50616-99-4Relevant articles and documents
Distinguishing between Polar and Electron-Transfer Mechanisms for Reactions of Anions with Alkyl Halides
Bordwell, F. G.,Wilson, Craig A.
, p. 5470 - 5474 (2007/10/02)
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 (eT-) components or radical-pair intermediates in reactions with alkyl halides.The trio members were shown to undergo eT- 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, SN2 products were formed in a rate order that was the reverse of that established as characteristic of eT-.Reactions of the more sterically hindered 9-i-Pr2N-Fl- ion with PhCH2Cl or i-BuI to give SN2 products were over 1400 times slower than those with 9-MeFl-, whereas the eT- 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 SN2 reactions, gave the reactivity order characteristic of eT-, and radical-type products were formed.For this reaction, a plot of log kobsd vs.Eox(A-) for seven 9-G-Fl- ions, wherein the bulk and radical-stabilizing ability of G was varied, was linear.This result shows that Eox(A-) is a good measure of eT- ability and that eT- rates are insensitive to steric effects in either the donor or acceptor that cause larger rate retardations in polar SN2 reactions.
