Distinguishing between Polar and Electron-Transfer Mechanisms for Reactions of Anions with Alkyl Halides

Article abstract of DOI:10.1021/ja00252a027

9-Substituted fluorenide carbanions in the series 9-MeFl^-, 9-MeO-Fl^-, and 9-Me2N-Fl^-, which have nearly the same basicities but become progressively easier to oxidize, were selected as a test trio to probe for electron-transfer (e_T^-) components or radical-pair intermediates in reactions with alkyl halides.The trio members were shown to undergo e_T^- at progressively faster rates (>10² rate span) with two different types of single-electron acceptors, as expected- On the other hand, with PhCH2Cl, i-BuBr, or i-BuI, S_N2 products were formed in a rate order that was the reverse of that established as characteristic of e_T^-.Reactions of the more sterically hindered 9-i-Pr2N-Fl^- ion with PhCH2Cl or i-BuI to give S_N2 products were over 1400 times slower than those with 9-MeFl^-, whereas the e_T^- rate with c-C6H10(NO2)Ts was eight times faster.On the other hand, reactions of the test trio with F3CCH2I, which is known to react slowly in S_N2 reactions, gave the reactivity order characteristic of e_T^-, and radical-type products were formed.For this reaction, a plot of log k_obsd vs.E_ox(A^-) for seven 9-G-Fl^- ions, wherein the bulk and radical-stabilizing ability of G was varied, was linear.This result shows that E_ox(A^-) is a good measure of e_T^- ability and that e_T^- rates are insensitive to steric effects in either the donor or acceptor that cause larger rate retardations in polar S_N2 reactions.