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...

The rate constants of solvolyses of 2,2,2-trichloro-1,1-dimethylethyl chloroformate (I) in 33 solvents can be well correlated using the extended Grunwald-Winstein equation, with incorporation of the NT solvent nucleophilicity scale and the YCl solvent ionizing scale, with sensitivities towards changes in the scale having values of 1.42 ± 0.09 for l and 0.39 ± 0.05 for m, respectively. The activation enthalpies are ∆H ≠ = 12.3 to 14.5 kcal·mol -1 and the activation entropies are -28.2 to -35.5 cal·mol -1 ·K -1 , consistent with the proposed bimolecular reaction mechanism. The kinetic solvent isotope effect of 2.14 in MeOH/MeOD is in accord with a bimolecular mechanism, probably assisted by general-base catalysis.

The solvolysis rate constants of allyl chloroformate (CH2=CHCH2OCOCl, , which is also consistent with the proposed bimolecular reaction mechanism. The solvent kinetic isotope effect (SKIE, kMeOH/kMeOD) of 2.16 was also in accord with the SN2 mechanism. The values of product selectivity (S) for the solvolyses of 3 in alcohol/water mixtures was 1.3 to 3.9, which is also consistent with the proposed bimolecular reaction mechanism.3

The solvolysis rate constants of 2,2,2-trichloroethyl chloroformate (Cl 3 CCH 2 , which is consistent with the proposed bimolecular reaction mechanism. The kinetic solvent isotope effect (k MeOH /k MeOD ) of 2.39 is also in accord with S N 2 mechanism probably assisted by general-base catalysis.

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The solvolysis rate constants of 5-dimethylamino-naphthalene-1-sulfonyl chloride ((CH 3 ) 2 NC 10 H 6 SO 2 Cl, 1) in 31 different solvents are well correlated with the extended Grunwald-Winstein equation, using the N T solvent nucleophilicity scale and Y Cl solvent ionizing scale with sensitivity values of 0.96 ± 0.09 and 0.53 ± 0.03 for l and m, respectively; the correlation coefficient value was 0.955. These l and m values can be considered to support an S N 2 reaction pathway having a transition state (TS) structure similar to that of the benzenesulfonyl chloride reaction. This interpretation is further supported by the activation parameters, i.e., relatively small positive ΔH ≠ (12.0 to 15.9 kcal·mol) and large negative ΔS ≠ (−23.1 to −36.3 cal·mol) values, and the solvent kinetic isotope effects (SKIEs, 1.34 to 1.88). Also, the selectivity values (S = 1.2 to 2.9) obtained in binary solvents are consistent with the proposed mechanism.

This report shows the rates of solvolyses for phenyl methanesulfonyl chloride (C 6 H 5 CH 2 SO 2 Cl, I) in ethanol, methanol, and aqueous binary mixtures incorporating ethanol, methanol, acetone, 2,2,2-trifluroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) are reported. Three representative solvents, studies were made at several temperatures and activation parameters were determined. The thirty kinds of solvents gave a reasonably precise extended Grunwald-Winstein plot, coefficient (R) of 0.954. The sensitivity values (l = 0.61 and m = 0.34, l/m = 1.8) of phenyl methanesulfonyl chloride (I) were smaller than those obtained for benzenesulfonyl chloride (C 6 H 5 SO 2 Cl, II; l = 1.01 and m = 0.61) and 2-propanesulfonyl chloride ((CH 3 ) 2 CHSO 2 Cl, III; l = 1.28 and m = 0.64). As with the two previously studied solvolyses, an S N 2 pathway with somewhat ionization reaction is proposed for the solvolyses of I. The activation parameters, ΔH ≠ and ΔS ≠ , were determined and they are also in line with values expected for a bimolecular reaction mechanism. The kinetic solvent isotope effect of 2.34 in CH 3 OH/CH 3 OD is in accord with a bimolecular mechanism, probably assisted by general-base catalysis.

Kinetic and product studies of the solvolyses of acyclic phosphorochloridates are extended to two cyclic diesters, 2-chloro-1,3,2-dioxaphospholane-2-oxide (1) and 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane-2-oxide (2). Slightly faster solvolyses are observed for 1 than for the acyclic dimethyl phosphorochloridate (3), and 2 solvolyzes somewhat slower than 3. An extended Grunwald-Winstein equation treatment shows similar sensitivities to changes in solvent nucleophilicity and solvent ionizing power for 1, 2, and 3, and a concerted S N 2 attack is proposed in each case. Product studies for the solvolyses of 2 in aqueous alcohols are presented. O O P O Cl MeO MeO P O Cl O P O Me Me O Cl (1) ( 2)