Montmorillonite KSF-catalysed regioselective trans-tert-butylation of tert-butylphenols

Article abstract of DOI:10.1016/S0040-4039(01)01317-X

ortho-Regioselective de-tert-butylation of poly-tert-butylphenols is performed by treatment with montmorillonite KSF as a cheap, green and reusable catalyst. The procedure can be utilised for converting an equimolecular mixture of 2,4-di-tert-butylphenol and phenol into the more precious 4-tert-butylphenol (77% yield, 93% selectivity).

Full text of DOI:10.1016/S0040-4039(01)01317-X

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TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6543–6545
Montmorillonite KSF-catalysed regioselective
trans-tert-butylation of tert-butylphenols
Franca Bigi, Maria Lina Conforti, Raimondo Maggi, Alessandro Mazzacani and Giovanni Sartori*
Dipartimento di Chimica Organica e Industriale dell’Universita`, Parco Area delle Scienze, 17/A, I-43100 Parma, Italy
Received 3 July 2001; revised 20 July 2001; accepted 21 July 2001
Abstract—ortho-Regioselective de-tert-butylation of poly-tert-butylphenols is performed by treatment with montmorillonite KSF
as a cheap, green and reusable catalyst. The procedure can be utilised for converting an equimolecular mixture of 2,4-di-tert-
butylphenol and phenol into the more precious 4-tert-butylphenol (77% yield, 93% selectivity). © 2001 Elsevier Science Ltd. All
rights reserved.
The tert-butyl group is frequently utilised as a posi-
tional protective group for the selective preparation of
functionalised aromatic compounds with particular
interest into the phenol derivatives.1 In fact, specific
experimental conditions have been found for the selec-
tive introduction and removal of this protective and/or
blocking group.2 In general removal of a tert-butyl
group from aromatic compounds requires the use of an
excess of AlCl3 or similar hard Lewis or protic acids.3
each) were separately heated in toluene (25 ml), utilised
as solvent and acceptor for the detached tert-butyl
group, at 110°C for 4 h in the presence of KSF clay (1.0
g).9
Products detected in the reaction mixtures were the
unreacted starting tert-butylphenol 1, the parent phenol
2 and tert-butyltoluenes.10 Production of isomeric and
poly-tert-butylphenols via an isomerisation–transalky-
lation reaction involving the starting tert-butylphenol
111 was completely prevented due to the use of a large
excess of toluene. Yield of phenol 2 produced versus
time is depicted in Fig. 2. The conversion of com-
The use of stoichiometric reactions and traditional syn-
thetic and purification procedures has been recognised
by R. A. Sheldon as the reason for the production of
impressively large amounts of waste in the manufacture
of fine chemicals and pharmaceuticals.4 Thus, cleaner
catalytic technologies are requested with particular
attention to the use of heterogeneous, easily available
and reusable catalysts.
OH
OH
OH
Our current interest in the exploitation of solid acids
for fine chemicals preparation,5 has led us to re-exam-
ine the tert-butyl transfer from tert-butylphenols pro-
moted by KSF montmorillonite clay.6 Kinetic studies
on the microwave-activated catalytic de-tert-butylation
of tert-butylphenols on KSF catalyst were recently
reported.7
1a
1b
1c
Figure 1. tert-Butylphenols utilised.
We decided to perform experiments without application
of microwaves since this technique causes a reduction in
the surface area of the solid catalyst and consequently
lowers its efficiency and prevents its reusability.8
In a first series of experiments ortho-, meta- and para-
tert-butylphenols 1a, 1b and 1c (Fig. 1) (1 mmol, 0.15 g
Figure 2. Phenol yield produced from ortho- ("), meta- (ꢀ)
and para-tert-butylphenol (ꢁ).
* Corresponding author. Tel.: +39 0521 905551; fax: +39 0521
905472; e-mail: sartori@unipr.it
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01317-X

