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...
