Phosphonium-based ionic liquids as efficient borane carriers

Article abstract of DOI:10.1071/CH06021

Phosphonium-based ionic liquids form new ionic liquids when treated with borane (BH₃), and these ionic liquids are useful for reduction reactions involving carbonyl compounds. Alane (AlH₃) reacts with the phosphonium cation, neverthe

Full text of DOI:10.1071/CH06021

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Aust. J. Chem. 2006, 59, 298–301
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Phosphonium-Based Ionic Liquids as Efficient Borane Carriers
A
A
A,B
Taramatee Ramnial, Michelle K. Hauser, and Jason A. C. Clyburne
A
Department of Chemistry, Simon Fraser University, Burnaby BC V5A 1S6, Canada.
Corresponding author. Email: clyburne@sfu.ca
B
Phosphonium-based ionic liquids form new ionic liquids when treated with borane (BH3), and these ionic liquids
are useful for reduction reactions involving carbonyl compounds. Alane (AlH3) reacts with the phosphonium
cation, nevertheless the resulting solutions are capable of reducing esters to aldehydes. Phosphonium-based ionic
liquids may serve as useful materials for reactive gas transport, and hence may serve as a fluid alternative to
porous materials and may provide a convenient method of safely transporting boron hydrides for a variety of
applications.
Manuscript received: 17 January 2006.
Final version: 27 April 2006.
Ionic liquids, and in particular imidazolium-based ionic liq-
uids, have undergone extensive investigation for a myriad of
applications over the last several years.[1–3] Studies examin-
ing quarternary phosphonium systems are much rarer, and
this may reflect the fact that commercial supplies and con-
as a heat sink for strongly exothermic reactions.[14] Recently,
we showed that phosphonium-based ionic liquids can sup-
[6,15]
port strong bases such as N-heterocyclic carbenes
Grignard reagents,
and
[
16]
and the stability of these basic com-
pounds could be couched in kinetic arguments since access
to the protic site of the phosphonium-based ionic liquids is
difficult. Deprotonation of the phosphonium-based ionic liq-
uid would lead to the formation of a phosphorane but this
reaction was not observed. Hence, it seemed obvious that
phosphonium-based ionic liquids would serve as an efficient
venient syntheses of these salts have only recently become
available.[
4,5]
We have examined some of the chemistry of
imidazolium ions and have shown that they are unsuitable for
[6]
reactions involving active metals (i.e. Na or K) or hydridic
−
− [7,8]
The imidazolium
sources such as [BH4] and [AlH4] .
ion is notoriously reactive at the C2 hydrogen site, and this
[9]
[17]
carrier of hydriditic reagents, specifically borane (BH3)
reactivity can be a blessing[ or a bane
10]
[11]
depending on
which is typically sold as an ethereal solution, most usually
in tetrahydrofuran (THF). These solvents have the disadvan-
tage of being flammable and volatile. Herein we report the
synthesis of new phosphonium-based ionic liquid mixtures
possessing borane hydride anions, and show their use in the
reduction of aldehydes, ketones, and acid chlorides. We also
show the reaction ofAlH3 with phosphonium-based ionic liq-
uids leading to the decomposition of the phosphonium cation
and the formation of a solution capable of reducing esters to
aldehydes in good yields.
the reactions examined. Phosphonium-based ionic liquids,
as shown in Fig. 1, are attractive since they are easily pre-
pared, more robust, and thermally more stable than either
quarternary ammonium or imidazolium salts.[
4]
We have become interested in the materials aspects of
ionic liquids and these properties include the ability to stabi-
lize reactive species, as well as their fluid properties. More
traditional aspects of ionic liquids include their solvent prop-
erties, and ionic liquids are considered to be among some
of the ‘greener’ solvents.[ The link between ionic liquids
and green chemistry is largely due to their non-volatility,
1]
Commercially available phosphonium-based ionic liquids
1 and 2 were purified by washing the solvent with sodium
bicarbonate solution and subsequent extraction with water
and hexanes followed by drying by means of azeotropic distil-
lation with toluene. Stable compounds, having the empirical
and yet they are still good solvents for many organic, inor-
ganic, and metalloorganic compounds.[
12,13]
Perhaps of more
importance are their non-flammability and their ability to act
ꢀ
formula [R RP][XBH3], can be prepared either by pass-
3
ing gaseous B2H6 through the phosphonium-based ionic
[
18,19]
C14H29
liquids
or by mixing one equivalent of BH3·THF solu-
X
1 X ϭ Cl
X ϭ C H19CO
tion with the phosphonium-based ionic liquid followed by
completeremovaloftheTHFbyexhaustiveevacuation.These
new ionic liquids are recyclable. After reduction and extrac-
tion of the products, the ionic liquids were purified by using
the method described above, and these ionic liquids were
P
2
9
2
C H
6
13
C H
6
13
C H
6
13
Fig. 1. Selected phosphonium-based ionic liquids.
©
CSIRO 2006
10.1071/CH06021
0004-9425/06/050298