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6544
F. Bigi et al. / Tetrahedron Letters 42 (2001) 6543–6545
pounds 1 in phenol 2 follows the order 1a (97%) >1c
(38%)ꢀ1b (traces) in agreement with results obtained
in AlCl3-promoted similar reactions;1,2,12 the selectivity
is always higher than 98%.
These results prompted us to investigate the regiose-
lective tert-butyl transfer from the ortho position of 3
to the para position of 2 to give 1c. This reaction
shows considerable practical interest since the two low
value compounds 3 and 2 can be simultaneously con-
verted into the more precious compound 1c. Com-
pounds 3 (2 mmol, 0.41 g) and 2 (2 mmol, 0.19 g)
were heated at 130°C for 24 h in the presence of KSF
clay (1 g) in different solvents.
The more rapid increase of phenol production from
1a compared with 1c may be explained by the influ-
ence of the OH group, which favours the ortho-de-
alkylation more than the para by the ‘proximity
effect’ as in the case of the preferential ortho-regiose-
lective tert-butylation of phenol substrates promoted
by an OH group.13
Results from Table 1 suggest that the selectivity
towards the formation of 1c was very high in all
cases. Moreover, the catalytic efficiency is slightly
influenced by the polarity parameter of the solvent
provided that solvents with ET(30)14 <40 are utilised
(entries 1–5). The low reactivity of the present system
in highly polar solvents could be attributed to the
preferential adsorption on the catalyst surface of sol-
vent molecules with respect to the phenolic reagents.
However, the best results were obtained by carrying
out the reaction without a solvent, which represents
more environmentally friendly conditions.
The reactivity of 2,4-di-tert-butylphenol 3 was succes-
sively examined with the aim of achieving selective
mono- or bis-de-tert-butylation reaction (Scheme 1).
Compound 3 (2.5 mmol, 0.52 g) was heated in tolu-
ene (25 ml) at 110°C under stirring in the presence of
KSF clay (1 g). Results of the conversion of 3 and
yields of 1c and 2 versus time are depicted in Fig. 3.
The de-tert-butylation is much faster at the ortho
position in agreement with similar reactions per-
formed under homogeneous catalysis. Consequently,
compound 1c could be produced in 75% yield and
96% selectivity after 14 h, whereas production of phe-
nol 2 represents a side reaction occurring for a longer
reaction time. Under harder conditions (namely 140°C
in xylenes for 4 h and with 1 g of KSF clay per
mmol of 3) phenol 2 is quantitatively produced by
complete bis-de-tert-butylation of 3.
Finally, we faced the problem of catalyst recycling in
the model reaction with ortho-tert-butylphenol 1a.
Montmorillonite KSF, recovered at the end of the
reaction by Bu¨chner filtration, washed with hot
methanol and dried under vacuum at rt, was reused
for at least two further cycles showing the following
yields (selectivities): 96 (97), 94 (98).
In conclusion, treatment of tert-butylphenols with the
cheap and easily available montmorillonite KSF
results in the mild, safe and selective de-tert-butyla-
tion of mono and di-tert-butylphenols. Moreover,
para-tert-butylphenol can be produced by selective
trans-alkylation from 2,4-di-tert-butylphenol to phe-
nol itself. Finally, the solid catalyst can be filtered off
and reused showing similar high yield and selectivity.
OH
Montmorillonite KSF
Toluene, 110 °C
OH
1c
OH
Montmorillonite KSF
3
Xylenes, 140 °C
Table 1. Solvent effect in transalkylation reaction involv-
ing 2,4-di-tert-butylphenol and phenol
2
OH
OH
OH
Scheme 1.
Montmorillonite KSF
+
130 °C, 24 h
3
2
1c
Entry
Solvent
ET(30)
1c Yielda [sel.b] (%)
1
2
3
4
5
6
Decaline
Chlorobenzene
Tetrachloroethylene
DMSO
DMF
None
31.2
36.8
39.4
45.1
43.8
–
62 [94]
66 [98]
68 [98]
9 [97]
13 [96]
77 [93]
a [1c/(3+2 introduced)]×100.
b [1c/(3+2 reacted)]×100.
Figure 3. Influence of reaction time on the selectivity of the
process.

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F. Bigi et al. / Tetrahedron Letters 42 (2001) 6543–6545
6545
Acknowledgements
6. For an extensive review in the use of acid clays as
catalysts in organic syntheses, see: Balogh, M.; Laszlo, P.
Organic Chemistry using Clays; Springer Verlag: New
York, 1993.
We acknowledge the support of the Ministero dell’Uni-
versita`
e
della Ricerca Scientifica
e
Tecnologica
7. Ha´jek, M.; Radoiu, M. T. J. Mol. Catal. A: Chem. 2000,
160, 383–392.
(MURST), Italy, and the University of Parma
(National Project ‘Processi puliti per la Chimica Fine’).
The authors are indebted to Dr. Goetz Burgfels and
Dr. Uwe Flessner (Su¨d-Chemie) for helpful suggestions
on the deactivation of Montmorillonite KSF. The help
rendered by Dr. M. Ghillani is also acknowledged.
8. It is reported that solid inorganic materials undergo
surface area lowering when subjected to MW irradiation:
Ananthakumar, S.; Krishnapriya, G.; Damodaran, A. D.;
Warrier, K. G. K. Mater. Lett. 1998, 35, 95–99. Indeed to
the best of our knowledge, the reuse of the solid catalyst
in MW-promoted reactions has never been reported.
9. Montmorillonite KSF (Fluka) is a natural clay treated
with sulfuric acid, showing the following chemical com-
position (average value): SiO2 (54.0%), Al2O3 (17.0%),
Fe2O3 (5.2%), CaO (1.5%), MgO (2.5%), Na2O (0.4%),
K2O (1.5%); we used this catalyst from a freshly opened
container since it is demonstrated that KSF clay sponta-
neously deactivates on standing at rt for several months.
10. Detailed analysis of reaction mixture of ortho-tert-
butylphenol 1a revealed that 70% of the starting tert-
butyl group was transferred to toluene whereas the
remaining 30% was lost as isobutylene.
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