of a number of aryl hydroxybenzene-p-sulfonates revealed that
a dissociative mechanism involving the unprecedented quin-
onoid sulfene species 5 shown in Scheme 3 takes place, showing
hydrolysis follows a S 2 mechanism, rather than with the 4-
N
hydroxybenzenesulfonate, which has been shown to react via an
2a,b
E1cB pathway. In other words, according to the well known
reaction sensitivities for sulfonyl transfer reactions and sub-
stituent constants for the p-amino and p-acetylamino groups,
electron releasing substituents in the acyl moiety such as the
4
4
-amino group (either neutral or ionised) and the more acidic
-acetylamino group are expected to increase reaction rates
significantly if a dissociative route is followed. In contrast, a
decelerating effect should be observed if an associative mechan-
ism was occurring, as found in the present case. Also the
observation of a negative, high value of the activation entropy
is fully consistent with this hypothesis. Indeed, in sulfonyl
transfer reactions, strongly negative activation entropy values
are typical of associative mechanisms. In contrast, positive or,
sometimes, low negative values are commonly found when
Scheme 3
that the phenoxide ion group is extremely efficient as the
2
internal nucleophilic centre.
However, it is also conceivable that other lone pair donor HZ
groups may behave as internal nucleophilic centres, either in
their neutral or ionised form, giving rise, respectively, to dis-
1
dissociative mechanisms are followed.
Aiming at enforcing the system under study to react through
dissociative pathways, we directed our interest to the more
reactive sulfonyl fluorides and chlorides. It is well known,
indeed, that the dissociative mechanisms are strongly favoured
over the associative ones as the nucleofugality of the leaving
sociative mechanisms of the S 1 type (by virtue of intermediate
N
stabilization by charge delocalisation) or of the E1cB type (on
account of intermediate stabilisation by charge neutralization).
In order to better understand the rôle played by the nature of
the internal nucleophile on the E1cB mechanism of the arene-
sulfonyl group transfer, we have carried out an investigation
into the alkaline hydrolysis of 2,4-dinitrophenyl esters as well as
halides (i.e., chlorides, fluorides) of sulfanilic and N-acetyl-
sulfanilic acid. Our aim was to check if, in alkaline solution,
p-amino and p-acetylamino groups could divert the arene-
sulfonyl group transfer mechanism from the usually occurring
SN2 to a dissociative one. In that case, the sulfoquinone imines
5
group increases. Rate constants for the alkaline hydrolysis of
chlorides and fluorides of sulfanilic and N-acetylsulfanilic acid
are shown in Table 2, as well as ratios between them. For the
sake of comparison, data are also shown for toluene-p-sulfonyl
and phenylmethanesulfonyl chlorides and fluorides, which are
models for the associative and dissociative mechanisms respect-
ively. Since, as stated before, sensitivity to leaving group basicity
is much higher for the dissociative than for the associative
mechanisms, rate ratios are expected to be significantly higher
for the former than for the latter ones: these ratios, therefore,
may represent a useful mechanistic probe.
6
and 7 could be involved as reaction intermediates.
As regards p-acetylaminosulfanilyl halides, the chloride to
fluoride reactivity ratio is virtually the same (3.4) as that (3.7)
found for the tosyl derivatives (which hydrolyse through the
associative, S 2(S) mechanism) and it is much lower than the
N
one (45) related to the two phenylmethanesulfonyl halides,
which have been suggested to react through a dissociative
6
mechanism, most probably an ‘asynchronous’ E2. In the case
of the two sulfanilyl halides, the reactivity ratio is 700, and this
value is even higher than the one found for the corresponding
two phenylmethanesulfonic acid derivatives. Clearly, this out-
come is not consistent with sulfanilyl chloride reacting through
Actually, reports on these species can be found in the liter-
ature. It was claimed, indeed, that the yellow colour arising
from a strong absorption band with a maximum at 426 nm,
which quickly developed on dissolving sulfanilyl chloride in
pyridine, was due to accumulation of 6 in solution. This species
was described as ‘stable’, since it seemed to disappear quite
the S 2(S) mechanism. Comparison of the rate constants
N
within each group of arenesulfonyl halides (i.e., chlorides and
fluorides) further substantiates this conclusion. The reactivity
3
slowly to give poly(p-benzenesulfonamide).
order found for fluorides (p-AcNH > p-Me > p-NH ) is fully
2
A subsequent, careful reinvestigation of this reaction, how-
consistent with the occurrence of the associative, S 2(S) mech-
N
4
ever, has shown that this claim was in error. However, since,
7
anism (for which the Hammett ρ constant is positive). The
sometimes, the rôle played by solvent polarity may be crucial in
governing the choice of the mechanism of the sulfonyl transfer
process, the possibility that in aqueous alkaline solution the
same is not found in the chloride series, where the reactivity
order is p-NH2 > p-AcNH > p-Me. Once again, this fact
suggests that, unlike sulfanilyl fluoride, sulfanilyl chloride
decomposes in alkaline solution through a pathway different
from that followed by the other two members of the same
group.
S 1–E1cB mechanism, rather the commonly observed S 2(S)
N
N
one, might take place, is still open.
In order to gain a better understanding of this point, we
resorted to competition experiments, in which sulfanilyl and
Results and discussion
The alkaline hydrolyses of the 2,4-dinitrophenyl esters of sulf-
anilic and N-acetylsulfanilic acid were first investigated. Excel-
lent pseudo-first order kinetics were followed over at least 90%
of the entire reaction in the pH range employed. Second-order
rate constants (which represent reactivity towards hydroxide
ions) and Arrhenius parameters are shown in Table 1. For the
sake of comparison, data taken from the literature for two
model substrates, i.e. the 2,4-dinitrophenyl esters of benzene-
sulfonic acid and 3,5-dimethyl-4-hydroxybenzenesulfonic acid
are also shown. Comparison of reactivity and activation
entropy data reveals that there is strict similarity between the
first two substrates and the benzenesulfonate, whose alkaline
N-acetylsulfanilyl chlorides were allowed to react with excess,
i
equimolar amounts of isopropylamine (Pr NH ) and tert-
2
t
butylamine (Bu NH ). Our idea was that, owing to their differ-
2
ent steric requirements, the associative, S 2(S) mechanism
N
would involve larger discrimination between the two amines
than the dissociative one. Indeed, steric crowding at the nucleo-
philic centre would be hardly felt when amine attacks the highly
reactive, planar sulfene-type intermediate as compared to the
much less reactive, more hindered tetrahedral sulfonyl chloride.
To find support for this idea we have carried out this
experiment also on toluene-p-sulfonyl and phenylmethane-
sulfonyl chloride, which, as already stated before, react via the
6
12
J. Chem. Soc., Perkin Trans. 2, 2002, 611–614