PHOTOINDUCED FREE-RADICAL CHEMISTRY OF THE ACYL TELLURIDES - GENERATION, INTERMOLECULAR AND INTRAMOLECULAR TRAPPING, AND ESR SPECTROSCOPIC IDENTIFICATION OF ACYL RADICALS

Acyl tellurides are prepared in good to excellent yield by the reactio n of sodium aryl tellurides with acyl chlorides, or mixed anhydrides, and are found to be moderately air-stable substances. In contrast to p revious literature reports, acyl tellurides of aryl and vinyl carboxyl ic acids are found to be excellent sources of acyl radicals on photoly sis with a simple white light source. The acyl radicals so generated m ay be trapped intermolecularly by dichalcogenides, or by TEMPO in exce llent yield. Trapping by N-tert-butyl-alpha-phenyl nitrone produces a stable nitroxide radical which has been characterized by ESR spectrosc opy. The very efficient trapping of acyl radicals by acyl tellurides t hemselves is demonstrated by a crossover experiment. Acyl tellurides a re shown to participate in very efficient radical cyclization reaction s onto alkenes with the formation of five-, six-, and eight-membered r ings. The immediate products of the cyclizations onto alkenes are alph a-[(aryltelluro)methyl] ketones and the chemistry of these relatively unstable species is briefly described. Treatment with hydrogen peroxid e affords alpha-methylene ketones in high yield. When elimination of t he aryl telluro group is not possible the alpha-[(aryltelluro)methyl] ketones are stable species that may subsequently be employed in furthe r radical chain reactions, for example with tributyltin hydride and me thyl acrylate. Cyclization onto alkynes yields alpha-[(aryltelluro)met hylene] ketones which are stable species and which take part in substi tution reactions with higher order cuprates or with diphenyl diselenid e.