Base-Catalyzed Isomerization of 3-Butenenitrile. Relative Nucleophilicities of Carbanions, Nitranions, Oxanions, and Thianions toward Hydrogen

Article abstract of DOI:10.1021/ja00302a023

Rate constants for base-catalyzed isomerization of CH2=CHCH2CN to CH3CH=CHCN in Me2SO solution by families of carbanions (C(1-)), nitranions (N(1-)), oxanions (O(1-)), and thianions (S(1-)) have been measured.The relative catalytic effect of these anions at the same basicity is S(1-), O(1-) > N(1-) > C(1-).This order is the same as that observed previously for attack of families of anions of these types on a β-hydrogen atom in elimination reactions from cyclohexyl bromide, but it differs from that for attack on carbon in S_N2 reactions: S(1-) >> C(1-) > O(1-) > N(1-).The Broensted coefficients, wich measure the sensitivity of the rates to changes in basicity, were in the range 0.8-1.These large values are consistent with previous values determined for deprotonation of α-CN carbon acids, but not with the predictions of the Leffler-Grunwald rate-equilibrium equation.A review of the literature failed to provide support for the Leffler-Grunwald postulate that β will approach zero for exergonic proton transfers from carbon acids and unity for endergonic proton transfers.The results are rationalized by a two-step mechanism for proton transfers involving a hydrogen-bonded carbanion intermediate.The failure of substituent effects on protonations of carbanions to conform to those observed in the deprotonation of the corresponding carbon acids provides evidence against a one-step mechanism.Our observations argue against the application of the Hammond-Leffler postulate to most organic reactions over the usual reactivity ranges studied.