Reactions of PhSCH2Li and NCCH2Li with benzaldehyde and benzophenone: Whendoes the mechanism change from ET to polar?

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effect were measured for the reaction of phenylthiomethyllithium (PhSCH2Li, 1) wi th benzaldehyde and benzophenone, and cyanomethyllithium (NCCH2Li, 2)with b enzaldehyde, and the results were compared with those for other lithium rea gents such as MeLi, PhLi, CH2=CHCH2Li, and CH2=C(OLi)C(CH3)(3). It was prev iously shown that the reactions of MeLi, PhLi, and CH2=CHCH2Li proceed via a rate-determining electron transfer (ET) process whereas the reaction of l ithium pinacolone enolate goes through the polar (PL) mechanism. The reacti on of 1 with benzaldehyde gave no carbonyl-carbon KIE ((12)k/(13)h = 0.999 +/- 0.004), similar to that measured previously for the MeLi reaction with benzophenone ((12)k/(14)k = 1.000). The effect of substituents of the aroma tic ring of benzaldehyde and benzophenone on the reactivity gave very small Hammett p values of 0.17 +/- 0.03 and 0.26 +/- 0.06, respectively. These s mall p values are again similar to that observed for the reaction of MeLi. Likewise the reactions of 2 with benzaldehydes gave small KIE and the p val ue ((12)k/K-13 = 0.996 +/- 0.004, p = 0.14 +/- 0.02). Dehalogenation and en one-isomerization probe experiments for 2 showed no evidence for the presen ce of radical-ion pair of sufficient lifetime during the course of the reac tion. It is concluded that the reactions of 1 and 2 with the aromatic carbo nyl compounds proceed via the electron transfer-radical coupling mechanism with rate-determining ET as in the reactions of MeLi, PhLi, and CH2=CHCH2Li .