176
REACTIVITY OF NUCLEOPHILES TOWARDS CYCLIC SULFAMATES
Scheme 2
effect of changes in geometry on pKa .
ion or imidazole concentrations were in large excess over
the cyclic sulfamate concentrations. High-pressure kinetic
measurements were made using a laboratory-built high-
pressure stopped-flow spectrophotometer.9 For reactions in
moderate concentrations of organic solvent, Kel-F drive
syringes could be used, but these were substituted by glass
syringes at the highest concentration of acetonitrile. The
temperature was controlled and the primary data were
acquired and processed by methods identical with those for
the atmospheric pressure kinetics. Plots of the logarithm of
the rate constants (kobs ) versus pressure were obtained, and
⌬V* was calculated using a standard method. The plots, an
example of which is shown in Figure 1, showed no evidence
of curvature and therefore the values of ⌬V* cited refer to a
pressure of 0·10 MPa.
The reason for chemoselectivity (exocyclic versus endo-
cyclic nucleophilic attack) in cyclic sulfamates 1a–g is still
far from understood. Therefore, to investigate these systems
further, kinetic studies of the reaction of hydroxide ion with
cyclic sulfamates 1c (5-chloro), 1f (5-bromo), 1g (5-nitro)
and 1h (6-nitro) have been carried out at atmospheric and
elevated pressures and, in some cases, in different solvent
compositions. The results are described and analyzed. This
represents a more thorough and comprehensive study than
that in which some preliminary findings were reported.5
In addition, the kinetics of reaction of imidazole with the
5-nitro (1g) and the 6-nitro compounds (1h), again at both
atmospheric and elevated pressures, have been investigated
and are reported. The rate laws were obtained and are
simple, allowing a mechanistic interpretation of the derived
activation volumes, ⌬V*. The value of this parameter as a
diagnostic indicator of mechanism in both inorganic and
organic reactions has been realized.7, 8
RESULTS
The order of reaction for hydroxide ion reacting with cyclic
sulfamates has previously been established as second
overall,5 first order in each of the cyclic sulfamate and
hydroxide ion concentrations. The choice of solvent compo-
sition was dictated by solubility considerations. Cyclic
sulfamates are virtually insoluble in water, as is the sodium
hydroxide, at the concentrations used, in solvent mixtures
containing a high percentage of acetonitrile. Earlier work
has demonstrated the viability of application of the high-
pressure kinetic technique to this type of reaction. Rate
constants for reaction of hydroxide with 1g and 1h at
different pressures in different solvent mixtures are listed in
Table 1. The reactivity of hydroxide ion toward the halogen
derivatives 1c and 1f was also examined at different
pressures in a 50% acetonitrile–water mixture (Table 2). A
typical example of a plot of ln kobs versus pressure used to
calculate the activation volumes is presented in Figure 1.
In a solution of excess imidazole, 1g and 1h undergo
rapid reactions requiring the stopped-flow method for
kinetic measurements. A compilation of rate constants
obtained at atmospheric pressure is given in Table 3.
EXPERIMENTAL
Materials. The cyclic sulfamates were those used in
previous studies, or were new samples having identical
characteristics and properties as described earlier.5 Solutions
of sodium hydroxide were made from standard high-
concentration vials (Merck, Titrisol) of NaOH and Milli-Q
water. Imidazole was obtained from Sigma. Acetonitrile and
ethanol were of analytical reagent grade. Concentrations of
mixed aqueous solvents used in the kinetic studies, prepared
with Milli-Q water and the appropriate solvent, are
expressed in volume percent before mixing.
Methods. The UV–visible spectra of products of the
reactions were obtained using a Shimadzu Model 250
spectrophotometer. The reactions are too rapid at room
temperature for conventional time range instruments, and
were studied kinetically using a laboratory-built stopped-
flow
spectrophotometer
or
a
Dionex
D-110
spectrophotometer, at the wavelengths characterizing the
product(s) absorption given in the tables. Temperature
control was maintained to within ±0·1 °C by a thermostated
circulating fluid. Data were acquired using an Apple IIe
computer and processed by using kinetics software. All
reactions were first order (for at least three half-lives), under
the conditions of the experiments in which the hydroxide
On plotting kobs versus the imidazole concentration, a
straight line, which passes through the origin within
experimental error, is obtained for both nitro isomers
(Figure 2), indicating that the reaction is first order in both
imidazole concentration and cyclic sulfamate concentration.
Since there is no intercept, a reverse reaction or a parallel
solvolysis reaction are non-existent or negligible.
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 175–181 (1997)