9582 J. Am. Chem. Soc., Vol. 120, No. 37, 1998
Blans and Vigroux
the presence of 16% of acetonitrile33 (see Experimental Section).
However, it means that there is a significant charge buildup on
the phenolic oxygen in going from the initial to the transition
state consistent with S-O bond cleavage.34 The “effective
charge”35 on the phenolic oxygen of the reactant ester 1- is
unknown, but its value is expected to be close to that observed
on the phenolic oxygen of related esters such as, e.g., monoan-
ions of aryl phosphate (+0.7436) and aryl sulfate (+0.737)
monoesters.38 Assuming that the “effective charge” is +0.7 in
1- and, therefore, > +0.7 in 1, then the âlg value of -0.54
implies that there must be +0.16 or >+0.16 unit of “effective
charge” on the aryl oxygen at the transition state depending on
whether the reactant species is 1- or 1, respectively. Such
positive values are consistent with the fact that the rate constants
kSO2 are well correlated with σ constants.
(6) There seems to be good evidence that the leaving group
expulsion from either 1- or 1 is assisted by proton transfer as
shown in II, III, or IV. The overall change of the “effective
charge” (ec) on the aryl oxygen in going from the reactant
species 1-/1 (ec g ca. +0.7 see above) to the final species ArOH
(ec ) 0) or ArO- (ec ) -1) is ca. -0.7 or -1.7, respectively.
A âlg of -0.54 associated to an overall charge change of -1.7
would indicate that the “effective charge” on the aryl oxygen
at the transition state is some 32% of the difference between
that in ground (compound 1) and product (ArO-) states. In
comparison, the âlg value for ArO- expulsion from 1- is -1.39
(vide infra) indicating that the S-O bond fission is well
advanced (82%) in the transition state. The fact that the
transition state for the expulsion of ArO- from 1 would be some
61% earlier than that from 1- is not consistent with expecta-
tion: 1- has more driving force than 1 to expel ArO- so that
the transition state for ArO- expulsion should be earlier for
1-. This “contradiction” with expectation can be resolved if
there is protonation of the leaving group as in mechanisms II,
III, or IV. In that case, the âlg value of -0.54 is associated to
an overall “effective charge” change of ca. -0.7 consistent with
a late transition state.
Figure 4. Plot of the log of kp, the spontaneous hydrolysis reaction
for aryl N-(methoxycarbonyl) sulfamates 1a-h at 50 °C (µ ) 1.0 M
with KCl), against pKa of the corresponding leaving group ArOH. The
linear regression equation obtained with compounds 1a-f is log kp )
(-1.39 ( 0.1)pKArOH + (2.43 ( 0.7), r ) 0.987. For compounds 1d-
h, values of kp were extrapolated at 50 °C from Eyring plots obtained
between 100 and 90 °C.
s-1 for leaving phenols of pKa < ca. 9.4 (at 50 °C),28 that is,
only a concerted mechanism is possible for compounds 1. Paths
a and b represent the two types of concerted general acid
catalysis that may be considered to “bypass” the zwitterionic
species in eq 9. As mentioned in an earlier section, unambigu-
ous distinction between inter- and intramolecular general acid
catalysis (i.e., between paths a and b, respectively) will depend
on the observation or not of buffer acid catalysis. While the
observation of buffer catalysis should readily resolve the kinetic
ambiguity in favor of path a, the failure to observe any buffer
effect, as in the case of compounds 1, should maintain the kinetic
ambiguity between the two paths.
“Neutral” Hydrolysis. In contrast to the acidic hydrolysis
reaction ka, the pH-independent “neutral” hydrolysis kp (corre-
sponding to the plateau regions around pH 7 in Figure 1) takes
place exclusively via S-O bond fission (see Results). The
hydrolysis rates for the anions of N-(methoxycarbonyl)sulfamate
esters 1a-f depend very strongly on the basicity of the leaving
group, the slope of log kp against pKlg being -1.39 (Figure 4).
This figure is consistent with a transition state in which the
Intermolecular (Path a) or Intramolecular (Path b)
General Acid Catalysis? The concerted general acid catalysis
of path a or b (Scheme 2) for the reaction with S-O bond
cleavage appears to be enforced39,40 by the disappearance of
the barrier for leaving group expulsion from the zwitterionic
species in eq 9. By extrapolating the Bro¨nsted line shown in
breaking of the bond to the leaving group is well advanced.
2- 41
This is typical of many phosphoryl (-PO3
)
and sulfuryl
- 42
(-SO3
)
group transfer reactions including hydrolysis. The
low Bro¨nsted exponent for attack of sodium azide and substi-
tuted pyridines on the sulfur center of 1a (ânuc ) 0.14, Figure
S2 in the Supporting Information) indicates weak interaction
with nucleophile. Thus, although the second-order kinetics
demands that the nucleophiles take part in the rate-limiting step,
rate constants are virtually independent of the basicity of the
nucleophiles, from pyridines with pKa of 1.45 to pyridines with
pKa of 9.20 (Tables S1 and S2 in the Supporting Information).
Again, this behavior is typical of phosphoryl and sulfuryl transfer
reactions.41,42 The large value of âlg associated with the small
Figure 4 (vide infra), in the same way as indicated in footnote
26, we find that the rate constant kz (eq 9) is greater than 1013
(33) For the dissociation of benzoic acids, a maximum increase of 141%
is observed for F at 25 °C in going from 100% water to 100% acetonitrile.
See: Kolthoff, I. M.; Chantooni, M. K. J. Am. Chem. Soc. 1971, 93, 3843.
(34) The exact charge on the aryl oxygen is not known and not directly
comparable to pure water, but the differences in conditions are small and
do not invalidate the conclusion that there is significant S-O bond cleavage
at the transition state.
ânuc coefficient is consistent with an “exploded” transition state
(35) Williams, A. AdV. Phys. Org. Chem. 1991, 27, 1.
for substitution that may occur by a concerted or stepwise
preassociation mechanism.43 Although there is evidence for free
intermediates of type O2SdNR when R ) H and Me,14b,24 we
failed in the present case to demonstrate that the putative
(36) Bourne, N.; Williams, A. J. Org. Chem. 1984, 49, 1200.
(37) Hopkins, A. R.; Day, R. A.; Williams, A. J. Am. Chem. Soc. 1983,
105, 6062.
(38) The ArO- species is defined as possessing 1 unit of negative charge
on its oxygen, in comparison with zero in the neutral phenol (ArOH). Thus,
the “effective charges” of +0.74 and +0.7 on the phenolic oxygen of
phosphate and sulfate monoesters, respectively, mean that -PO3H- and
-SO3- groups are effectively more electron withdrawing than the hydrogen
(proton) when covalently linked to an aryl oxygen.
(41) Cox, J. R.; Ramsay, O. B. Chem. ReV. 1964, 64, 317 and references
therein. Benkovic, S. J.; Schray, K. J. In The Enzymes; 3rd ed.; Boyer, P.
D., Ed.; Academic Press: New York, 1973; Vol. 8, pp 201-238 and
references therein.
(39) Jencks, W. P. Acc. Chem. Res. 1976, 9, 425.
(40) Jencks, W. P. Acc. Chem. Res. 1980, 13, 161.
(42) Williams, A.; Douglas, K. T. Chem. ReV. 1975, 75, 627 and
references therein.