engleski

The photochemistry of 2-vinylstilbenes. 2. Photoreactions of 2-vinyl- and 2-propenylstilbene and of o-substitution products

Irradiation of 2-vinylstilbene 19 results in the formation of exo-5-phenylbenzobicyclo[2.1.1]hex-2- ene-(exo-21) as the main product (70%). As contrasted with the results with o-divinylbenzene no benzobicyclo[3.1.0 ]hex-2-ene derivative (23) is formed. I rradiation of 2-propenylstilbene (20) gives rise to both types of products, viz., 30% endo-5-methyl-exo-6-phenylbenzobicyclo[2.1.1]hex- 2-ene (27) and l0% exo-4-methyl-endo-6- phenylbenzobicyclo[3.1.0]hex-2-ene (28). Irradiation of 2-vinylstilbenes substituted at one ortho position of the  ring gives a qualitatively similar result as the parent compound ; the main product is an exo-6-phenylbicyclo[2.1.1]hex-2-ene derivative. On introduction of an ortho substituent in the  ring a similar product is obtained, but an endo-6-phenylbicyclo[3.1.0]hex-2- ene derievative is isolated as a second product. 2-Vinylstilbenes having two ortho substituents in the  ring give neither of these photoproducts. The different behavior has been explained by the supposition that the photoproducts mainly arise from the cis isomer of the starting compounds, and that distinct products originate from different conformations of the cis isomers. The supposition has been supported by stereochemical considerations, calculation of Mulliken overlap populations, and NMR data. Most probably the photoreactionos occur from the S1 states. The bicyclo[2.1.1]hexene derivatives are formed by a radical reaction whereas the bicyclo[3.1.0]hexenes may arise from a concerted process. Only when the former reaction is not much faster than the latter are both types of products found.

photochemistry ; 2-vinylstilbene ; propenylstilbene ; ortho-substitution ; cycloaddition

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