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A study of aryl radical cyclization in enaminone esters
Authors:Navarro-Vázquez Armando  García Alberto  Domínguez Domingo
Institution:Departamento de Química Orgánica y Unidad Asociada al CSIC, Facultad de Química, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain.
Abstract:Aryl radical cyclization in N-phenyl, N-benzyl, and N-phenethyl enaminone esters 1a-f was studied. N-Benzyl and N-phenethyl enaminones afforded 5-exo and 6-exo cyclization products, respectively, but radical cyclization did not occur in N-phenyl enaminones. The rate constants for the 5-exo and 6-exo cyclization processes in secondary enaminones were estimated as being on the order of 10(7) s(-1) at 353 K; since DNMR experiments showed the rate constant for rotation around the enaminone C3-N bond to be on the order of 10(4) s(-1) at this temperature, the initial enaminone configuration is maintained throughout the cyclization process. PM3 calculations suggested that the nonoccurrence of endo and 4-exo cyclizations is due to the corresponding transition structures involving significant distortion of the conjugated enaminone system.
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