Competition between nucleophilic addition and electron-transfer process in the reaction of 9-diazo-10-antrone with Grignard reagents

9-diazo-10-anthrone reacts with RMgX (R = Me, Et, Bu n, 5-hexenyl, Pr/, benzyl, Bu t) essentially yielding 9-alkylazo-10-hydroxy derivatives, which are isolated in their tautomeric quinoid structure as alkylhydrazones of 9,10-anthraquinone. The yields of these compounds decrease as the oxidation potentials (Eox) of the Grignards decrease: at the same time additional compounds, formed through a radical mechanism, are obtained in higher yields. The reaction has been interpreted as a competition between single electron transfer (SET) and nucleophilic attack, which occur with ratios varying with the oxidation potentials of the Grignard reagents. Evidences for the SET pathway have been found performing an experiment in the presence of 2,2.6,6- tetramethylpiperidine-l-oxyl (TEMPO) as a scavenger of C-centered radicals.