Quantum Yields of the Initiation Step and Chain Propagation Turnovers in SRN1 Reactions. Photostimulated Reaction of Neophyl Iodide with Carbanions in DMSO

JUAN E. ARGÜELLO; ALICIA B. PEÑÉÑORY; ROBERTO A. ROSSI

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Año: 2000 vol. 65 p. 7175 - 7182

0022-3263

Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively.The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 _ 105 M-1 s-1, by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 106 M-1 s-1. Although the overall quantum yield determined (ì ) 350 nm) for the studied reactions is below 1, the chain lengths (¼propagation) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively