Iodine-catalyzed one-pot three-component synthesis of homoallyl benzyl ethers from aldehydes

Article abstract of DOI:10.1016/j.tetlet.2009.02.052

Iodine has been found to be an effective catalyst for one-pot synthesis of homoallyl benzyl ethers under mild reaction conditions. Various homoallyl benzyl ethers were synthesized in moderate to high yield by three-component condensation of aldehydes, ben

Full text of DOI:10.1016/j.tetlet.2009.02.052

Tetrahedron Letters 50 (2009) 1958–1960
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Tetrahedron Letters
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Iodine-catalyzed one-pot three-component synthesis of homoallyl
benzyl ethers from aldehydes
*
Dolly Kataki, Prodeep Phukan
Department of Chemistry, Gauhati University, Guwahati 781 014, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Iodine has been found to be an effective catalyst for one-pot synthesis of homoallyl benzyl ethers under
mild reaction conditions. Various homoallyl benzyl ethers were synthesized in moderate to high yield by
three-component condensation of aldehydes, benzyloxytrimethylsilane, and allyltrimethylsilane in pres-
ence of iodine (10 mol %) in dichloromethane at 0 °C.
Received 19 December 2008
Revised 1 February 2009
Accepted 9 February 2009
Available online 12 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Iodine
Aldehydes
Benzyloxytrimethylsilane
Allyltrimethylsilane
Homoallyl benzyl ether
The reaction of an allyltrimethylsilane with a carbonyl com-
pound in the presence of Lewis acid, known as the Sakurai–Hosomi
reaction,1 has been extensively studied and successfully applied in
organic synthesis.2 The allylation of acetals using allyltrimethylsi-
lane3 is a useful method because the resulting homoallylethers and
homoallyl acetates can be subjected to further synthetic manipula-
tion. Synthesis of homoallylethers from aldehydes can also be
achieved by using an alkoxysilane and allyltrimethylsilane in pres-
ence of a catalytic amount of iodotrimethylsilane.4 Seebach had
developed another variation of the method for synthesis of homo-
allylethers by treating an aldehyde with dialkoxydichlorotitanium
and subsequent addition of allyltrimethylsilane.5 Several catalysts
such as TiCl4,3 AlCl3,6 BF3ÁEt2O,6 tritylperchlorate,7 diphenylboryl
triflate,7 montmorillonite,8 Pb/Al,9 (CH3)3SiI,10 TMSOTf,11 TiCp2(CF3-
SO3)2,12 trimethylsilylbis(fluorosulfonyl)imide,13 CF3COOH,14 BiBr3,15
Sc(OTf)3,16 indium metal,17 TMSNTf2,18 Bi(OTf)3ÁxH2O19 and TMSOF20
have been used to investigate the reaction of acetals with allyltrimeth-
ylsilane. However, these methods have several drawbacks. For exam-
ple: (1) synthesis of acetals from corresponding benzaldehyde is a
prerequisite as most of these acetals are not commercially available
and further synthetic manipulation of the homoallylmethyl or ethyl
ether is not practicable due to the inactivity of the aliphatic ether link-
age; (2) many of these methods use corrosive and moisture sensitive
catalysts such as TMS; (3) Metal triflates are expensive and toxic. In
most of the organic synthetic methods, benzyl ethers are preferred
to their alkyl counterparts as the benzyl group can be easily deprotec-
ted. However, the synthesis of homoallyl benzyl ethers has received
scant attention in the literature, with only a few methods out-
lined.20–24 Recently, Mohan et. al. developed a three-component pro-
cedure for the synthesis of homoallyl benzyl ethers using Bi(OTf)3 as a
catalyst.24 However, bismuth triflate is expensive and toxic. In recent
years, iodine is finding extensive applications as a catalyst for organic
transformations.25 The advantages in the use of iodine are (i) mild
neutral conditions (ii) cost effectiveness and ready availability (iii)
no stringent dry conditions required. As a part of our ongoing work
on iodine-catalyzed reactions,26 we report herein an one-pot three-
component method for the synthesis of homoallyl benzyl ether from
aldehydes using iodine as a catalyst (Scheme 1).
Initially, we investigated the allylation reaction under various
conditions for benzaldehyde as a model substrate. In order to
accomplish the synthesis, allyltrimethyl silane (1.2 mmol) was
added to a solution of benzaldehyde (1 mmol), benzyloxytrimeth-
ylsilane (1.2 mmol), and iodine (0.1 mmol) in acetonitrile at room
temperature. After 24 h of reaction at room temperature, the reac-
tion afforded the corresponding homoallyl benzyl ether in 60%
yield (Table 1, entry 1). To optimize the reaction conditions, a
screening was performed on several parameters such as solvents,
catalyst concentration, and the amount of benzyloxytrimethylsi-
lane and allyltrimethylsilane. Initially, the reaction was examined
by taking different solvents (Table 1). Dichloromethane was found
to be superior to other solvents.
Thereafter, the reaction was examined carefully under the
influence of different concentrations of the catalyst using dichlo-
romethane as solvent (Table 2). The use of 10 mol % of iodine
(based on benzaldehyde) gave the best result with a yield of
* Corresponding author. Tel.: +91 9435111114.
E-mail address: pphukan@yahoo.com (P. Phukan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.052

