Sulfonylation in Chloroaluminate Ionic Liquids
J . Org. Chem., Vol. 66, No. 25, 2001 8619
reaction went almost to completion may be due to
differential basicities of TsCl and sulfone. To check
whether the equilibrium of the reaction 4 can be driven
to the left in the presence of excess of substrate and TsCl,
the reaction was carried out where the molar proportion
of the ionic liquid was adjusted so that it provided less
[Al2Cl7]- than the amount of substrate. A reaction that
contained 1,3,5-trimethylbenzene, TsCl, and ionic liquid
in a molar proportion of 1.1:1.0:0.5, respectively, afforded
incomplete conversion to corresponding sulfone. Interest-
ingly, the percentage conversion in this reaction was
found to be slightly greater than the [Al2Cl7]- would
permit if consumed but at a slow rate. The reaction
carried out by adding 1,3,5-trimethylbenzene (1.2 equiv)
to the equimolar mixture of TsCl and [bmim]Cl-AlCl3, N
) 0.67 maintained at reduced pressure of 0.01 atm,
showed a similar spectrum as Figure 3. This shows that
it is practically difficult to drive HCl out of the reaction
mixture before it reacts with [Al2Cl7]-, even under
reduced pressure. HCl formed in the reaction mixture
has a high tendency to react with [Al2Cl7]-, and moreover
the medium is considerably viscous to allow the easy
removal of HCl. The interactions of the product sulfone
with [AlCl4]- and [Al2Cl7]- ions are inevitable. The
oxidizing and the protic impurities known to be present
in such ionic liquids may further complicate the pro-
cesses, rendering them difficult to explain. Substantial
experimental evidences of physicochemical nature may
prove fruitful in this regard.
regenerated. Later, after the regeneration of [Al2Cl7]-
species, the interaction of [Al2Cl7]- with HCl in the
presence of the acylated product and the electrophilic
substitution can become competitive. Probably because
of these reasons, the reaction is observed to go beyond
the extent of what is expected from it if it consumes
equimolar amount of [Al2Cl7]- species. AlCl3 is known to
form a complex with the carbonyl group of the acylated
product in the conventional Friedel-Crafts acylations.
This complex formation could render the complete re-
combination of AlCl3 with [AlCl4]- difficult, to give
[Al2Cl7].- However, the 27Al NMR of the mixture of
[bmim]Cl-AlCl3, N ) 0.67, acetyl chloride, and 1,4-
dimethylbenzene in molar proportion of 1.0:1.0:1.1, re-
spectively, shows only one peak at δ 79.3 ppm corre-
sponding to [AlCl4]- ion, which reflects that no other Al-
containing species is present in significant concentration
to be detected by NMR. Thus it seems that the equilib-
rium constant of the reaction 6 is high enough to allow
the complex formation. No such complex formation with
AlCl3 + [AlCl4]- h [Al2Cl7]-
(6)
the formed sulfone even in case of the sulfonylation was
observed in NMR (Figure 3). The complex of 1,3,5-
trimethyl-2-[(4-methylphenyl) sulfonyl]-benzene with AlCl3
in a control experiment in NMR showed a peak at 70.9
ppm.
On the basis of the conclusions drawn from the above
experiments the plausible mechanisms of sulfonylation
in [bmim]Cl-AlCl3, N ) 0.67 liquid are as follows:
Friedel-Crafts alkylation21 and acylation22,23 have been
investigated in such ionic liquids. The reaction between
acetyl chloride and the acidic ionic liquid was followed
1
by H NMR by Wilkes et al.22 The results suggested a
TsCl + [bmim][Al2Cl7] h
stoichiometric reaction between CH3-COCl and [Al2Cl7]-
[Ts][AlCl4] + [bmim][AlCl4]
ion. (reaction 5). Indeed, it is possible to isolate the solid,
[Ts][AlCl4] + Ar-H h Ar-Ts + HCl + AlCl3
AlCl3 + [bmim][AlCl4] h [bmim][Al2Cl7]
CH3-COCl + [bmim][Al2Cl7] h
[CH3-CO][AlCl4] + [bmim][AlCl4] (5)
which is suspected to be [CH3-CO] [AlCl4], from the ionic
liquid. We however believe that, after the electrophilic
substitution with the acyl cation, even if [Al2Cl7]- species
is regenerated, it can no longer be completely available
for further reaction as a result of its interaction with
liberated HCl in the presence of acetylated product.
Preliminary investigations employing 27Al NMR spec-
troscopy has confirmed this belief. The acylation using
acetyl chloride and 1,4-dimethylbenzene in [bmim]Cl-
AlCl3, N ) 0.67 liquid was followed by 27Al NMR
spectroscopy, which after 10 min of reaction showed no
detectable amount of [Al2Cl7]- species. The control ex-
periment of the acylated product in [bmim]Cl-AlCl3, N
) 0.67 liquid after treatment with dry HCl as in case of
sulfonylation showed the predominance of [AlCl4]- spe-
cies. Had this not been the case, the ionic liquids would
have revolutionized the field of Friedel-Crafts chemistry
and several other synthetically important reactions
driven by the Lewis acid catalysts. In the initial phases
of the reaction, some amount of HCl formed escapes into
the atmosphere even before the [Al2Cl7]- species is
Ar-Ts + HCl + [bmim][Al2Cl7] h
[Ar-TsH]+ + [bmim]+ + 2[AlCl4]-
and
TsCl + [bmim][Al Cl ] h TsCl‚‚‚‚[Al2Cl7]-[bmim]+
2
7
TsCl‚‚‚‚[Al2Cl7]-[bmim]+ + Ar-H h
Ar-Ts + HCl + [bmim][Al2Cl7]
Ar-Ts + HCl + [bmim][Al2Cl7] h
[Ar-TsH]+ + [bmim]+ + 2[AlCl4]-
To generalize the procedure, the sulfonylation reactions
were carried out on different substrates such as benzene
and substituted benzenes. The reactions worked well, and
the results are illustrated in Table 2. The optimization
of time, temperature, and yields was carried out for the
mole ratio 1.1:1.0:1.2 of substrate, TsCl, and ionic liquid,
respectively. The reactions were continued until the
quantitative conversions at room temperature or a little
higher, bearing in mind the ambient quality of the
procedure. However, the experimentation revealed that
the rates of the reactions in ionic liquid increased with
the corresponding increase in the molar proportion of the
(21) Koch, V. R.; Miller, L. L.; Osteryoung, R. A. J . Am. Chem. Soc.
1976, 98, 5277.
(22) Boon, J . A.; Levisky, J . A.; Pflug, J . L.; Wilkes, J . S. J . Org.
Chem. 1986, 51, 480.
(23) Surette, J . K. D.; Green, L.; Singer, R. D. Chem. Commun. 1996,
2753.