Mechanistic Studies of the Azomethine Ylide-Forming Photoreactions of N-(Silylmethyl)phthalimides and N-Phthaloylglycine

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• 作者：Yasutake Takahashi ; Tsutomu Miyashi ; Ung Chan Yoon ; Sun Wha Oh ; Maria Mancheno ; Zhuoyi Su ; Daniel F. Falvey ; and Patrick S. Mariano
• 刊名：Journal of the American Chemical Society
• 出版年：1999
• 出版时间：April 28, 1999
• 年：1999
• 卷：121
• 期：16
• 页码：3926 - 3932
• 全文大小：96K
• 年卷期：v.121,no.16(April 28, 1999)
• ISSN：1520-5126

文摘

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) resultsin the production of cycloadducts. In addition, irradiation of these substances in aqueous MeCN leads toformation of N-methylphthalimide. Laser flash photolysis and fluorescence spectroscopy have now beenemployed to investigate the mechanistic details of these novel excited-state processes. The results of this effortshow that azomethine ylides are the key reactive intermediates in these processes. In addition, the investigationsprovide information about the dynamics of several ylide decay pathways and the nature of the excited statesresponsible 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 asazomethine ylides. Kinetic analysis of the decay of the ylides in the presence of dipolarophiles, methyl acrylateand acrylonitrile, provides the rates of the dipolar cycloaddition reactions. Reactions of methyl acrylate withthe 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 promotesthe decay of the N-[(trimethylsilyl)methyl]phthalimide-derived ylide by a process which is second order inMeOH and has a kinetic OD-isotope effect of 4.3. In contrast, quenching of this ylide by acetic acid is firstorder in AcOH. The results suggest that the mechanism for MeOH-promoted decay involves initial and reversibleformation of a silylate complex via nucleophilic addition of MeOH to the ylide. This is then followed byrate-limiting proton transfer from MeOH to the carbanionic center in the silylate complex either in concertwith 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 theylide derived by irradiation of N-phthaloylglycine by a process which is first order in MeOH and has a kineticOD-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-transferreactions of the (silylmethyl)phthalimides can occur in both the singlet and triplet excited-state manifolds.