Quantum yields of the initiation step and chain propagation turnovers in S(RN)1 reactions: Photostimulated reaction of 1-iodo-2-methyl-2 phenyl propane with carbanions in DMSO

Neophyl radicals were generated by photoinduced electron transfer (PET) fro m 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) afforde d mainly the reduction products tert-butylbenzene (5) and the rearranged is obutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photos timulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, a s a radical trap, afforded adduct 10 in 57% yield. The photoinduced reactio n of the enolate anion of acetophenone (3) with 1 gave the substitution pro ducts 11 (50%) and 12 (16%), which arise from the coupling of 3 with radica ls 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 x 10(5) M-1 s(-1), by the use of the rearrang ement of this radical as a clock reaction. The anion of nitromethane (4) wa s 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 o nly 2% of the rearranged product 14. When the ratio of 4 to 1 was diminishe d, it was possible to observe both substitution products 13 End 14 in 16% a nd 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 10(6) M-1 s(-1). Although the overall quantum yield determi ned (lambda = 350 nm) for the studied reactions is below 1, the chain lengt hs (Phi (propagation)) for the reaction of 1 with anions 3 and 4 are 127 an d a, respectively.