Fries rearrangement in ionic melts

Article abstract of DOI:10.1016/S0040-4039(01)00029-6

1-Butyl-3-methylimidazolium chloroaluminate, [BMIm]⁺Al₂Cl₇^-, was used as a solvent as well as a Lewis acid catalyst in Fries rearrangement reactions of phenyl benzoates. The rate of consumption of phenyl benzoate obeyed first-order kinetics. Good yields and high selectivity are the features observed in this unconventional but interesting aprotic solvent.

Full text of DOI:10.1016/S0040-4039(01)00029-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1979–1981
Fries rearrangement in ionic melts
Jitendra R. Harjani, Susheel J. Nara and Manikrao M. Salunkhe*
Department of Chemistry, The Institute of Science, 15 Madam Cama Road, Mumbai 400 032, India
Received 16 December 2000; accepted 5 January 2001
Abstract—1-Butyl-3-methylimidazolium chloroaluminate, [BMIm]+Al2Cl7−, was used as a solvent as well as a Lewis acid catalyst
in Fries rearrangement reactions of phenyl benzoates. The rate of consumption of phenyl benzoate obeyed first-order kinetics.
Good yields and high selectivity are the features observed in this unconventional but interesting aprotic solvent. © 2001 Elsevier
Science Ltd. All rights reserved.
Ionic melts were first prepared by US scientists Frank
Hurley and Tom Weir in the late 1940s. Initial phases
of research on room temperature ionic melts centred on
their potential applications in the field of electrochem-
istry.1 However, recently scientists have started looking
upon them as ideal solvents blessed with the ability to
solvate a wide variety of organic substrates, defying the
rule of thumb, ‘like dissolves like’. Apart from the fact
that they can solvate organic, organometallic and inor-
ganic compounds, their negligible vapour pressure, ease
of handling and potential for recycling, circumvent
many of the problems associated with volatile, molecu-
lar organic solvents. Their properties can be altered by
the fine-tuning of other parameters, such as the choice
of organic cation, inorganic anion and alkyl chain
attached to the organic cation. These properties have
been exploited in reactions such as hydrogenation,2
alkylation,3 Diels–Alder reactions,4 and many other
reactions that have been reviewed.5
which can be brought about by changing the molar
ratio of the two components,6 viz. the organic halide
and AlCl3. In chloroaluminate ionic melts, the Lewis
acid species present in AlCl3-rich compositions is well
−
established and known to be Al2Cl7 . Composition of
the ionic liquids is expressed as the apparent AlCl3
mole fraction, N. Accordingly, they are classified as
basic, neutral and acidic melts where N is 0–0.5, 0.5 and
0.5–0.67, respectively. The Fries rearrangement reaction
of phenyl benzoate was carried out in N=0.33, 0.5 and
0.67 melts. Positive results were obtained only in the
case of an acidic ionic melt as expected. This rearrange-
ment reaction is known to be catalysed by Lewis acids;
thus, in our attempt to envisage its effect on the extent
of conversion, we carried out a series of experiments on
phenyl benzoate (Scheme 1) at 120°C for 2 h by varying
the melt composition in terms of AlCl3, i.e. N over the
range 0.56–0.67.
Halogeno and alkylhalogenoaluminate(III) ionic melts
fascinated us the most owing to their remarkable Lewis
acidity, which prompted us to try Fries rearrangements
in such a system. Lewis acidity of such ionic melts is a
function of the mole fraction of AlCl3 present. We
herein report for the first time Fries rearrangement
reactions in 1-butyl-3-methylimidazolium chloroalumi-
nate, [BMIm]+Al2Cl7 . Substantial rate enhancement,
−
good yields and high selectivity by control of tempera-
ture are the features observed in this system. One of the
interesting properties possessed by this system is the
wide range of Lewis acidity attainable, variation of
Scheme 1. Fries rearrangement of phenyl benzoate in
[BMIm]Cl·xAlCl3.
* Corresponding author. Tel./fax: (91-22) 2816750; e-mail:
mmsalunkhe@hotmail.com
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00029-6

