Lanthanide(III) triflates as recyclable catalysts for atom economic aromatic nitration

Article abstract of DOI:10.1039/a700546f

Lanthanide(III) triflates catalyse (1-10 mol%) the nitration of a range of simple aromatic compounds in good to excellent yield using stoichiometric quantities of 69% nitric acid; the only by-product is water and the catalyst can be readily recycled by simple evaporation.

Full text of DOI:10.1039/a700546f

View Article Online / Journal Homepage / Table of Contents for this issue
Lanthanide(iii) triflates as recyclable catalysts for atom economic aromatic
nitration
Francis J. Waller,*a Anthony G. M. Barrett,*b D. Christopher Braddockb and Dorai Ramprasada
a Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA
b Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AY
Lanthanide(III) triflates catalyse (1–10 mol%) the nitration
of a range of simple aromatic compounds in good to
excellent yield using stoichiometric quantities of 69% nitric
acid; the only by-product is water and the catalyst can be
readily recycled by simple evaporation.
precipitated and the organic phase became yellow, and after
12 h no phase boundary was apparent. The solution was allowed
to cool and diluted with water. The yellow organic phase was
dried (MgSO4) and evaporated to give nitrotoluene (390 mg,
95%). The colourless aqueous phase was evaporated to give
ytterbium(iii) triflate as a white free-flowing solid (183 mg,
98%).
Nitration of aromatic compounds is an immensely important
industrial process.1 The nitroaromatic compounds so produced
are themselves widely utilised and act as chemical feedstocks
for a great range of useful materials such as dyes, pharmaceuti-
cals, perfumes and plastics. Unfortunately nitrations typically
require the use of potent mixtures of concentrated or fuming
nitric acid with sulfuric acid leading to excessive acid waste
streams and added expense. Alternatively, nitric acid may be
used in conjunction with strong Lewis acids such as boron
trifluoride.2 The Lewis acid is used at or above stoichiometric
quantities and is destroyed in the aqueous quench liberating
large amounts of strongly acidic by-products. With chemists
under increasing pressure to perform atom economic processes3
with minimal or no environmentally unfriendly by-products,
development of novel catalyst systems that facilitate aromatic
nitrations in this manner should be of great importance.4
Lanthanides have found increasing use as mild and selective
reagents in organic synthesis.5 In particular, lanthanide(iii)
triflates6,7 have been used to good effect as Lewis acids in
Diels–Alder,8 Michael,9 Friedel–Crafts10 and Mukaiyama11
reactions. For the Mukaiyama reaction the optimum solvent
system was found to be aqueous THF; the catalyst was recycled
via aqueous work-up and used repeatedly with little detriment to
rate or yield. The compatibility of lanthanide(iii) triflate salts
with water and yet their apparent ability to function as strong
Lewis acids is somewhat paradoxical. We sought to harness this
water tolerant Lewis acidity and herein report the use of
catalytic quantities of lanthanide(iii) triflates for the nitration of
a range of simple aromatic compounds in good to excellent
