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 of electron-defecient olefins (e.g., methyl acrylate) results in the production of cycloadducts. In addition, irradiation of these substances in aqueous MeCN leads to formation of N-methylphthalimide. Laser flash photolysis and fluorescence spectroscopy have now been employed to investigate the mechanistic details of these novel excited-state processes. The results of this effort show that azomethine ylides are the key reactive intermediates in these processes. In addition, the investigations provide information about the dynamics of several ylide decay pathways and the nature of the excited states responsible for the ylide-forming silyl-migration (singlet and triplet) and decarboxylation (triplet) reactions. Pulsed irradiations of MeCN solutions of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine (2) give rise to transients whose absorption and decay properties are consistent with their assignment as azomethine ylides. Kinetic analysis of the decay of the ylides in the presence of dipolarophiles, methyl acrylate and acrylonitrile, provides the rates of the dipolar cycloaddition reactions. Reactions of methyl acrylate with the ylides produced by pulsed irradiation of N-[(trimethylsilyl)methyl]phthalimide (1) and N-phthaloylglycine (2) occur with respective bimolecular rate constants of 8.9 × 106 and 2.7 × 107 M-1 s-1. Methanol promotes the decay of the N-[(trimethylsilyl)methyl]phthalimide-derived ylide by a process which is second order in MeOH and has a kinetic OD-isotope effect of 4.3. In contrast, quenching of this ylide by acetic acid is first order in AcOH. The results suggest that the mechanism for MeOH-promoted decay involves initial and reversible formation of a silylate complex via nucleophilic addition of MeOH to the ylide. This is then followed by rate-limiting proton transfer from MeOH to the carbanionic center in the silylate complex either in concert with or preceding desilylation. The mechanism for AcOH-induced ylide decay has these steps reversed; i.e., rate-limiting proton transfer precedes AcOH-induced desilylation. Also, MeOH catalyzes 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 effect of 1.5. Finally, the observations (1) of complete loss of fluorescence of the 1,8- and 2,3-naphthalimide chromophores upon changing the N-substituent from methyl to (trimethylsilyl)methyl and (2) that ylide formation from 1 can be xanthone triplet sensitized suggest that the ylide-forming, silyl-transfer reactions of the (silylmethyl)phthalimides can occur in both the singlet and triplet excited-state manifolds.