1980
J. R. Harjani et al. / Tetrahedron Letters 42 (2001) 1979–1981
ment did not decisively prove intermolecularity since
the possibility of transesterification or acyl/aroyl inter-
change could not be ruled out. Thus, phenyl benzoate
was subjected to the rearrangement reaction in the
presence of anisole at 120°C for 2 h, which gave
p-methoxybenzophenone as the major product. This
provided evidence for intermolecularity of the rear-
rangement reaction. Earlier endeavours to elucidate the
mechanism of the Fries reaction led to three different
mechanisms,8 one of which emphasised the dissociation
of the phenyl ester to furnish phenolate ions and an
acid chloride which subsequently, via acylation, gave
products. We carried out experiments on phenyl
pivalate at 120°C; the reaction mixture gave off copious
amounts of carbon monoxide. This indicated dissocia-
tion of the substrate to the oxocarbonium ion, Me3C-
CO+, and phenolate ion, out of which the former
underwent subsequent decomposition to the more sta-
ble tert-carbocation, +CMe3. However, in this case,
trace amounts of a ketonic material were formed; there-
fore, to increase its yield by suppressing the decomposi-
tion of the acyl cation, the reaction was carried out at
low temperature. The reaction at low temperature gave
marginal yields; the other products obtained were phe-
nol and the unreacted ester. The ketone that was
obtained was characterised as the o-hydroxyketone,
which suggested a probable intramolecular rearrange-
ment via a polarised co-ordination complex of the ester
and chloroaluminate species of the ionic melt.
Figure 1. First-order plots of the Fries rearrangement of
phenyl benzoate in [BMIm]+Al2Cl7−; (") reaction at 80°C;
(ꢀ) reaction at 100°C. Conversion was monitored by GC.
Our studies revealed that the extent of conversion is a
function of Lewis acidity of the ionic melt. This demon-
strated the increase in catalytic activity of the melt due
to the consequent increase in the concentration of the
catalytic species Al2Cl7 as calculated.7 On carefully
−
monitoring the o/p ratio while the Lewis acidity was
varied, it was observed that, at low Lewis acidity, more
of the o-hydroxy benzophenone was formed but in
melts of high Lewis acidity the proportion of p-hydroxy
benzophenone increased as shown in Scheme 1.
The melt corresponding to N=0.67 gave maximum
conversion (97%) and hence was utilised for further
studies. The molar ratio of the phenyl benzoate to
The reaction was studied on various substrates in
[BMIm]+Al2Cl7 was optimised and was found to be
−
[BMIm]+Al2Cl7 melt9 at 120°C for 2 h. Good yields of
−
1:1.2, beyond which the increase in molar ratio resulted
in no significant rise in the percentage conversion.
the rearranged products were seen in the case of sub-
strates bearing electron-donating groups, whereas in
substrates bearing electron-withdrawing groups consid-
erable debenzoylation was observed. The results are
summarised in Table 1.
In order to investigate the rate of reaction in [BMIm]+
−
Al2Cl7 , we carried out a series of experiments with
constant amounts of phenyl benzoate and [BMIm]+
−
In conclusion, [BMIm]+Al2Cl7 proved to be a useful
−
Al2Cl7 , as described in the experimental procedure (see
References), at different temperatures. Results revealed
that the rate of consumption of phenyl benzoate obeyed
first-order kinetics (Fig. 1).
medium for Fries rearrangements, eliminating the use
of solvent by playing a dual role of solvent as well as
Lewis acid catalyst. The substrates show significant
increase in reactivity, minimising the reaction times and
improving the yields substantially. The experimental
procedure10 is quite simple, avoiding tedious procedures
Surprising results were observed when the effect of
temperature on the o/p ratio of the rearranged product
of phenyl benzoate at 80°C was studied. After 1 h of
the reaction, 66% of the total product (26%) formed
was found to be ortho, i.e. the thermodynamically-
favoured product at low temperature. Higher regiose-
lectivities were observed in this system at different
temperatures. At 120°C, 81% p-product was obtained
after 2 h whereas at 180°C, 85% o-product was
obtained after 4 h. Apart from this, the variation of the
o/p ratio with temperature and time is illustrated in Fig.
2.
In an attempt to contemplate the mechanistic details of
the Fries rearrangement reaction in [BMIm]+Al2Cl7 ,
−
several experiments were carried out. Crossover experi-
ments of phenyl benzoate and p-methylphenyl acetate
were carried out at 120°C for 2 h, which resulted in
typical products as expected from an intermolecular
rearrangement reaction. However, the above experi-
Figure 2. Fries rearrangement of phenyl benzoate in [BMIm]+
Al2Cl7−: variation of o/p ratio with temperature and time; (")
reaction at 140°C; (ꢀ) reaction at 160°C; (ꢁ) reaction at
180°C.

J. R. Harjani et al. / Tetrahedron Letters 42 (2001) 1979–1981
1981
−
Table 1. Fries rearrangement of phenyl benzoates in [BMIm]+Al2Cl7
and cumbersome work-up11 involving the removal of
solvents like nitrobenzene. Further investigations con-
cerning the mechanism of this reaction in the melt are
currently in progress.
1964; Vol. III, p. 502.
9. The melt was prepared as previously described: (a)
Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L.
Inorg. Chem. 1982, 21, 1263; (b) Abdul-Sada, A. K.;
Greenway, A. M.; Seddon, K. R.; Welton, T. Org. Mass
Spectrom. 1993, 28, 759.
References
10. Experimental procedure: To the weighed quantity of
[BMIm]+Al2Cl−7 melt (6 mmol), the aryl benzoate (5
mmol) was added and the mixture was stirred at different
temperatures. The same quantities were used in the
kinetic experiments. All additions were carried out in
an inert atmosphere glove box. The reactions were
quenched by adding 6 M HCl. The neutralised reaction
mixture was then extracted with diethyl ether (3×5 mL).
The combined organic layers were dried using sodium
sulphate and evaporated under reduced pressure. The
products, obtained after extraction, were then analysed
by GC/MS, chromatographed using a silica gel column
and further characterised by IR, NMR and physical
constants.
1. Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L.
Inorg. Chem. 1982, 21, 1263.
2. Dyson, P. J.; Ellis, D. J.; Parker, D. G.; Welton, T.
Chem. Commun. 1999, 25.
3. Earle, M. J.; McCormac, P. B.; Seddon, K. R. Chem.
Commun. 1998, 2245.
4. Fisher, T.; Sethi, A.; Welton, T.; Woolf, J. Tetrahedron
Lett. 1999, 40, 793.
5. Review: Welton, T. Chem. Rev. 1999, 99, 2071.
6. Hussey, C. L. Pure Appl. Chem. 1988, 60, 1763.
7. Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. J.
Org. Chem. 1986, 51, 480.
8. Friedel–Crafts and Related Reactions; Olah, G. A., Ed.
11. Baltzly, R.; Ide, W. S.; Phillips, A. P. J. Am. Chem. Soc.
1955, 77, 2522.
Acylation and related reactions; Interscience: New York,
.
.