yield using a stoichiometric amount of 69% nitric acid where
the only by-product is water. Furthermore the catalyst is readily
recycled by a simple evaporative process.
A series of other metal triflate salts were screened for
catalytic activity (Table 2). Scandium triflate was found to have
comparable catalytic activity to that of the corresponding
ytterbium salt, but praseodymium, europium and lanthanum
triflates, although active, were found to be somewhat less
effective.
The recovered catalyst was found to be active for further
nitrations with no reduction in rate or yield and with no change
in selectivity. The results from four successive nitrations with
m-xylene are shown in Table 3. On one occasion the white
precipitate obtained from the nitration mixture with lantha-
num(iii) triflate consisted of crystals of the known lantha-
num(iii) triflate nonahydrate.12† These crystals were also found
to have an identical diffuse reflectance IR spectrum to that of the
amorphous recovered material and also that of the commercial
Table 1 Nitration of aromatics in the presence of ytterbium(iii) triflatea
Product distribution (%)c
Conversion
Arene
(%)b,c
ortho
meta
para
PhH
> 75 (75)
> 95 (95)
89
92
0
—
52
38
44
—
—
7
trace
trace
—
—
41
62
56
—
PhMe
PhPh
PhBr
PhNO2
p-xylene
p-dibromobenzene
m-xylene
> 95
8
> 95
> 95
—
—
—
—
—
—
4-NO2: 85
1-NO2: 91
2-NO2: 15
2-NO2: 9
naphthalene
Ytterbium(iii) triflate was selected as the most promising
candidate for the proposed nitrations. Accordingly a range of
simple aromatic compounds were treated with 1 equiv. of 69%
nitric acid in 1,2-dichloroethane heated at reflux in the presence
of 10 mol% ytterbium(iii) triflate.The results are shown in Table
1. No dinitrated products were observed in any case and in
accord with this the system failed to nitrate nitrobenzene. In the
control experiments with no catalyst, only slow reaction
occurred and no more than 10% of nitrated products were
observed. Ytterbium triflate was found to be effective even at
catalyst loadings as low as 1 mol% and with m-xylene the
reaction had proceeded to 80% conversion after 12 h.
The following procedure is representative. Nitric acid (69%;
192 ml, 3.0 mmol) was added to a stirred suspension of
ytterbium(iii) triflate (186 mg, 0.30 mmol) in 1,2-dichloro-
ethane (5 ml). The suspension dissolved to give a two phase
system in which the aqueous phase was the more dense. Toluene
(240 ml, 3.0 mmol) was added and the stirred mixture was
heated at reflux for 12 h. During the reaction a white solid
a All reactions carried out on a 3 mmol scale with 10 mol% ytterbium(iii)
triflate and 1 equiv. of 69% nitric acid in refluxing 1,2-dichloroethane (5 ml)
for 12 h. b Isolated yields in parenthesis. c GC and/or 1H NMR analysis.
Table 2 Effect of lanthanide(iii) triflate on nitration of m-xyelenea
Product distribution (%)c
b
M(OTf)3
Conversion (%)c
4-NO2
2-NO2
La
Eu
Pr
Sc
Yb
22
36
65
85
88
86
86
88
85
85
14
14
12
15
15
a
All reactions performed with 10 mol% lanthanide(iii) triflate, 1 equiv.
each of m-xylene and 69% nitric acid in refluxing dichloromethane (15 ml)
for 18 h. b Commercially available hydrated forms. c GC analysis.
Chem. Commun., 1997
613

