Mechanistic studies of the azomethine ylide-forming photoreactions of N-(silylmethyl)phthalimides and N-phthaloylglycine

In earlier studies we have shown that irradiation of MeCN solutions of N- [ (trimethylsilyl)methyl]phthalimide and N-phthaloylglycine in the presence o f electron-defecient olefins (e.g., methyl acrylate) results in the product ion of cycloadducts. In addition, irradiation of these substances in aqueou s MeCN leads to formation of N-methylphthalimide. Laser flash photolysis an d fluorescence spectroscopy have now been employed to investigate the mecha nistic details of these novel excited-state processes. The results of this effort show that azomethine ylides are the key reactive intermediates in th ese processes. In addition, the investigations provide information about th e dynamics of several ylide decay pathways and the nature of the excited st ates responsible for the ylide-forming silyl-migration (singlet and tripler ) and decarboxylation (triplet) reactions. Pulsed irradiations of MeCN solu tions of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine ( 2) give rise to transients whose absorption and decay properties are consis tent with their assignment as azomethine ylides. Kinetic analysis of the de cay of the ylides in the presence of dipolarophiles, methyl acrylate and ac rylonitrile, provides the rates of the dipolar cycloaddition reactions. Rea ctions of methyl acrylate with, the ylides produced by pulsed irradiation o f N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine (2) occu r with respective bimolecular rate constants of 8.9 x 10(6) and 2.7 x 10(7) M-1 s(-1). Methanol promotes the decay of the N-[(trimethylsilyl)methyl]ph thalimide-derived by a process which is second order in MeOH and has a kine tic OD-isotope effect of 4.3. In contrast, quenching of this ylide by aceti c acid is first order in AcOH. The results suggest that the mechanism for M eOH-promoted decay involves initial and reversible formation of a silylate complex via nucleophilic addition of MeOH to the ylide. This is then follow ed:by rate-limiting proton transfer from MeOH to the carbanionic center in the silylate complex either in concert wither preceding desilylation. The m echanism for AcOH-induced ylide decay has these steps reversed; i.e., rate- limiting proton transfer precedes AcOH-induced desilylation. Also, MeOH cat alyzes the decay of the ylide derived by irradiation of N-phthaloylglycine by a process which is first order in MeOH and has a kinetic OD-isotope effe ct of 1.5. Finally, the observations (1) of complete loss of fluorescence o f the 1,8- and 2,3-naphthalimide chromophores upon changing the N-substitue nt from methyl to (trimethylsilyl)methyl and (2) that ylide formation from 1 can be xanthone tripler sensitized suggest that the ylide-forming, silyl- transfer reactions of the (silylmethyl)phthalimides can occur in both the s inglet and tripler excited-state manifolds.