Ionic liquids: A convenient solvent for environmentally friendly allylation reactions with tetraallylstannane

Article abstract of DOI:10.1039/a903661j

Ionic liquids based on the 1-butyl-3-methylimidazolium cation have been used as solvents for the preparation in good yield of homoallylic alcohols from tetraallylstannane and a range of aldehydes.

Full text of DOI:10.1039/a903661j

Ionic liquids: a convenient solvent for environmentally friendly allylation
reactions with tetraallylstannane
Charles M Gordon* and Adam McCluskey*b
a
a
Department of Pure and Applied Chemistry, the University of Strathclyde, Thomas Graham Building, 295
Cathedral Street, Glasgow, UK G1 1XL. E-mail: c.m.gordon@strath.ac.uk
Department of Chemistry, the University of Newcastle, University Drive, Callaghan, NSW 2308, Australia.
b
E-mail: amcclusk@mail.newcastle.edu.au
Received (in Liverpool, UK) 6th May 1999, Accepted 15th June 1999
Ionic liquids based on the 1-butyl-3-methylimidazolium
cation have been used as solvents for the preparation in good
yield of homoallylic alcohols from tetraallylstannane and a
range of aldehydes.
([bmim][PF
6
]), which were prepared by literature methods.10
On completion of the allylation reaction the ionic liquid was
observed to be cloudy. Work-up was achieved simply by
2
extraction of the reaction mixture with Et O followed by
removal of the solvent.† This approach generally allowed
isolation of products of excellent purity, unreacted starting
material being the only contaminant in some cases, as discussed
below. The cloudy residue was assumed to be the tin by-
In recent years chemists have striven to develop new synthetic
methodologies and techniques which are more environmentally
benign, work generally referred to under the title of ‘clean
technology’. Organostannanes are extremely useful reagents in
organic synthesis, but are also notoriously environmentally
unfriendly due to the presence of toxic tin residues. There have
been a number of attempts to develop cleaner organostannane
reagents and reaction conditions. Examples include the devel-
product, and this did not transfer into the Et
yields obtained are listed in Table 1. Product characterisation
2
O extracts. The
1
13
was carried out using H and C NMR spectroscopy.
As can be seen in Table 1, excluding entry 5 the yields varied
from 66–93% depending on the nature of the aldehyde
employed. Such values are in good agreement with those
previously reported for the equivalent reaction in MeOH
1
opment of polymer supported organostannanes, the use of
2
3
water as solvent, and water-soluble reagents.
9
Room temperature ionic liquids are attracting increasing
interest as environmentally benign reaction media for organic
and organometallic synthetic chemistry, for example Friedel–
solution. Reactions were judged to have gone to completion if
1
13
the H and C NMR spectra showed no signals for the aldehyde
group. It should be noted that the small scale on which the
reactions were carried out meant that mechanical losses were
proportionately quite large, so larger scale preparations would
be expected to give higher yields. The allylation of benzalde-
4
5
Crafts and hydrogenation reactions. These solvents possess a
number of interesting properties, notably their lack of vapour
pressure, ease of reuse, lack of flammability, and large
accessible temperature range. Furthermore, by alteration of the
cation and anion it is possible to adjust the solubility properties
of an ionic liquid, for example making it immiscible with water
or certain organic solvents. Recent studies on Diels–Alder
6
hyde in [bmim][PF ] was investigated on a larger scale to
confirm this. When 2.50 g of 1 (8.87 mmol) was reacted with
3.765 g of benzaldehyde (35.46 mmol) in 10 ml of ionic liquid,
an isolated yield of 5.17 g (96%) of the homoallylic alcohol was
obtained.
The reaction was observed to proceed to completion with
simple aromatic aldehydes like benzaldehyde (entry 1). Similar
behaviour was observed when electron-withdrawing sub-
stituents were present (e.g. p-Cl, entry 2), and the presence of
weakly electron-donating substituents did not appear to hinder
the reaction significantly. Thus good yields were obtained in the
reactions of tolualdehyde (entry 3) and anisaldehyde (entry 4).
reactions in ionic liquids based on the 1-alkyl-3-methylimidazo-
2
lium cation in conjunction with a variety of anions (e.g. PF
6
2
and BF ) have suggested reactivity patterns similar to those
4
6,7
observed in alcoholic solvents. Herein we report for the first
time the combination of organostannane chemistry and ionic
liquid methodology.
The allylation of aldehydes to produce homoallylic alcohols
is a useful organic transformation, which has attracted consider-
8
able attention in recent years. The most common method is the
use of metal–allyl complexes, and one of us (AM) has recently
reported the use of tetraallylstannane (1) for allylation under
very mild conditions (Table 1).9
4 6
Table 1 Yields of homoallylic alcohol using [bmim][BF ] and [bmim][PF ]
ionic liquids
The reaction was found to be most efficient when carried out
in MeOH solution. 1 has a number of advantages as a reagent
compared with the allyltrialkylstannanes traditionally used for
such reactions. In particular, all of the allyl ligands are
transferred, leaving the tin by-product in the form of inorganic
salt which is much more easily separated from the reaction
mixture than organostannane residues resulting from allyltri-
alkylstannane reactions. Most importantly from a clean chem-
istry standpoint, as each molecule of 1 will react with four
separate aldehyde molecules the reaction is extremely ‘atom
efficient’, since the only reagent atom not incorporated into the
product is tin.
Yield (%)
Entry
R
[bmim][BF
4
]
[bmim][PF
6
]
1
2
3
4
5
6
7
8
9
a
Ph
p-ClC
p-MeC
p-MeOC
p-Me
Pr
C
79a
82a
a
a
6
H
4
93
82
72
76
b
b
6
H
4
80
a
b
6
H
4
84
2
NC
6
H
4
0
70
72
69
0
The reaction of a range of aromatic and aliphatic aldehydes
with 1 was investigated. In a typical experiment, the chosen
aldehyde was stirred at room temperature for 16 h with 0.25
equiv. of 1 dissolved in 2 ml of the chosen ionic liquid. The
ionic liquids employed were 1-butyl-3-methylimidazolium
b
b
70
74
66
b
b
5
H
12
b
b
trans-PhCNCMe
(S)-Me CNCH(CH
Isolated yield. b NMR yields.
b
b
2
2
)
2
CHMeCH
2
73
78
4
tetrafluoroborate ([bmim][BF ]) and hexafluorophosphate
Chem. Commun., 1999, 1431–1432
1431

