The photochemistry of 1-vinyl-2-pyridones and their acyclic analogs

Abstract

520-00In view of the wide variety of photoreactions potentially available to the enamide chromphore, we have investigated the excited state behavior of a number of enamide systems including 1-vinyl-2-pyridones and their acyclic analogs, the N-vinlyacetamides. These studies have led to the discovery of two previously undocumented enamide photoprocesses. Irradiatons of the isobutenyl and propenyl-2-pyridones in solvents of low polarity yield the familiar Dewarpyridones and ?Ç4 + ?Ç4 dimers. When these compounds are irradiated in polar solvents such as methanol or water, reaction occurs from the singlet state to generate the corresponding pyrodonylethanols. Irradiations of aqueous solutions of these alkenylpyridones containing perchloric acid leads to production of the oxazolo[3,2-a] pyridinium perchlorates. Accumulated information suggests the operation of mechanisms icolcing initial electrocyclization to azomethine ylids which are trapped by water or acid to give oxazolopyridinium slats. The hydroxide salts, formed when water is the proton donor, are then transformed into the pyridonylethanols upon work-up. Detailed kinetic analyses of the singlet reaction and decay pathways for these 2-pyridones has yielded individual rate constants for singlet-pyridone conversion to Dewar-pyridone and pyridinium ylid, singlet decay and emission and ylid trapping by water and return to ground state puridones. The solvent polarity dependence of these 2-pyridone photoreactions have been explored using photophysical techniques and interpreted in terms of a solvent polarity effect on the rate of conversion of the singlet pyridines to azomethine ylids. Similar studies have been conducted on a series of N-vinylacetamide. Irradiations of N-phynyl-N-isobutenylacetamide and N-propyl-N-isobutenylacetamide in acetonitrile solutions gave the ??-iminoketones as the result of a [1,3]-acyl shift...