Nucleophilic attacks on carbon–carbon double bonds. Part XVII. Base-catalysis in the displacement of vinylic ethoxy-, fluoro-, and cyano-leaving groups by amines

Abstract

The observed second-order rate coefficients k²_obs for the displacement of the leaving group X of p-Me₂N·C₆H₄·C(X):C(CN)₂[(1) when X = OEt, CN, and F] by piperidine and morpholine in MeCN, and of compound (I; X = OEt) also in EtOH and PrⁱOH and of (I; X = OEt) with di-isobutylamine in MeCN, increase with the increase in the amine concentration. Two types of kinetic behaviour are (a) a linear dependence, k²_obs=k′+k″[Amine], observed for all the reactions of compounds (I; X = CN) and (I; X = F) and for (I; X = OEt) with piperidine in MeCN and EtOH, and (b) curved plots of k²_obs against [Amine], which yield linear inversion plots of 1/k²_obs against 1/[Amine], found for the other reactions. The reactions of (I; X = OEt) in MeCN are not catalysed by pyridine, p-phenylenediamine, and N-methylpiperidine, but the reaction with piperidine + morpholine is faster than the combination of the two separate reactions. The reaction of the cyano-compound (I; X = CN) with di-isobutylamine is faster in 1:3 CCl₄–MeCN than in 1:1 CCl₄–MeCN. A mechanism is suggested in which the amine attacks the double bond (k₁) forming a zwitterion, which can either revert to the reactants (k_–1), expel the leaving group in an uncatalysed route (k₂), or react with a second amine molecule in a catalysed (k_3B) route, where a rapid proton transfer equilibrium is followed by a slow electrophilically assisted detachment of the leaving group by the ammonium ion. Linear plots are obtained when k_-1k₂+k₃[Amine] and curved plots which yield linear inversion plots when k_–1∼k₂+k₃[Amine]. The catalysed route predominates in MeCN (k_3B/k₂= 5—>2300) but the uncatalysed route becomes more important in the alcohols. The effects of the changes in the nucleophile, the solvent, and the leaving group [relative k_3B in MeCN: (I; X = OEt), 1 < (I; X = CN), 10⁴ < (I; X = F), 10⁷] are consistent with the mechanism suggested, and are in accord with a multistep mechanism for nucleophilic vinylic substitution.