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.