Trimethylsilyl trifluoromethanesulfonate catalyzed one-pot method for the conversion of aldehydes to homoallyl ethers in an ionic liquid

Article abstract of DOI:10.1055/s-2005-872112

A mild method for the trimethylsilyl trifluoromethanesulfonate (TMSOTf) catalyzed one-pot synthesis of homoallyl ethers from aldehydes has been developed in the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim] [OTf]). The advanta

Full text of DOI:10.1055/s-2005-872112

SHORT PAPER
2661
Trimethylsilyl Trifluoromethanesulfonate Catalyzed One-Pot Method for the
Conversion of Aldehydes to Homoallyl Ethers in an Ionic Liquid
C
atalyze
d
O
ne
e
-Pot Conve
t
rsion
o
e
f
Aldehyde
r
s
into Homoall
W
yl Ethers in an Ionic Liq
.
uid Anzalone, Ram S. Mohan*
Laboratory for Environment Friendly Organic Synthesis, Department of Chemistry, Illinois Wesleyan University,
Bloomington, IL 61701, USA
E-mail: rmohan@iwu.edu
Received 3 April 2005
nyl)imide,8 iodotrimethylsilane,9 trimethylsilyl trifluoro-
Abstract: A mild method for the trimethylsilyl trifluoromethane-
sulfonate (TMSOTf) catalyzed one-pot synthesis of homoallyl
ethers from aldehydes has been developed in the ionic liquid 1-bu-
methanesulfonate (TMSOTf),10 dicyclopentadienyl-
bis(triflyloxy)titanium(IV) [Ti(Cp)2(CF3SO3)2],11 trifluo-
roacetic acid,12 bismuth tribromide,13 trimethylsilyl
tyl-3-methylimidazolium
trifluoromethanesulfonate
([bmim]
[OTf]). The advantages of this method include the use of a recycla- bis(trifluoromethanesulfonyl)amide (TMSNTf2),14 scan-
dium(III) trifluoromethanesulfonate,15 indium metal,16
ble ionic liquid, facile product isolation without employing excess
and bismuth(III) trifluoromethanesulfonate.17 One draw-
organic solvent, elimination of an aqueous waste stream, and mild
reaction conditions.
back of using acetals for the synthesis of homoallyl ethers
is that they often have poor shelf lives or are not commer-
Keywords: ionic liquids, homoallyl ethers, aldehydes, allylations
cially available and need to be synthesized from the cor-
responding aldehyde. Hence, we were interested in
Room temperature ionic liquids are becoming increasing- developing a mild, one-pot synthesis of homoallyl ethers
ly popular as solvents in organic synthesis for several rea- from aldehydes.18
sons.1,2 They are practically nonvolatile and, hence, do not
We now report a TMSOTf-catalyzed one-pot synthesis of
pose the risks associated with volatile organic com-
homoallyl ethers from aldehydes in the ionic liquid
pounds. In addition, they are nonflammable and can be re-
[bmim][OTf] that allows facile product isolation and
cycled easily without any significant loss in activity.
avoids an aqueous workup, thus eliminating an aqueous
Although ionic liquids have several attractive features,
waste stream. When these reactions were done under sol-
they do have some drawbacks. Ionic liquids are very ex-
vent-free conditions, often an exothermic reaction oc-
pensive and often, reactions carried out in them require
curred accompanied by the formation of black tar.
the use of generous amounts of organic solvents for prod-
Therefore, ionic liquids offer a convenient medium for
uct isolation. Hence, ionic liquids are attractive alterna-
these reactions. The results are summarized in Table 1.
tives to organic solvents only when their use promotes
The reaction was effective with both allyltrimethylsilane
new reaction pathways or allows for milder reaction
and methallyltrimethylsilane (e.g., Table 1, entry 1g).
conditions than would be otherwise possible. Herein, we
Since the number of trialkylorthoformates that are com-
report a versatile one-pot method for the conversion
mercially available is limited, we also investigated the use
of aldehydes to homoallyl ethers in the ionic liquid 1-
of alkoxytrimethylsilanes in the reaction; this allowed a
butyl-3-methylimidazolium trifluoromethanesulfonate
wider range of homoallyl ethers to be synthesized. In ad-
([bmim][OTf]), an alternative to dichloromethane, which
dition, the reaction could be carried out with just 1.2
is the typical solvent for allylations using allyltrimethylsi-
equivalents of the alkoxysilane (while 2.0 equivalents of
lane. In addition, unique solubility properties of ionic liq-
the trialkylorthoformate were needed). The versatility of
uids are taken advantage of to allow facile product
this method is illustrated by the synthesis of allyl homoal-
isolation.
lyl ethers (Table 1, entries 4a and 4b), which can serve as
Homoallyl ethers are versatile functional groups amena- useful precursors to cyclic enol ethers via ring-closing
ble to further synthetic manipulation and, hence, their syn- metathesis reactions.19
thesis has attracted considerable attention. The allylation
The reaction is rapid at room temperature. The aldehydes
of acetals using organosilicon reagents is a useful method
are soluble in the ionic liquid, but the crude reaction prod-
to generate homoallyl ethers and several catalysts have
uct is only partially soluble and, hence, the bulk of the
been used to effect this transformation. These include tita-
product separates rapidly from the ionic liquid as the reac-
nium(IV) chloride,3 aluminum trichloride,4 boron trifluo-
tion progresses. Besides the expected homoallyl ether, the
ride–diethyl ether complex,4 trityl perchlorate,5
crude product was found to contain unreacted allyltri-
diphenylboryl trifluoromethanesulfonate,5 montmorillo-
methylsilane and trialkylorthoformate or alkoxysilane.
nite,6 lead/aluminum,7 trimethylsilyl bis(fluorosulfo-
Although the bulk of crude product separates as a distinct
layer, the recovered ionic liquid did contain up to 10% of
SYNTHESIS 2005, No. 16, pp 2661–2663
Advanced online publication: 04.08.2005
DOI: 10.1055/s-2005-872112; Art ID: M02005SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
5
the homoallyl ether product. In order to achieve complete
separation, the ionic liquid was extracted with a minimal
amount of diethyl ether (3 × 5 mL). If a homoallyl ether

