2-vinylstilbene (1) accompanied by the cis- and trans-isomers of the starting compounds. Evidence is found for the occurrence of an ionic intermediate during the photolysis of 20 in methanol. At 360 nm, an additional, small amount of 1-chloro-endo- and 1-chloro-exo-6-phenylbenzobicyclo[3.1.0]hex-2-ene (39) is formed from 20. The elimination of hydrogen chloride also occurs during the irradiation of 2-(2-chlorovinyl)stilbene (23) and leads to 2-ethynylstilbene (41). The acetylenic compounds are photocyclized into 2-phenylnaphthalene (38). However, 2-(1-chlorovinyl)stilbene (22) reacts without loss of chlorine to give 1-chloro-exo-5-phenylbenzobicyclo[2.1.1]-hex-2-ene (40). This exceptional reaction mode is due to a much higher rate of photocyclization resulting from less distortion in the stilbene moiety and a preference for a conformation better suited to the reaction of the 2-(1-chlorovinyl)stilbene molecule. It is shown that both isomers of 22 can photocyclize into 40.