Table 2 Results obtained using recycled ionic liquids
At present the mechanism of the reaction is not clear.
Previous studies on the reaction in MeOH have suggested the
presence of an eight-membered ring transition state involving
Yield (%)
9
both reactants and the solvent. The results presented here do
Entry
R
Cycle
[bmim][BF
4
]
6
[bmim][PF ]
not give any direct clue as to the role of the ionic liquid in this
reaction. Further mechanistic investigations are currently in
progress.
a
a
1
2
3
a
Ph
Ph
Ph
1
2
3
79
82
81
83
a
a
a
a
82
78
In conclusion, ionic liquids appear to be excellent solvents in
which to carry out allylation reactions using tetraallylstannane.
The reaction proceeds most readily with aromatic aldehydes,
provided that no strongly electron donating substituent is
present. The reaction is less rapid in aliphatic aldehydes, but it
should be stressed that all of the investigations reported here
were carried out at 15 °C. Allylation of an aldehyde containing
a chiral centre gave little evidence for stereoselectivity in the
reaction, although the example chosen was relatively unfavour-
able. Separation of the products from the ionic liquid is very
straightforward, as is recycling of the liquid. The latter will be
an important concern if ionic liquids are truly to be considered
environmentally friendly solvents. Future work will address the
reaction of a wider range of substrates, and a mechanistic
investigation will be attempted, part of which will be an attempt
to identify the insoluble tin by-product of the reaction.
We would like to thank the Royal Society of Edinburgh for
the award of a BP Research Fellowship (C. M. G.), the
University of Newcastle (A. M.) for financial support, and Dr
Phil Dennison for assistance with the 1 Sn NMR spectros-
copy.
Isolated yield.
2
When a strongly electron-donating substituent such as Me N
was present, however, only starting materials were obtained on
work-up (entry 5). The greatly reduced reaction rate with
electron-donating substituents was also observed in the reac-
tions carried out in MeOH solution.9
The presence of significant amounts of unreacted 1 in the
organic extracts from the reactions involving simple aliphatic
aldehydes suggested that these did not go to completion (entries
6
and 7). No traces of unreacted aldehyde were observed in
these cases, but these are probably sufficiently volatile to be lost
during work-up. Traces of 1 were also found in the reaction
products of trans-2-methylcinnamaldehyde (entry 8), suggest-
ing that here too the reaction was incomplete. Yields obtained in
all three cases were reasonable despite this.
The product of the (S)-(2)-citronellal reaction (entry 9) can
contain both threo- and erythro-isomers. The NMR spectra
suggested that any selectivity was modest at best, with peaks
corresponding to each isomer being of almost identical
intensity. This is perhaps not surprising given the combination
of the small size of the methyl group and its distance from the
reactive site. Similarly small degrees of selectivity have been
reported in the crotylstannationation of citronellal carried out in
19
Notes and references
† Typical procedure for the allylation of alkanals by tetraallylstannane in
ionic liquids. Benzaldehyde (106 mg, 1 mmol) was placed in a 5 ml reaction
(MeCHNCHCH
2
)SnCl.11
vial with a spin vane. To this was added 2 ml of [bmim][BF ] followed by
4
aqueous solution using (E/Z)-Bu
2
tetraallylstannane (70.7 mg, 0.25 mmol), the septum cap was replaced, and
the mixture stirred vigorously at room temperature (typically 15 °C) for
In general it was found that there was no contamination of the
products with tin by-products when the reaction proceeded to
completion. This was confirmed by recording 1 Sn NMR
spectra of the organic products, which indicated that no tin was