2662
P. W. Anzalone, R. S. Mohan
SHORT PAPER
Table 1 One-Pot Method for the Conversion of Aldehydes to Homoallyl Ethers in [bmim][OTf]a
OR1
HC(OR1)3 or R1OTMS
O
TMS
TMSOTf (3–10 mol%)
R
R
H
[bmim][OTf]
r.t.
Entry
Aldehyde
R
TMSOTf (mol%) Reagent
Homoallyl etherb Time (min)
R1
Yieldc (%)
1a
1b
1c
1d
Ph
5
5
5
3
HC(OMe)3
Me
Me
Me
Me
10
30
25
5
78
88
82
77
Tol
HC(OMe)3
HC(OMe)3
HC(OMe)3
4-ClC6H4
O
O
1e
PhCH2CH2
10
5
HC(OMe)3
HC(OMe)3
Me
120
80
48
44
1fd
OMe
O
1ge
Ph
0.5
HC(OMe)3
10
74
OMe
Ph
2f
Tol
3
6
3
3
MeOTMS
EtOTMS
Me
10
10
5
74
97
81
83
3
3-BrC6H4
Ph
Et
4ag
4bg
R1
R1
=
OTMS
OTMS
4-ClC6H4
5
=
a Two equivalents of trialkylorthoformate or allyltrimethylsilane were used, unless otherwise mentioned.
b All products have been reported previously in the literature and were characterized by 1H and 13C NMR spectroscopy (see ref. 18e for entry
1e and 18f for all others).
c Refers to the yield of the isolated and purified product.
d Reaction was carried out using cyclohexanone as substrate; the full substrate and product are shown in this entry.
e Reaction was carried out using methallyltrimethylsilane in place of allyltrimethylsilane and 0.5 mol% TMSOTf; the full product is shown in
this entry.
f Reaction was carried out using 1.2 equivalents of MeOTMS and 1.2 equivalents of allyltrimethylsilane.
g Reaction was carried out using 1.2 equivalents of allyloxytrimethylsilane and 1.2 equivalents of allyltrimethylsilane.
product is needed on a large scale, this procedure is espe- ethers has been developed using TMSOTf as the catalyst
cially attractive because the product can be easily separat- in the ionic liquid [bmim][OTf].
ed from the ionic liquid using a separatory funnel. To the
best of our knowledge, this is the first example of a reac-
1-Butyl-3-methylimidazolium trifluoromethanesulfonate was pur-
tion in an ionic liquid wherein product isolation can be
achieved by taking advantage of the differential solubility
of the starting material and product in the ionic liquid. The
recovered ionic liquid was found to be effective in cata-
lyzing subsequent reactions, although with each run the
reaction took longer unless additional TMSOTf (1–2
mol%) was added, suggesting that some catalyst was be-
ing removed in the diethyl ether extractions.
chased from Acros Chemicals and stored in a vacuum desiccator
(15 mmHg) at r.t. It was dried prior to use at 80 °C (0.1 mmHg) for
12 h. Reaction progress was followed by gas chromatography using
a Varian CP 3800 capillary GC instrument equipped with a fused
silica column (30 m × 0.25 mm ID). Products were analyzed by
NMR spectroscopy on a JEOL 270 MHz (67.5 MHz for 13C NMR)
NMR spectrometer. Crude products were purified by flash column
chromatography using silica gel.
Methyl 1-Phenylbut-3-enyl Ether; Typical Procedure:
The results with aliphatic aldehydes were less promising
and the allylation required the use of 10 mol% catalyst. A
moderate yield of the expected homoallyl ether product
was obtained from 3-phenylpropionaldehyde (Table 1,
entry 1e). Similarly, the results were less promising using
cyclohexanone (Table 1, entry 1f) and complete conver-
sion to the homoallyl ether could not be obtained even
with increased catalyst loading. In summary, a mild meth-
od for the direct conversion of aldehydes into homoallyl
A mixture of benzaldehyde (0.509 g, 4.80 mmol), HC(OMe)3 (1.05
mL, 1.02 g, 9.59 mmol), and allyltrimethylsilane (1.51 mL, 1.10 g,
9.59 mmol) in [bmim][OTf] (2.0 mL, 2.59 g) was stirred at r.t. in a
flame-dried round-bottom flask under N2 as TMSOTf (43.4 mL,
0.2399 mmol) was added. After 10 min, the product (top layer) was
separated from the ionic liquid using a separatory funnel. The ionic
liquid was extracted with Et2O (3 × 5 mL). The combined Et2O ex-
tracts and the product layer were stirred with solid Na2CO3 (0.3 g)
for 5 min, and then filtered and concentrated on a rotary evaporator
to yield the crude product (1.13 g) as a clear, yellow oil [in an alter-
Synthesis 2005, No. 16, 2661–2663 © Thieme Stuttgart · New York

