PHOTOCHEMISTRY OF CYCLOALKENES .V. EFFECTS OF RING SIZE AND SUBSTITUTION

The effects of ring size and substitution on the photochemical behavior of cyclic olefins have been investigated. Cyclohexene and -heptene undergo photosensitized addition of methanol in a fashion similar to that of the 1-methyl and 1-methoxyl analogs, but only in the presence of added mineral acid. Photoprotonation of the 1-phenyl derivatives 9b and c can be readily effected in methanol by either direct irradiation or sensitization with acetophenone, but in either case added acid is required. By contrast, 1-methylcyclopentene showed no tendency to undergo photoprotonation under conditions in which 1-methylcyclohexene underwent extensive protonation. The photoproducts from 1-methylcyclopentene, methylenecyclopentane and methylcyclopentane, apparently arise via a radical process. Radical behavior is also exhibited by 2-norbornene (12), which affords a mixture of norbornanemethanol (16), 2,2′-binorbornane (17), and norbornane (18), as well as the photodimers 13 and 14. Analogous behavior is exhibited by the 2-methyl derivative 22, which affords a mixture of 2-methylnorbornane (23), 2-methylenenorbornane (24), and the mixture of methanol adducts 25. Extension of the study to other bicyclic systems revealed that bicyclo[2.2.2]oct-2-ene (8) is photostable in acidic methanol, the bicyclo[3.1.1]hept-2-ene analog α-pinene (28) undergoes isomenzation to cis-ocimene (29) under a variety of conditions, and the homologous bicyclo[3.2.1]oct-2-ene (35) undergoes facile photoprotonation in acetic acid or acidic methanol or t-butyl alcohol. Thus, aside from the two exceptions of bicyclo[2.2.2]oct-2-ene (8) and α-pinene (28), the photochemical behavior of cyclic olefins is a subtle function of the degree of flexibility available about the double bond, with radical behavior being exhibited by more highly constrained olefins and ionic behavior by olefins having approximately the same flexibility as cyclohexene or -heptene. The mechanistic implications of these results are discussed. © 1969, American Chemical Society. All rights reserved.