ELECTRON-TRANSFER-INDUCED PHOTOCHEMICAL-REACTIONS OF (SILYLALLYL)IMINIUM AND BENZYLPYRROLINIUM SALTS BY DUAL DIRADICAL AND DIRADICAL CATION CYCLIZATION PATHWAYS

The electron-transfer-induced photocyclization reactions of a series of N- and C-silylmethallyl-substituted iminium and 1 -benzyl-1 -pyrrolinium salts have been investigated in order to gain information about their mechanistic course and to probe their synthetic potential. The results obtained from these studies demonstrate that diradical cation intermediates formed in these processes can be transformed to products via two competing mechanistic pathways involving radical coupling or loss of allylic or benzylic electrofugal groups. Studies with the ortho CH3and ortho CH2Si-t-BuMe2substituted N-benzyl-2- methylpyrrolinium perchlorates show that diradical cation coupling is the predominant mode followed for reaction of the intermediate singlet and triplet cation diradicals in either MeCN, MeOH, or Me2CO. This pathway leads to exclusive formation of benzopyrrolizidine products. In contrast, the ortho CH2SiMe3analogue photocyclizes in MeCN to form predominantly a benzoindolizidine product via a route involving desilylation of the intermediate cation diradical. The predominance of diradical cation desilylation over radical coupling in the ortho CH2SiMe3system is lessened for reaction in MeOH and enhanced when acetone triplet sensitization is employed. Photocyclizations of the meta OH, OMe, OSi-t-BuMe2, CH3, CH2Si-t-BuMe2, and CH2SiMe3substituted N-benzyl-2-methylpyrrolinium perchlorates proceed mainly or exclusively by cation diradical coupling mechanisms and provide the corresponding benzopyrrolizidines in modest to excellent yields. Cyclization in these systems is preferred at the position para to the arene ring substituent, and their selectivity increases as the substituent size increases. The results demonstrate that N-benzyliminium salts participate in a new and general photoinduced cyclization reaction that mimics the ground-state Pictet-Spengler analogue and has unique applications in the area of isoindoline synthesis. Similar mechanistic conclusions have been reached in studies with N- or C-(silylmethallyl)iminium salts containing deuterium labels that enable distinction between photocyclization pathways involving either desilylation or radical coupling of intermediate cation diradicals. In triplet-sensitized reactions of these substances, the desilylation pathway is followed exclusively while, in the direct-irradiation, singlet processes the desilylation and radical coupling modes are competitive. Solvent and substituent effects on the rates of cation diradical coupling have been observed and interpreted in terms of charge localization and frontier orbital considerations. © 1990, American Chemical Society. All rights reserved.