Reduction of aldehydes using trialkylboranes in ionic liquids
George W. Kabalka* and Rama R. Malladi
Departments of Chemistry and Radiology, The University of Tennessee, Knoxville, TN 37996-1600 USA.
E-mail: kabalka@utk.edu
Received (in Corvallis OR, USA) 1st September 2000, Accepted 25th September 2000
First published as an Advance Article on the web 31st October 2000
Table 2 Reduction of aldehydes with tributylborane in [Emim][PF6]
Non-aqueous ionic liquids, molten salts, have been found to
enhance organoboron mediated reductions of aldehydes.
Aldehydes
Time/h
Yield (%)c
In recent years, chemists have developed new synthetic
methodologies which are environmentally benign.1 For exam-
ple, ionic liquids have been found to be useful in many
environmentally sensitive industrial applications.2 More re-
cently, ionic liquids have attracted interest as reaction media in
organic and organometallic synthetic manipulations such as
hydrogenation,3 allylation,4 Heck-vinylation,5 epoxidation,6
Diels–Alder reactions,7 and Suzuki cross-coupling reactions.8
These solvents possess a number of interesting properties,
which include a lack of vapor pressure, ease of reuse, absence of
flammability and a tolerance for large temperature variations.
The reduction of aldehydes by organoborane reagents is an
important organic transformation. Generally, boron hydrides
are utilized as reducing agents due to their facile reactivity.9
Trialkylboranes, most notably the pinanyl derivatives, have
also been found to be especially useful reducing
reagents.10,11 However, reductions involving simple trialk-
ylboranes generally require reaction temperatures in excess of
approximately 150 °C.12 We have discovered that ionic liquids,
such as 1-butyl-3-methylimidazolium tetrafluoroborate
([Bmim][BF4]),13 1-ethyl-3-methylimidazolium tetrafluorobor-
ate ([Emim][BF4]), and 1-ethyl-3-methylimidazolium hexa-
fluorophosphate ([Emim][PF6]) enhance the rate of trialk-
ylborane reductions. For example, tributylborane reduces
benzaldehyde at rt in [Emim][PF6] although the reaction can be
carried out more rapidly at 100 °C (Table 1).
C6H5CHO
16a
16a
16a
16a
16a
24a
16a
16a
48b
24a
48b
100
100
100
100
100
40
98
96
100
100
100
p-BrC6H4CHO
m-BrC6H4CHO
o-FC6H4CHO
p-FC6H4CHO
p-CH3OC4CHO
o-CH3C6H4CHO
p-CH3C6H4CHO
C10H20
O
O
C10H20
(CH3)3CCHO
a Reactions were carried out at 100 °C. b Reactions were carried out at rt.
c NMR yield.
mg, 1.00 mmol) was added and the mixture stirred at rt for 48 h.
The product was extracted into ether (2 3 5 mL) the extracts
were combined and dried over anhydrous MgSO4. The solvent
was removed under reduced pressure and then purified by silica
gel chromatography to yield 102 mg (94%) of benzyl alco-
hol.14
Ionic liquids are excellent solvents in which to carry out
reductions using trialkylboranes. The reaction proceeds readily
with aromatic and aliphatic aldehydes. Separation of the
products from the ionic liquid is straightforward as is recycling
of the solvent.
We wish to thank the U.S. Department of Energy and the
Robert H. Cole Foundation for support of this research.
Both aromatic and aliphatic aldehydes are reduced by
tributylborane in ionic liquids (Table 2). The presence of a para-
substituted electron-donating group appears to hinder the
reaction. Only one alkyl group is utilized, and thus an equimolar
ratio of aldehyde and tributylborane is required. Presumably
this is a consequence of the weaker Lewis acidity of the
dialkylborinic ester generated after the first reduction, reaction
1.
Notes and references
1 G. W. Kabalka and R. M. Pagni, Tetrahedron, 1997, 53, 7999.
2 Michael Freemantle, Chem. Eng. News, May 15, 2000, 37.
3 P. J. Dyson, D. J. Ellis, D. G. Parker and T. Welton, Chem. Commun.,
1999, 25.
4 C. M. Gordon and A. McCluskey, Chem. Commun., 1999, 1431.
5 V. P. W. Bohm and W. A. Hermann, Chem. Eur. J., 2000, 6, 1017.
6 C. E. Song and E. J. Roh, Chem. Commun., 2000, 837.
7 (a) M. J. Earle, P. B. McCormac and K. R. Seddon, Green Chem., 1999,
1, 23; (b) T. Fisher, A. Sethi, T. Welton and J. Woolf, Tetrahedron Lett.,
1999, 40, 793.
8 C. J. Mathews, P. J. Smith and T. Welton, Chem. Commun., 2000,
1249.
9 N. M. Yoon, Pure Appl. Chem., 1996, 68, 843.
10 M. M. Midland, A. Tramontan and S. A. Zderic, J. Am. Chem. Soc.,
1977, 99, 5211.
(1)
The fact that the organic products are readily removed from
the ionic liquids via extraction is especially appealing. We
routinely recycled the ionic solvents and found no decrease in
reduction yields.
The reduction of benzaldehyde is representative: benzalde-
hyde (106 mg, 1.00 mmol) and [Emim][PF6] (250 mg) were
placed in a 10 mL round-bottomed flask. Tributylborane (182
11 M. M. Midland, Chem. Rev., 1989, 89, 1553.
12 B. M. Milkhailov, Yu. N. Bubnov and V. G. Kiselev, J. Gen. Chem.
USSR, 1966, 36, 65.
Table 1 Reduction of benzaldehyde with tributylborane in different ionic
liquids
13 J. S. Wilks and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 1992,
965.
Ionic liquid
Time/h
Yield (%)c
14 In the absence of the molten salt, essentially no reduction occurs.
[Bmim BF4]
[Emim BF4]
[Emim PF6]
[Emim PF6]
16a
16a
16a
48b
93
90
96
94
a Reactions were carried out at 100 °C. b Reaction was carried out at rt.
c Isolated yield.
DOI: 10.1039/b007190k
Chem. Commun., 2000, 2191
This journal is © The Royal Society of Chemistry 2000
2191
Kabalka