present in any form. In cases where partial conversion was
observed, however, some 1 was found in the organic extracts.
No contamination of the products with ionic liquid was
observed, however. The reaction between 1 and benzaldehyde
1
6 h. After this time the mixture was extracted with Et
2
O (3 3 10 ml), the
19
organic extracts were combined and dried over anhydrous MgSO
4
. The
Et
‡
2
O was removed in vacuo to yield a pale oil, 117 mg (79%).
Recycling of ionic liquids. After complete reaction and work-up as
described above, the ionic liquid could be used with no further treatment.
Since this would result in the build-up of tin residues over a period of time,
however, the procedure generally employed was to dissolve the ionic liquid
in EtOAc (10 ml), and wash with water (2 3 5 ml) and brine (5 ml).
was monitored using 119Sn NMR spectroscopy to attempt to
determine the fate of the tin residues. A signal corresponding to
Addition of Et
2
O (20 ml) caused two layers to form, the lower being
essentially pure ionic liquid.
1
was observed at 245.8 ppm before addition of the
benzaldehyde. This disappeared rapidly when the reagents were
mixed, but no further 1 Sn signals were observed in either the
ionic liquid or the organic product. The reaction was accom-
panied by the formation of an insoluble white residue, more
noticeable than in the preparative experiments due to the lower
volume of ionic liquid employed. Attempts were made to
1
2
3
4
W. P. Neumann and J. Junggebauer, Tetrahedron, 1997, 53, 1361.
A. McCluskey, Green Chem., 1999, 1, 167.
R. Breslow and J. Light, Tetrahedron Lett., 1990, 31, 2957.
C. J. Adams, M. J. Earle, G. Roberts and K. R. Seddon, Chem.
Commun., 1998, 2097.
19
5 P. J. Dyson, D. J. Ellis, D. G. Parker and T. Welton, Chem. Commun.,
1999, 25.
6 (a) M. J. Earle, P. B. McCormac and K. R. Seddon, Green Chem., 1999,
identify this species, but it proved to be relatively insoluble in
all solvents investigated, and no signals were seen in the 119Sn
1
, 23; (b) T. Fischer, A. Sethi, T. Welton and J. Woolf, Tetrahedron
spectrum. It should be noted that these results contrast strongly
with 1 Sn investigations carried out on the reaction in MeOH
solution, where no insoluble product was observed, and signals
were seen in the region 2600 to 2640 ppm which were
assigned to polymeric tin(iv) methoxide species.9c
Lett., 1999, 40, 793.
J. A. Berson, Z. Hamlet and W. A. Mueller, J. Am. Chem. Soc., 1962, 84,
19
7
8
2
97.
For reviews see: Y. Yamamoto and N. Asao, Chem. Rev., 1993, 93,
207; J. A. Marshall, Chem. Rev., 1996, 96, 31
2
Little difference was found in the yields obtained using the
two different solvents. Recycling of the ionic liquid was carried
out for the reaction of benzaldehyde following an extremely
straightforward protocol.‡ The results gained are shown in
Table 2. It can be seen that no decrease in yield was observed in
runs carried out using ‘old’ ionic liquid, and furthermore the
9 (a) A. Yanagisawa, H. Inoue, M. Morodome and H. Yamamoto, J. Am.
Chem. Soc., 1993, 115, 10 356; (b) T. M. Cokley, R. L. Marshall, A.
McCluskey and D. J. Young, Tetrahedron Lett., 1996, 37, 1905; (c)
T. M. Cokley, P. J. Harvey, R. L. Marshall, A. McCluskey and D. J.
Young, J. Org. Chem., 1997, 62, 1961; (d) A. McCluskey, I. Wayan
Muderawan, Muntari and D. J. Young, Synlett, 1998, 8, 909.
1
0 J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun.,
products obtained were of the same purity as in the first run. In
1
992, 965.
2
the case of the recycled [PF
6
] salt the purification removed all
1
1 D. Furlani, D. Marton, G. Tagliavini and M. Zordan, J. Organomet.
Chem., 1988, 341, 345.
of the cloudy residue, while some cloudiness was observed in
2
the recycled [BF
4
]
salt, but this did not seem to impair the
performance of the liquid.
Communication 9/03661J
1432
Chem. Commun., 1999, 1431–1432