Iron(III) p-toluenesulfonate catalyzed synthesis of homoallyl ethers from acetals and aldehydes

Article abstract of DOI:10.1016/j.tetlet.2007.10.026

Iron(III) p-toluenesulfonate, Fe(OTs)₃·6H₂O, is an inexpensive, versatile and commercially available catalyst for the allylation of acetals using allyltrimethylsilane to yield homoallyl ethers in moderate to good yields. The one-pot

Full text of DOI:10.1016/j.tetlet.2007.10.026

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Tetrahedron Letters 48 (2007) 8665–8667
Iron(III) p-toluenesulfonate catalyzed synthesis of homoallyl
ethers from acetals and aldehydes
Matthew J. Spafford, Erin D. Anderson, Joshua R. Lacey, Ann C. Palma
and Ram S. Mohan*
Laboratory for Environmentally Friendly Organic Synthesis, Department of Chemistry, Illinois Wesleyan University,
Bloomington, IL 61701, USA
Received 12 September 2007; revised 4 October 2007; accepted 4 October 2007
Available online 9 October 2007
Abstract—Iron(III) p-toluenesulfonate, Fe(OTs)3Æ6H2O, is an inexpensive, versatile and commercially available catalyst for the allyl-
ation of acetals using allyltrimethylsilane to yield homoallyl ethers in moderate to good yields. The one-pot conversion of aldehydes
to homoallyl ethers using alkoxysilanes has also been accomplished using Fe(OTs)3Æ6H2O as a catalyst. The use of mild reaction
conditions and a relatively non-corrosive catalyst make this method an attractive option for the synthesis of a range of homoallyl
ethers.
Ó 2007 Elsevier Ltd. All rights reserved.
The synthesis of homoallyl ethers from acetals and alde-
hydes has attracted considerable attention. The allyl-
ation of acetals using organosilicon reagents is a useful
method to generate homoallyl ethers and hence several
catalysts have been used to effect this transformation.
These include TiCl4,1 AlCl3,2 BF3ÆEt2O,2 trityl perchlo-
rate,3 diphenylboryltriflate,3 montmorillonite,4 trimethyl-
silylbis(fluorosulfonyl)imide,5 (CH3)3SiI,6 TMSOTf,7
TiCp2(CF3SO3)2,8 tris(p-bromophenyl)aminium hexa-
chloroantimonate,9 triarylpyrilium salts,10 trimethyl-
these methods require the use of corrosive catalysts such
as TMS triflate, TiCl4 or rather moisture-sensitive cata-
lysts such as AlCl3 or Sc(OTf)3. An environmentally
friendly approach to the synthesis of homoallyl benzyl
ethers in which the acetal is generated in situ from the
corresponding aldehyde using FeCl3 as the catalyst has
been reported.16 Our continued interest in developing
synthetic methodology using mild and efficient Lewis
acid catalysts17 prompted us to investigate the utility
of iron(III) p-toluenesulfonate, Fe(OTs)3Æ6H2O, as a
catalyst for allylation reactions using allyltrimethyl-
silane. Although the synthesis of several transition metal
tosylates has been described18 and iron(III) tosylate is
commercially available, there are only a few reports of
the use of transition metal tosylates as catalysts in
organic synthesis. Herein we report the use of iron(III)
tosylate as an efficient catalyst for the allylation of ace-
tals and also for the one-pot conversion of aldehydes to
a range of homoallyl ethers. The mild reaction condi-
tions and the use of a relatively non-toxic catalyst make
this method attractive for the synthesis of homoallyl
ethers.
silylbis(trifluoromethanesulfonyl)amide,11
BiBr3,12
Sc(OTf)3,13 and Bi(OTf)3ÆxH2O.14 Many acetals are not
commercially available and must be synthesized from
the corresponding aldehyde. In addition, most of these
methods only report allylation of dimethyl or diethyl
acetals, which results in the formation of homoallyl
methyl or ethyl ethers. Although the alkene moiety of
homoallyl alkyl ethers can be easily transformed into
other groups, alkyl ethers are somewhat inert and not
amenable to easy synthetic manipulation. Another dis-
advantage of this approach is that many acetals have
poor shelf lives. Hence the synthesis of homoallyl ethers
directly from aldehydes would be advantageous. How-
ever, there are very few reports of the one-pot syntheses
of homoallyl ethers from aldehydes.15 Further, many of
We first investigated the allylation of acetals using
allyltrimethylsilane catalyzed by Fe(OTs)3Æ6H2O (2.0–
10.0 mol %) in CH3CN at room temperature (Table 1).
The reaction worked with a range of acetals in relatively
short reaction times. The reactions worked better in
CH3CN than in CH2Cl2.
*
Corresponding author. Tel.: +1 309 556 3829; fax: +1 309 556
3864; e-mail: rmohan@iwu.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.026