View Article Online
and Dr David Williams and Dr Andrew J. P. White for the single
crystal X-ray structure determination of lanthanum(iii) triflate
nonahydrate.
available lanthanide salt.‡ As expected the crystals were found
to catalyse the nitration reaction.
The nature of the de facto nitration agent is not known at
present. The different rates with various lanthanide triflates
indicate that the metal centre is involved. Nitric acid–triflic acid
mixtures are known to effect nitration13 but free triflic acid
seems unlikely in our system since ytterbium(iii) triflate yields
a near-neutral solution when dissolved in water.§ The relative
ratios of the various nitroaromatic isomers produced are fully
Footnotes
† A single crystal X-ray diffraction showed the material to be identical with
the known lanthanum(iii) triflate nonahydrate where the nine water
molecules occupy the first co-ordination sphere in a tricapped trigonal prism
arrangement with triflate counterions
‡ Aldrich chemical company
§ The pH of a 0.08 m solution of ytterbium(iii) triflate in water was
determined to be 6.40.7
+
consistent with electrophilic attack by NO2 or more probably
+
an NO2 carrier of the type 11. However, bidentate lanthanide
nitrates 2 are well characterised compounds,14 and nitration
References
Ln
H
O–
O
O
O
+
O
+
N
+
N
1 G. A. Olah, R. Malhotra and S. C. Narang, Nitration: Methods and
Mechanisms, VCH, New York, 1989; C. K. Ingold, Structure and
Mechanism in Organic Chemistry, Cornell University Press, Ithaca,
New York, 2nd edn., 1969; G. A. Olah and S. J. Kuhn, in Friedel-Crafts
and Related Reactions, ed. G. A. Olah, Wiley-Interscience, New York,
1964, vol. 2; K. Schofield, Aromatic Nitrations, Cambridge University
Press, Cambridge, 1980.
Ln
O
1
2
may proceed via direct attack of such a (protonated) species.
Bidentate metal nitrates have been implicated in nitrations with
titanium tetranitrate15 and ceric ammonium nitrate16 where the
arene is thought to attack the metal bound nitrate species
directly.
In conclusion, we have demonstrated the catalytic use of
lanthanide(iii) triflates for the atom economic nitration of
simple aromatic substrates where the only by-product is water.
The catalyst is readily recyclable and we believe this to be a
major step forward in the area of clean technology for aromatic
nitration.
The Imperial College group thank Air Products and Chem-
icals, Inc. for support of our research under the auspices of the
joint Strategic Alliance, Glaxo Group Research Ltd. for the
most generous endowment (to A. G. M. B), the Wolfson
Foundation for establishing the Wolfson Centre for Organic
Chemistry in Medical Science at Imperial College, the EPSRC
2 R. J. Thomas, W. F. Anzilotti and G. F. Hennion, Ind. Eng. Chem., 1940,
32, 408; G. F. Hennion, US Pat., 2314212, 1943.
3 B. M. Trost, Angew. Chem., Int. Ed. Engl., 1995, 34, 259.
4 For a recent nitration using clean methodology, see K. Smith,
A. Musson and G. A. DeBoos, Chem. Commun., 1996, 469. However
this method suffers from the disadvantage of stoichiometric quantities of
acidic by-products.
5 T. Imamaoto, in Lanthanides in Organic Synthesis, Academic Press,
London, 1994.
6 R. W. Marshmann, Aldrichim. Acta, 1995, 28, 77.
7 S. Kobayashi, Synlett, 1994, 689.
8 S. Kobayashi and H. Ishitani, J. Am. Chem. Soc., 1994, 116, 4083;
I. E. Marko and G. R. Evans, Tetrahedron Lett., 1994, 35, 2771.
9 S. Kobayashi, I. Hachiya, T. Takakori, M. Akida and H. Ishitani,
Tetrahedron Lett., 1992, 33, 6815.
10 A. Hachiya, M. Moriwaki and S. Kobayashi, Bull. Chem. Soc. Jpn.,
1995, 68, 2053; A. Kawada, S. Mitamura and S. Kobayashi, Chem.
Commun., 1996, 183; A. Kawada, S. Mitamura and S. Kobayashi,
Synlett, 1994, 545; A. Kawada, S. Mitamura and S. Kobayashi, J. Chem.
Soc., Chem. Commun., 1993, 1157.
Table 3 Recycled ytterbium(iii) triflatea
11 S. Kobayashi and I. Hachiya, J. Org. Chem., 1994, 59, 3590 and
references cited therein.
12 J. M. Harrowfield, D. L. Kepert, J. M. Patrick and A. H. White, Aust.
J. Chem., 1983, 36, 483; A. Chatterjee, E. N. Maslen and K. J. Watson,
Acta Crystallogr., Sect. B,, 1988, 44, 381.
13 C. L. Coon, W. G. Blucher and M. E. Hill, J. Org. Chem., 1973, 38,
4243.
14 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley,
New York, 4th edn., 1980, pp. 173, 981–1004.
15 D. W. Amos, D. A. Baines and G. W. Flewett, Tetrahedron Lett., 1973,
3191.
16 S. Dincturk and J. H. Ridd, J. Chem. Soc., Perkin Trans. 2, 1982,
961.
Conversion Mass of
Run (%)b
catalyst/mgc
1
2
3
4
89
81
90
88
190 ( > 100)
152 (82)
127 (68)
115 (62)
a All runs performed with 3 mmol m-xylene, 10 mol% ytterbium triflate (run
1) and 1 equiv. of nitric acid in refluxing 1,2-dichloroethane (5 ml) for 5 h.
b
GC analysis. The isomeric ratio of 4- and 2-nitroxylene was unchanged
throughout (85:15 respectively). c Mass of ytterbium(iii) triflate recovered
from each run. The figures in parenthesis indicate the percentage recovery
which were not optimised.
Received, 23rd January 1997; Com. 7/00546F
614
Chem. Commun., 1997