Phosphonium-Based Ionic Liquids
299
reused for borane reduction of benzaldehyde to give 90%
yield of benzyl alcohol.
Elemental analyses of the new ionic liquids are in accord
with the expected values. Unambiguous assignment of the
carbonyl compounds with the new 1·BH3 or 2·BH3 ionic
liquids at room temperature. Yields were determined by gas
chromatography–mass spectrometry analysis (GC-MS) of
the extracts. The data are presented in Table 1. For com-
parison, the products were also extracted by Kulgelröhr
distillation from the ionic liquids and the yields were compa-
rable to those obtained in the GC-MS analyses of the extracts
as well as to reactions performed in molecular solvents such
1
H NMR signals in the new ionic liquid anion of 1·BH3 and
2
·BH3 have not been made due to the complexity, overlap,
and high intensity of the proton peaks in the phosphonium-
based ionic liquids. The phosphonium-based ionic liquids
1
troscopy and these spectra generally exhibit broad resonances
and interestingly a clear feature consistent with the pres-
ence of [BH4] ;
1
1
[24]
·BH3 and 2·BH3 were also analyzed using B NMR spec-
as THF.
We note that phosphonium ions have been used as catalysts
[
25,26]
in several reaction types,
and phosphonium derivatives
−
[20]
[27,28]
other peaks were not assigned. The
have been used historically for phase-transfer catalysis.
IR spectra for the molecules exhibit peaks in the B–H
stretching region (2000–2400 cm ). Specifically, the new
Of relevance to this report is that quarternary phosphonium
−
1
[29]
cations activate carbonyl compounds.
In order to probe
phosphonium-based ionic liquid 1·BH3 exhibits absorptions
this interaction, we performed some simple experiments.
A large degree of vapour pressure lowering was observed
(namely, much less than predicted by Raoult’s Law) when a
known concentration of carbonyl or alcohol was added sep-
arately to phosphonium-based ionic liquid 1. The deviation
order is of ethanol > propionaldehyde > acetone, suggestive
of the degree of association in solution.
−
1
at 2037 (m), 2212 (m), and 2298 (s) cm whereas 2·BH3
−
1
exhibits peaks at 2139 (m), 2224 (m), and 2270 (s) cm .The
anion of 2 forms a new ionic liquid with BH3 showing a shift
inthecarboxylatestretchintheIRspectrum.Thesolutionsare
dynamic, and we note that ligand exchange reactions occur
−
for the anion [BH3Cl] to give a variety of boron hydride
−
chlorides such as [BH2Cl2] which further complicate the
We realize that there are two probable types of inter-
action between the solute (carbonyl compound) and the
solvent (phosphonium centre), namely coordination through
the quarternary cationic phosphonium site through [P· · · O]
interactions or through hydrogen bonding to the hydrogen
positioned α to the phosphonium cation. We performed some
NMR titration experiments in order to probe the associ-
ation between solute and solvent molecules. Addition of
a carbonyl compound, such as benzaldehyde and propi-
onaldehyde, to phosphonium-based ionic liquid 1 has little
effect on the 31P NMR spectra; however H NMR studies
indicate a downfield shift of the signal representing the pro-
ton α to the phosphorous with an increased concentration
of the carbonyl compound. This observation suggests that
the mode of association between solute and solvent likely
involves a hydrogen bonding interaction. Finally, we note
that the shortest cation anion contact in the simple phenoxide
