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Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Brönsted plots are linear with enhanced substituent constants, σp -, and basicities, pK a -, for strong para π-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C6H5 than of CH3. The proposed mechanism is supported by a relatively small βX (≅0.3) and by the lower ΔH ⧧ (6.7 kcal mol-1) and ΔS ⧧ (−44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para π-acceptor (X = p-CN) in pyridine.

The kinetics of reactions between Z-thiophenyl 4-nitrobenzoates and X-pyridines in acetonitrile at 55.0 degrees C are investigated. The Brönsted plots obtained for the pyridinolysis of thiophenyl benzoates are curved, with the center of curvature at pK(a) approximately 4.2 (pK(a)(0)). The Brönsted plots for these nucleophilic reactions show a change in slope from a large (beta(X) congruent with 0.64-0.72) to a small (beta(X) congruent with 0.19-0.23) value, which can be attributed to a change in the rate-determining step from breakdown to formation of a tetrahedral intermediate in the reaction path as the basicity of the pyridine nucleophile increases. This mechanism is supported by the change of the cross-interaction constant rho(XZ) from a large positive (rho(XZ) = 1.41) for the weakly basic pyridines to a small negative (rho(XZ) = -0.32) value for the strongly basic pyridines. The reactivity-selectivity principle (RSP) holds for the rate-limiting breakdown but fails for the formation of the intermediate. The aminolysis of thiophenyl benzoate with deuterated 4-chlorobenzylamine catalyzed by glymes has primary deuterium kinetic isotope effects (PKIEs), k(H(cat))/k(D(cat)) congruent with 1.28-1.62.

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Selectivity plots, log (k,,/k,,) vs. a,, have been obtained for various nucleophilic substitution reactions: (i) with anilines (N) in methanol (S), k,/k,, using literature data for YC6H4CH2CI, YC6H4COCI, YC6H4SO2CI, YC6H4COCti,Br, YC6H4CH (CHJ CI and YC6H4CH [C(CH,),] OS02C6H4N0,; and (ii) with kinetic solvent isotope effects (KSIE) in water and methanol, kSOH/kSOD, determined in this work for the same compounds (except for the latter two). The t w o selectivity plots are shown t o be equivalent in applying mechanistic criteria based on four reactivity-selectivity regions identified by the slopes of the selectivity plots, Apy. Straightforward applications were possible in most cases except for the aminolysis of phenacyl bromides and 1 -phenylethyl chlorides. For these two, detailed analyses of kinetic results were required, in addition to the reactivity-selectivity considerations, before deciding the reaction mec ha n ism.

Chloroformate esters (ROCOCl, 1) are known to hydrolyse much more slowly than other acyl chlorides (RCOCl). Initial-state stabilization through π-electron resonance (2) has been suggested to be responsible for the decreased reactivity of 1, since such resonance interaction is not possible for RCOCl.1 This resonance effect would be more significant as the R in 1 becomes a stronger electron donating group but be insignificant when R is a strong electron withdrawing group. The electronic nature of the R group would also cause a change in the reaction mechanism, i.e., from rate limiting breakdown of a tetrahedral intermediate to rate-limiting formation an intermediate upon introducing a strong electron donating group, R. 2To gain further understanding of the mechanism of acyl transfer, we have carried out a kinetic study on the solvolysis of 9-fluorenylmethyl chloroformate (3) in a variety of pure and binary solvents at 45.0 (2)], together with the enthalpies and entropies of activation,and solvent deuterium kinetic isotope effect (KIE). We have also compared our current results with those reported previously for solvolysis of 2-adamantyl chloroformate (2-AdOCOCl, 4), 4 phenyl chloroformate (C 6 H 5 OCOCl, 5), and benzyl chloroformate (C 6 H 5 CH 2 OCOCl, 6) 6 to investigate acyl-transfer reaction mechanisms. Results and DiscussionThe rate constants (k) for solvolysis of 3 in 34 pure and binary solvents at 45.0 o C are summarized in Table 1. As shown in the Table 1, k increases as the water content in the medium increases. This indicates that solvent-ionizing power is more important than solvent nucleophilicity, since water has much stronger ionizing power than other solvent employed in this study. 3The rate constant for ethanolysis of 3 measured in this study at 25.0 o C is 1.37 × 10 −4 s −1 (Table 2), which is an order of magnitude smaller than that reported previously for phenyl chloroformate (PhOCOCl, 5) (k = 2.60 × 105 This suggests that the 9-fluorenylmethyl group is less electron-withdrawing than the phenyl group (i.e., less favorable for promoting nucleophilic attack at the electrondeficient carbonyl carbon).The activation parameters, ΔH ≠ and ΔS ≠ , calculated from the k values at 4-different temperatures for solvolysis of 3 in pure EtOH and MeOH, and in 80%EtOH and 50%TFE are shown in Table 2. One can find relatively low ΔH ≠ (11.2 to 13.1 kcal·mol) and large negative ΔS ≠ values (−31.1 to −38.6 cal·mol), which are within the ranges of those reported previously for a normal S N 2 reaction.5,7 In deuterated methanol (MeOD), a kinetic solvent isotope effect (k MeOH /k MeOD ) of 2.20 ± 0.03 is observed. This is within the KIE values of 1.58-2.31 reported for the corresponding methanolyses of n-propyl chloroformate and a series of benzenesulfonyl chlorides at 25.0 o C, in which the reactions are believed to be S N 2 in character. 8,9The extended Grunwald-Winstein equation [eqn. (2)] is useful to determine the extent of nucleophilic participation by the solvent, since the magnitude of l and m in equation (2) i...

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