Dualism of Ion-Pairing Effects in Nucleophilic Vinylic Substitution with Transition-Metal Carbonyl Anions.

Abstract

Ion pairing with cations has a marked but nonuniform effect on the chemistry of carbonylmetalate anions and can either inhibit or facilitate their reactions. A study of the effect of cation-solvating agents ([18]crown-6, hexamethylphosphoramide) on the reactions of carbonylmetalates [Na(K)][M(CO)n L] with vinyl halides activated with strong electron-withdrawing groups, which at the same time could serve as hard donor centers for sodium or potassium cations is presented. The addition of a cation-solvating agent to the solution in THF produced a sharp drop in the rate of the reactions of Na[Re(CO)5 ] or K[Mn(CO)5 ] with the vinyl chlorides studied; the effect (kobs (THF)/kobs (THF+[18]crown-6)) increased along the series: PhCClC(CN)2 (4)<(Z)-PhC(CN)CHCl (17)<PhCClC(CO2 Et)2 (80)<(E)-ClCHCHCO2 Me (200). This strong cation assistance is contrasted by "normal" effects observed for the reaction of K[Mn(CO)5 ] with 1-chloro-2-methyl perfluorocyclopentene, accelerated by [2.2.2]cryptand, as well as for the other fluorine-activated vinyl halides studied previously. The characteristic increase in cation assistance effect in the order CO2 R>CN≫CF allows us to explain assistance from cation coordination to the donor centers in these groups, which stabilizes the transition state. The effect of ion pairing is also dependent on the leaving group in the vinyl halide, almost disappearing for (Z)-PhC(CN)CHBr, with only minor cation assistance observed for (E)-PhCOCHCHI. This prompted us to examine the effect of the leaving group in the reactions of carbonylmetalates with Z and E isomers of Ph(CN)CCHHal (Hal=I, Br, Cl, or F).