506
Published on the web April 20, 2011
Ionic Liquid-accelerated Allylation of Carbonyl Compounds with a Catalytic Amount
of Indium Generated from In Situ Reduction of InCl3 with Aluminum
Tsunehisa Hirashita,* Yuki Sato, Dai Yamada, Fusako Takahashi, and Shuki Araki
Omohi College, Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555
(Received February 9, 2011; CL-110110; E-mail: hirasita@nitech.ac.jp)
Table 1. Allylation of carbonyl compounds in ionic liquida
In ionic liquids allylation of carbonyl compounds with a
InCl3 (10 mol%)
Al(0) (160 mol%)
catalytic amount of indium(III) trichloride proceeds smoothly to
give the corresponding homoallylic alcohol in the presence of
aluminium. The corresponding dimeric ethers are also formed.
O
OH
Br
+
Ph
H
Ph
solvent, rt
2
1
3
160 mol%
Entry
Solvent
Time/h
Yield/%
Indium-mediated allylations of carbonyl compounds have
been extensively studied during the last two decades.1-4
Allylindium reagents can be prepared from stoichiometric
amounts of metallic indium and allylic halides in polar solvents,
including water, and can be used for allylation of carbonyl
compounds in a Barbier-type manner. We previously reported
the allylation of carbonyl compounds with a catalytic amount of
indium(III) trichloride and a stoichiometric amount of alumi-
nium(0) in aqueous THF.5 This method permits us an easy
access to reactions of allylic indium reagents with a reduced
amount of relatively expensive indium metal. However, this
allylation proceeds slowly; a couple of days are needed to
complete the reaction. Ionic liquids have emerged in organic
synthesis as a recyclable solvent and reactions of organometallic
reagents with carbonyl compounds can be performed in ionic
liquids.6-9 Although formation of allylindium reagents in ionic
liquids has been reported,10-12 allylation with a catalytic amount
of indium has not appeared.
1
2b
3
4
5
6
7
8c
bmim¢PF6
bmim¢PF6
100
8
15
7
0.5
4
4
0
0
95
72
100
69
bmim¢PF6-H2O (5:1)
bmim¢PF6-H2O (5:1)
bmim¢PF6-H2O (5:2)
bmim¢PF6-H2O (1:1)
H2O
trace
0
bmim¢PF6-H2O (5:2)
0.5
aAll reactions were carried out with allyl bromide (1.6 mmol),
benzaldehyde (1.0 mmol), InCl3 (0.10 mmol), and Al (1.6
mmol) in bmim¢PF6 (1 mL) and water. bWith In(0) powder
c
(100 mol %). Without InCl3.
Table 2. Allylation of benzaldehyde in ionic liquids
InCl3 (10 mol%)
Al(0) (160 mol%)
+
1
2
3
Ionic liquid-H2O (5:2)
rt, 0.5 h
160 mol%
We initiated a reaction of allyl bromide and benzaldehyde
with a catalytic amount of indium(III) trichloride in 1-butyl-3-
methylimidazolium hexafluorophosphate (bmim¢PF6). The re-
action, just changing the solvent from aqueous THF as we
reported previously, did not proceed (Table 1, Entry 1). Even a
stoichiometric amount of metallic indium gave no coupling
product (Entry 2). Addition of water was found to show a
significant effect on the allylation; the biphase reaction with a
proper amount of water afforded the corresponding homoallylic
alcohol 3 in high yields in shorter time than those performed in
aqueous THF (Entries 3-6). The use of water alone inhibited
the allylation (Entry 7). Without InCl3, no reaction proceeded
(Entry 8).
We next screened ionic liquids for this allylation (Table 2).
Bmim¢BF4 and 1-ethyl-3-methylimidazolium tetrafluoroborate
(emim¢BF4) gave 3 in good yields similar to that performed in
bmim¢PF6 (Table 2, Entries 1 and 2). 1-Butyl-4-methylpyridi-
nium hexafluorophosphate (bpy¢PF6) is also operative (Entry 3).
In contrast, the reaction in bis(trifluoromethylsulfonyl)imide
(NTf2) salts did not give 3 (Entries 4 and 5).
Entry
Ionic liquid
Yield/%
1
2
3
4
5
bmim¢BF4
emim¢BF4
bpy¢PF6
bmim¢Tf2N
Et2Me(MeOCH2CH2)N¢Tf2N
79
67
78
0
0
treatment entailed the addition of a catalytic amount of InCl3
prior to the next reaction. Although it is difficult to selectively
remove the aluminium hydroxide while maintaining the
indium(III) salt in the ionic liquid at this stage, the ionic liquid
can be recycled and reused for this allylation (1st: 86%, 2nd:
99%, 3rd: 81%).
The reaction of crotyl bromide with benzaldehyde in
bmim¢PF6 gave the corresponding homoallylic alcohol 4 with
modest diastereoselectivity (eq 1). Allylic indium reagents favor
intermolecular chelation with an oxygenated functional group
and a pyridyl moiety giving chelation-controlled products even
in the presence of water.13 Salicylaldehyde and 2-pyridine-
carbaldehyde often serve as good model compounds for this
purpose;14 however, the former gave modest syn-selectivity
similar to the reaction of benzaldehyde and with the latter
aldehyde the reaction did not proceed, indicating that chelation
control may not be operative under the present conditions.
Recycling the ionic liquid and the catalyst was examined.
The second use of the ionic liquid resulted in a low yield (40%)
and the third recycling gave no coupling product. After repeating
the allylation, the viscosity of the ionic liquid became higher. To
circumvent this problem, the ionic liquid must be washed with
water to remove the resulting aluminium hydroxide and this
Chem. Lett. 2011, 40, 506-507
© 2011 The Chemical Society of Japan
Hirashita, Tsunehisa