D. Kataki, P. Phukan / Tetrahedron Letters 50 (2009) 1958–1960
1959
O
I2 (10 mol%)
OSiMe3
O
SiMe3
+
H
+
R
DCM, 0 oC
R
Scheme 1.
Table 1
Synthesis of homoallyl benzyl ether in different solventsa
Table 3
Synthesis of homoallyl benzyl ethers using iodine as catalysta
Entry
Solvent
Time (h)
Yieldb (%)
Entry
Aldehyde
Time (min)
Yieldb (%)
86
1
2
3
4
5
6
7
CH3CN
CH2Cl2
CH3OH
Diethyl ether
CH3NO2
DMF
24
24
24
24
24
24
24
60
68
0
0
43
0
CHO
1
45
CHO
CHO
CHO
2
3
4
5
6
7
8
9
45
60
60
45
45
40
45
40
90
76
74
78
85
80
76
77
82
65
THF
0
Br
F
a
Reaction condition: benzaldehyde (1 mmol), benzyloxytrimethylsilane
(1.2 mmol), iodine (0.1 mmol), allyltrimethylsilane (1.2 mmol), solvent (2 mL), rt.
b
Isolated yield after chromatographic purification.
Table 2
Synthesis of homoallyl benzyl ether in dichloromethane under different conditions
Cl
Entry Benzaldehyde Benzyloxy-
Allyltri-
Iodine Temp Time Yield
(min) (%)
CHO
(mmol)
trimethylsilane methylsilane (mol %) (°C)
(mM)
(mM)
Cl
CHO
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1.2
1.2
1.2
1.1
1.1
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.5
10
20
5
10
10
10
10
27
27
27
27
0
210
120
300
210
40
67
52
45
70
82
86
75
Cl
0
0
45
30
CHO
CHO
67% after 3 h of reaction. Neither decreasing nor increasing the
iodine concentration helped in improving the yield. Although
the optimal amount of catalyst was found to be 10 mol %, the
yield was not satisfactory. Hence, the reaction was further stud-
ied by lowering the temperature to 0 °C. Surprisingly, the reac-
tion worked in a much better way at 0 °C. The reaction was
complete in 45 min with 86% yield of the corresponding homo-
allyl benzyl ether. Finally, the best result was obtained when
the ratio of benzaldehyde, benzyloxytrimethylsilane, allyltri-
methylsilane, and iodine was 1:1.2:1.2:0.1 at 0 °C in dichloro-
methane (Table 2, entry 6).
MeO
O2N
CHO
CHO
10
a
Reaction condition: aldehyde (1 mmol), benzyloxytrimethylsilane (1.2 mmol),
Next, we examined the scope of the reaction using various
aldehydes and the results are summarized in Table 3. In general,
high yields of homoallyl benzyl ethers were obtained with
10 mol % of iodine at 0 °C in dichloromethane.27 Aromatic sub-
strates bearing functional groups such as –CH3, –OMe, –Cl, –Br,
–NO2, and –F all reacted successfully to give the corresponding
homoallyl benzyl ethers (Table 3) in high yields irrespective of
the substituent position on the aromatic ring. This reaction
works with aliphatic aldehyde such as cinnamaldehyde but the
yield was relatively lower.
iodine (0.1 mmol), allyltrimethylsilane (1.2 mmol), dichloromethane (2 mL), 0 °C.
b
Isolated yield after chromatographic purification.
Acknowledgments
Financial support from the University Grants Commission
(Grant No. UGC-30-46/2004/SR) is gratefully acknowledged. D.K.
thanks CSIR (India) for a senior research fellowship.
In conclusion, we have developed an efficient protocol for the
one-pot three-component synthesis of homoallyl benzyl ether
starting from aldehyde in the presence of iodine as a catalyst.
The merits of this method are (a) very simple, one-pot, and high
yielding process which eliminates the need to isolate the interme-
diate and thus minimizes waste; (b) very cheap and easily avail-
able iodine; (c) low amount of catalyst (10%); (d) absence of
toxicity; (e) reactions are carried out in air since iodine is not mois-
ture sensitive. This method has wide scope for further applications
as the catalyst is cheap and easily available commercially.
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benzyloxytrimethylsilane (1.2 mmol) was added followed by the addition of
allyltrimethylsilane (1.2 mmol). The mixture was stirred at 0 °C for appropriate
time (TLC). After completion of the reaction, sodium thiosulfate (20 mg
approximately) was added and the reaction mixture was stirred for 20 min.
It was then extracted with ethylacetate, washed with brine, dried (Na2SO4),
and concentrated. Purification of the crude product by chromatography on
silica gel (60–120 mesh) with petroleum ether–EtOAc (5%) as eluent gave the
pure product.
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