NOVEL AND EFFICIENT AZOMETHINE YLIDE FORMING PHOTOREACTIONS OF N-(SILYLMETHYL)PHTHALIMIDES AND RELATED ACID AND ALCOHOL DERIVATIVES

An investigation of the photochemistry of N-(silylmethyl)phthalimides and related alpha-phthaloylacetic acids and 2-phenylethanol derivatives has uncovered new excited state processes resulting in the formation of azomethine ylide intermediates. Irradiation of N-[(trimethylsilyl)methyl]phthalimide in MeCN promotes C to O silyl transfer to generate the corresponding azomethine ylide which is efficiently trapped by reaction with water to yield the N-methylphthalimide or by dipolar cycloaddition with acetone, methyl acrylate, or acrylonitrile. Cycloadditions with the latter two dipolarophiles are both regioselective and endo-stereoselective. These processes can be triplet photosensitized by use of acetophenone. The related N-[(trimethylsilyl)methyl]-1,8-naphthalimide reacts in a similar manner upon irradiation in MeCN solutions containing the dipolarophiles methyl acrylate and acrylonitrile to produce cycloadducts which undergo spontaneous elimination of TMSOH, yielding alpha,beta-unsaturated ester or nitrile products. The ylide formed by irradiation of the (silylmethyl)phthalimide is trapped in a stereospecific (retention) manner by the dipolarophiles trans-hex-4-en-3-one, dimethyl maleate, and dimethyl fumarate. The effect of aryl ring substitution on the regiochemical course of the photoinduced C to O silyl migration process was probed by use of the 4-methoxy- and 4-carbomethoxyl-N-(silylmethyl)phthalimides. Irradiation of the former substance in an MeCN solution containing acrylonitrile gives rise to a single adduct whose structure suggests that silyl migration to oxygen of the carbonyl meta to the OMe substituent is highly favored. In contrast, the 4-carbomethoxyphthalimide is converted under these conditions to a mixture of regioisomeric adducts. Thus, silyl migration in the excited state of this substance is nonselective. In accord with hints found in earlier observations made by Kanaoka (Chem. Pharm. Bull. 1982, 30, 1263), N-phthalimide derivatives of the alpha-amino acids glycine, alanine, and phenylalanine undergo similar ylide forming photoreactions upon irradiation in MeCN solutions. The azomethine ylides produced by photodecarboxylation of these substances are efficiently trapped by dipolarophiles, and the overall photoreactions starting with the alanine and phenylalanine derivatives are highly stereoselective. Finally, the N-phthalimide derivative of 2-amino-1-phenylethanol also is transformed to a related ylide upon irradiation in MeCN. The nature, regiochemical and stereochemical course, and mechanistic interpretation of these new azomethine ylide forming photoreactions are discussed in this publication.