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

1973; Royal Society of Chemistry; Issue: 5 Linguagem: Inglês

10.1039/p29730000616

2050-8239

Zvi Rappoport, Pnina Peled,

Photochemistry and Electron Transfer Studies

The observed second-order rate coefficients k2obs 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, k2obs=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 k2obs against [Amine], which yield linear inversion plots of 1/k2obs 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 (k3B) 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 (k3B/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 k3B 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.