COMPETITION BETWEEN NUCLEOPHILIC-ADDITION AND ELECTRON-TRANSFER PROCESS IN THE REACTION OF 9-DIAZO-10-ANTHRONE WITH GRIGNARD-REAGENTS

9-diazo-10-anthrone reacts with RMgX (R = Me, Et, Bu(n), 5-hexenyl, Pr -i, benzyl, Bu(t)) essentially yielding 9-alkylazo-10-hydroxy derivati ves, which are isolated in their tautomeric quinoid structure as alkyl hydrazones of 9,10-anthraquinone. The yields of these compounds decrea se as the oxidation potentials (E(OX)) of the Grignards decrease: at t he 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 a ttack, which occur with ratios varying with the oxidation potentials o f the Grignard reagents. Evidences for the SET pathway have been found performing an experiment in tile presence of 2.2,6,6-tetramethylpiper idine-1-oxyl (TEMPO) as a scavenger of C-centered radicals. Copyright (C) 1996 Elsevier Science Ltd