Regioselectivity of Pyridine Ring Migration during Triplet-Sensitized Di-?-methane Photorearrangements of 5,8-Dihydro-5,8-methanoisoquinoline Derivatives. The Case for Heteroatom Control

Article abstract of DOI:10.1021/jo00355a020

Four 5,8-dihydro-5,8-methanoisoquinolines were synthesized and subjected to sensitized photoisomerization.For the parent heterocyclic system, the ring nitrogen atom was found to exert a directing influence favoring migration of C_para to the extent of 75percent.The "meta" -Cl and -OCH3 examples represent cases where the substituents should work cooperatively, and an additive effect was noted for the methoxyl case.Electron density values, in tandem with past photoelectron spectroscopy data, suggest that the overall regioselectivity pattern is the result of LUMO control.When the methoxy group is positioned "ortho", however, the substituents are positioned to direct in an antagonist fashion.The formation of a single photoproduct identified as 13 reveals that OCH3 totally controls the cours of the di-?-methane rearrangement.This behavior conforms to expectations based upon the HOMO as the discriminating regioselectivity factor.The parallelism between the responce of the title compounds and similarly substituted benzonorbornadienes is striking.