salt [Ph3PCH3][2,6-Ph2C6H3O] involves a novel [C–H· · · O]
situation,[ nevertheless reactivity studies using 1·BH3 and
21]
2
·BH3 withaknownexcessofbenzaldehydewerealsocarried
out and these showed that there are three hydrides available
for reduction.
These new phosphonium-based ionic liquids, specifically
1
·BH3 and 2·BH3, are air- and moisture-sensitive. Expo-
sure of these compounds to moist air results in hydrolysis
of the complex, giving a very large peak in the B–OH
stretching region (3330 cm ) of the IR spectrum.These new
−
1
1
ionic liquids are a convenient source of hydride for standard
chemical reductions.[
22,23]
They are, in our experience, non-
flammable, recyclable, and non-volatile, and represent a new
class of odourless source of borane.[ Moreover, these solu-
tions may provide a new method of hydride transport using
a flowing medium, possibly providing for a safer method of
using boranes as an energy carrier.
18]
We have used these new ionic liquids for classic reduc-
tions involving borane, namely the reduction of the carbonyl
functionality. A series of reactions were performed by com-
bining stoichiometric amounts (based on hydride) of the
[
30]
hydrogen bond.
We have also looked at the reactivity of other reducing
agents in phosphonium-based ionic liquids, namely NaBH4,
LiAlH4, and AlH3. NaBH4 in phosphonium-based ionic liq-
uid 1 reduces aldehydes, ketones, and acid chlorides to
the anticipated products, but we suggest that there are few
obvious advantages to using NaBH4 in phosphonium-based
ionic liquids over conventional solvents such as ethanol and
isopropanol. On the other hand, LiAlH4 and AlH3, which
are significantly more reactive than NaBH4 and clearly
incompatible with alcohols, react with 1 to give tetrade-
cyl(dihexyl)phosphine and hexene and with 2 to give, after
aqueous workup, tetradecyl(dihexyl)phosphine, hexane, and
decanol as determined by GC-MS, NMR, and MS stud-
Table 1. Reduction of carbonyl functionality
Phosphonium-based Reaction
ionic liquid
Yield
[%]
1
2
1
2
1
2
1
2
1
2
·BH3
Benzaldehyde → benzyl alcohol
Benzoyl chloride → benzyl alcohol
Benzophenone → benzhydrol
94
95
90
99
60
99
75
61
80
91
·BH3
·BH3
·BH3
·BH3
·BH3
·BH3
Cinnamaldehyde → cinnamyl alcohol
Benzaldehyde → benzyl alcohol
ies. Treatment of 2 with 10 mol-% of AlH for 2 h under
3
·BH3
an inert atmosphere and at room temperature results in the
generation of a new solution that shows 10–15% decompo-
sition of the ionic liquid, as shown by the presence of hexene
·10% BH3
·10% BH3

3
00
T. Ramnial, M. K. Hauser, and J. A. C. Clyburne
and phosphines in the solution,[31] consistent with decompo-
sition via a Hoffman-type decomposition.[ It should be
noted that no further decomposition of the phosphonium
ionic liquid with AlH3 occurs and this was tested by leav-
ing the new ionic liquid sitting for over one month. With the
decomposition of the phosphonium-based ionic liquid, there
is also the generation of a new aluminum hydride species.
The resulting solution is capable of reducing aldehydes and
ketones to alcohols (benzaldehyde 78%, 1-phenyl-ethanone
Acknowledgments
32]
Financial support was provided by the Natural Sciences and
Engineering Research Council of Canada (NSERC). We
thank Cytec Canada Inc. for the generous donation of the
phosphonium-based ionic liquids.
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1
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(
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