Reaction Pathways for o-Anilides Aryl Radicals: 1,5- or 1,6 O-Hydrogen Transfer versus

REY, VALENTINA; PIERINI, ADRIANA B.; PEÑEÑÓRY, ALICIA B.

JOURNAL OF ORGANIC CHEMISTRY

ACS

Lugar: Salt Lake City; Año: 2009 vol. 73 p. 1223 - 1230

0022-3263

The photoinduced reactions of o-iodoanilides (o-IC6H4N(Me)COR, 4a-d) with sulfur nucleophiles such as thiourea anion (1, -SCNH(NH2)), thioacetate anion (2, MeCOS-) and sulfide anion  (3,  S2-) follow different reaction channels, giving the sulfides by a radical nucleophilic substitution or the dehalogenated products by hydrogen atom transfer pathways. After an initial photoinduced electron transfer (PET) from 1 to iodide 4, the o-amide aryl radicals 12 are generated. These aryl radicals 12 afford alternative reaction pathways depending on the structure of the a-carbonyl moiety: a) 12b (R = Me) adds to 1 to render the methylthio-substituted compounds by quenching the thiolate anion intermediate with MeI after irradiation; b) 12c (R = -CH2Ph) follows a 1,5-hydrogen transfer to give a stabilized a-carbonyl radical (17); and c) 12d (R = t-Bu) affords 1,6-hydrogen transfer, followed by a 1,4-aryl migration to render an amidyl radical (20), which is reduced to the N-benzyl-N,2-dimethyl propanamide (10). Together with this last rearranged product, the ipso substitution derivative was also observed. Similar results were obtained in the PET reactions of 4d (R = t-Bu) with anions 2 and 3 under entrainment conditions with the enolate anion from cyclohexenone (5) or the tert-butoxide anion (6). From this novel rearrangement, and only under reductive conditions by PET reaction with anion 5, iodide 4d (R = t-Bu) affords quantitatively the propanamide 10. The energetic of the intramolecular rearrangements followed by radicals 12b-d were rationalized by B3LYP/6-31+G* calculations.