1,3-Dipolar Cycloaddition Reactions of Pyridinium Azomethine Ylides Containing 5,6-Dicyanopyrazines

Abstract

The intermolecular 1,3-dipolar cycloaddition reaction of azomethine ylides with carbon-carbon double or triple bonds represents an efficient and convergent method for the construction of heterocyclic structures containing a single nitrogen atom. 1 One of the most frequently employed methods for the generation of azomethine ylide involves the thermolysis of an aziridine. 2 However, the ring opening of aziridines to azomethine ylides is limited to those having substituent groups capable of stabilizing the dipole centers. Desilylation of α-trimethylsilylonium salts represents another convenient method of generating azomethine ylide. 3 More recently it has been found that nonstabilized azomethine ylides can be generated by the deprotonation of amine Noxides. 4 Azomethine ylides are too unstable to be isolated, and are typically used in situ. Of the various types of azomethine ylides, the pyridinium azomethine ylides are among the most attractive synthetically because they are easily prepared from the appropriate N-alkylpyridinium salt with base and their cycloadducts, the indolizines, have been used as synthetic building blocks. 5 In our continuing efforts to develop functional dyes based on dicyanopyrazine chromophores, we have become interested in pyridinium azomethine ylides containing dicyanopyrazines and their 1,3-dipolar cycloaddition reaction with dipolarophiles, because the indolizine cycloadducts, containing dicyanopyrazines, might meet our purpose. Dyes and pigments generally possess satisfactory color, and good color fastness properties against light and heat, and additional properties such as near infrared absorption, pleochroism, and photoconductivity. 6 The majority of these properties are caused by intramolecular charge-transfer interactions involving π-electrons and intermolecular π-π interactions of the dye chromophores. 7 In this paper, we report the generation of pyridinium azomethine ylide intermediates and the results of 1,3-dipolar cycloaddition reactions of the ylides with dipolarophiles to provide indolizines containing 5,6-dicyanopyrazine. The preparation of methyl or phenyl substituted 2-bromomethyl5,6-dicyanopyrazine 1 is described in a previous report. 6b