SHORT PAPER
Catalyzed One-Pot Conversion of Aldehydes to Homoallyl Ethers in an Ionic Liquid
2663
native workup, the reaction mixture was directly extracted with
Et2O (3 × 5 mL)]. The crude product was purified by column chro-
matography [silica gel (20 g), EtOAc–hexane 5:95] to yield the ho-
moallyl ether as a colorless oil; yield: 0.604 g (78%) (>98% pure by
1H and 13C NMR spectroscopy and GC). The recovered ionic liquid
(2.49 g) was stirred with powdered 4-Å sieves for 2 h, filtered, and
heated under vacuum overnight (80 °C, 0.1 mmHg) prior to re-use.
(7) Tanaka, H.; Yamashita, S.; Ikemoto, Y.; Torii, S.
Tetrahedron Lett. 1988, 14, 1721.
(8) Trehan, A.; Vij, A.; Walia, M.; Kaur, G.; Verma, R. D.;
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Mohan, R. S. Org. Lett. 2003, 5, 55.
Acknowledgment
(11) Hollis, T. K.; Robinson, N. P.; Whelan, J.; Bosnich, B.
We gratefully acknowledge funding from the National Science
Foundation (RUI grant) and Research Corporation (Cottrell College
Science Award). R.M. would also like to thank The Camille and
Henry Dreyfus Foundation for a teacher-scholar award.
Tetrahedron Lett. 1993, 34, 4309.
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Synthesis 2005, No. 16, 2661–2663 © Thieme Stuttgart · New York