Kinetics and Mechanism of the Aminolysis of O-Aryl S-Methyl Thiocarbonates

摘要

The reactions of secondary alicyclic (SA) amines-and quinuclidines (QUI) with 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2,respectively) and those of SA amines with 2,3,4,5,6-pentafluorophenyl S-methyl thiocarbonate (3) are subjected to a kinetic study in aqueous solution,at 25.0 deg C,and an ionic strength of 0.2 M (KC1).The reactions of thiocarbonates 1,2,and 3 were followed spectrophotometrically at 400,360,and 220 nm,respectively.Under amine excess,pseudo-first-order rate coefficients (k_(obsd)) are found.Plots of k_(obsd) vs amine concentration at constant pH are linear,with the slope (K_N) independent of pH.The Br0nsted-type plots (log k_N vs pK_a of aminium ions) are linear for all the reactions,with slopes beta = 0.9 for those of 1 with SA amines and QUI,beta = 0.36 and 0.57 for the reactions of 2 with SA amines and QUI,respectively,and beta = 0.39 for the reactions of SA amines with 3.The magnitude of the slopes indicates that both aminolyses of 1 are governed by stepwise mechanisms,through a zwitterionic tetrahedral intermediate (T~(+-)),where expulsion of the nucleofuge from T~(+-) is the rate-determining step.The values of the Bronsted slopes found for the aminolyses of thiocarbonates 2 and 3 suggest that these reactions are concerted.By comparison of the reactions under investigation between them and with similar aminolyses,the following conclusions arise: (i) Thiocarbonate 2 is more reactive than 1 toward the two amine series,(ii) The change of the nonleaving group from MeO in 4-nitrophenyl methyl carbonate to MeS in thiocarbonate 1 results in lower k_N values,(iii) The greater reactivity of this carbonate than thiocarbonate 1 is attributed to steric hindrance of the MeS group,compared to MeO toward amine attack.(iv) The change of a pyridine to an isobasic SA amine or QUI destabilizes the T~(+-) intermediate formed in the aminolyses of 2.(v) The change of 4-nitrophenoxy to 2,3,4,5,6-pentafluorphenoxy or 2,4-dinitrophenoxy as the leaving group destabilizes the tetrahedral intermediate formed in the reactions with SA amines,changing the mechanism from a stepwise process to a concerted reaction.