Regioselectivities in the Di-?-methane Photorearrangement of 2-Methylbenzonorbornadienes Carrying Methoxy- and Cyanoaryl Substituents. The Vinyl Methyl Effect

Article abstract of DOI:10.1021/jo00342a009

The triplet-state photoisomerisations of 2-methylbenzonorbornadiene and all eight possible aryl cyano- and methoxy-substituted derivatives have been investigated.The parent hydrocarbon underwent di-?-methane rearrangement with preferential (81 percent) benzo-vinyl bridging to C-3 (β bridging).The 5-CH3O, 6-CH3O, 5-CN, and 7-CN examples represent cases where the substituent effects should work cooperatively, and a single photoproduct was formed in each instance.In the remaining four examples, the substituents are positioned to direct in an antagonist fashion.Interestingly, the product ratios for the 7-CH3O and 7-CN systems are within experimental error of those determined on the basis of perfect substituent additivity effects.As concerns the 8-CH3O and 8-CN derivatives, the apparent directing effect of methyl for β/α bridging is seen to be slightly higher (96:4) than usual.Probable causes for this phenomenon are presented.Because of its role as a moderate controller of excited-state regioselectivity, the methyl substituent has proven to be a useful probe of such reactions.Synthetic approaches to the alkyl-substituted benzonorbornadienes are also detailed.