Kinetic and mechanistic study of the reactions of aryl chloroformates with quinuclidines

The reactions of quinuclidines with phenyl, 4-methoxyphenyl, 4-chlorophenyl, and 4-nitrophenyl chloroformates (PCIF, MOPCIF, CIPCIF, and NPCIF, respectively) in aqueous solution (25 °C, ionic strength 0.2 M KC1) are followed spectrophotometrically and evaluated kinetically. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsdversus [quinuclidine] are linear, with the pH-independent slope (kN). The Brønsted-type plots (log kNvs. pKa of quinuclidinium ions) are linear with slopes (β) of 0.32, 0.34, 0.31, and 0.23 for the reactions of PCIF, MOPCIF, CIPCIF, and NPCIF, respectively. The magnitude of the slopes suggests that these mechanisms are stepwise, with the formation of a zwitterionic tetrahedral intermediate (T ±) being the rate-determining step. The sensitivity of log kN to the basicity of the nonleaving group (βnlg) is -0.16. By comparing the reactions under investigation between each other and with similar aminolyses, the following conclusions can be drawn: (i) the mechanisms for the quinuclidinolysis of the four chloroformates studied are stepwise. (ii) The reactivity increases in the sequence MOPCIF < PCIF < CIPCIF < NPCIF. (iii) The change of the leaving group from 2,4-dinitrophenoxide to chloro changes the mechanism from concerted to stepwise. (iv) Quinuclidines are more reactive toward aryl chloroformates than isobasic secondary alicyclic amines. Copyright © 2006 John Wiley & Sons, Ltd.