8666
M. J. Spafford et al. / Tetrahedron Letters 48 (2007) 8665–8667
Table 1. Iron(III) tosylate catalyzed allylation of acetals19
OR2
Fe(OTs)3 6H2O
OR2
.
R1 OR2
R1
SiMe3
CH3CN, rt
Entry
R1a
R2
Allylsilane (equiv)
Mol % cat
Time
Yieldb,c (%)
1
2
3
4
5
6
Ph
Me
Et
Me
Me
Me
Me
2.0
1.5
1.5
1.5
2.0
1.7
10.0
5.0
5.0
2.0
5.0
2.0
1 h 20 min
1 h 10 min
4 h
1 h 30 min
18 h
7016
m-BrC6H4
p-ClC6H4
p-MeOC6H4
PhCH2–
89d,14b
7920
7616
61e,20
97d,21
CH3(CH2)10
–
19 h
OMe
7
Me
2.0
10.0
23 h
5520
OMe
a Acetals were purchased commercially or synthesized in laboratory by literature methods.22
b Isolated yield. Unless otherwise mentioned, the crude product was found to be >97% pure and hence further purification was deemed unnecessary.
c All products have been previously reported in the literature. Superscript numbers against yields refer to literature reference for the product.
d Purified by flash chromatography.
e Product was determined to be 95% pure by GC and NMR analysis.
The methodology was then extended to the one-pot
conversion of aldehydes to homoallyl ethers using
alkoxysilanes. Since a wide range of alkoxysilanes is
commercially available, this approach allowed the syn-
thesis of a variety of homoallyl ethers (Table 2). The
reaction worked with both aromatic and aliphatic alde-
hydes. The reaction of cinnamaldehyde (Table 2, entry
1d) afforded the corresponding monoallylation product.
In contrast, the use of TiCl4 as a catalyst for allylation
of cinnamaldehyde dimethyl acetal gave only a mixture
of diallylated products.2a Equal amounts of products
arose from both 1,2 and 1,4 additions. Although the
methodology was successful with cyclohexanone
dimethyl acetal (Table 1, entry 7), attempts to allylate
the dimethyl acetal of acetophenone resulted only in
deprotection of the acetal to give acetophenone. Cyclo-
hexanone (Table 2, entry 1f) was the only ketone with
which the one-pot methodology was successful. As can
be seen from Table 2, the methodology was successful
with several alkoxysilanes that allowed the convenient
Table 2. Iron(III) tosylate catalyzed conversion of aldehydes to homoallyl ethers23
R2OSiMe3
O
OR2
R1
H
R1
SiMe3
Fe(OTs)3.6H2O
CH3CN, rt
Entry
R1
Alkoxysilane R2 (equiv)
Allylsilane (equiv)
Mol % cat
Time
Yielda,b (%)
1a
1b
p-MeOC6H4
p-ClC6H4
Me, 2.0
Me, 1.5
2.0
1.7
1.0
5.0
2 h
1 h 30 min
79
86c
O
1c
Me, 1.7
1.7
5.0
5 h
69c,d,10
O
1d
1e
PhCH=CH
CH3(CH2)8–
Me, 1.7
Me, 2.0
1.7
2.0
5.0
5.0
15 h 15 min
18 h
6616
71c,24
1f
Me, 2.0
2.0
10.0
5 h
57
O
2
m-BrC6H4
p-BrC6H4
p-MeO2CC6H4
Ph
Et, 2.0
2.0
2.0
2.0
1.5
1.5
1.0
10.0
5.0
5.0
5.0
19 h
15 h 15 min
16 h
3 h
21 h
84
3a
3b
4a
4b
PhCH2, 2.0
PhCH2, 2.0
Allyl, 1.5
Allyl, 1.5
77c,16
72c,16
8825
8425
p-ClC6H4
a Isolated yield. Unless otherwise mentioned, the crude product was found to be >97% pure and hence further purification was deemed unnecessary.
b All products have been previously reported in the literature. Superscript number against yield refers to literature reference for the product.
c Purified by flash chromatography.
d Although the product has been reported in the literature, its spectral data have not been previously reported (CDCl3, 270 MHz) d 2.32–2.39 (m,
1H), 2.47–2.55 (m, 1H), 3.18 (s, 3H), 4.05 (t, 1H, J = 6.18 Hz), 4.97–5.06 (m, 2H), 5.71–5.78 (m, 1H), 5.94 (s, 2H), 6.72–6.80 (m, 3H); 13C NMR (12
peaks) (67.5 MHz) d 147.77, 146.91, 135.61, 134.67, 120.25, 116.79, 107.82, 106.67, 100.86, 83.31, 56.32, 42.42.

M. J. Spafford et al. / Tetrahedron Letters 48 (2007) 8665–8667
8667
Tetrahedron Lett. 1991, 32, 4779; (c) Wang, M. W.; Chen,
Y. J.; Wang, D. Synlett 2000, 385.
16. Watahiki, T.; Akabane, Y.; Mori, S.; Oriyama, T. Org.
Lett. 2003, 5, 3045.
synthesis of homoallyl methyl ethers, homoallyl ethyl
ethers, homoallyl allyl ethers and homoallyl benzyl
ethers.
17. We have previously reported the bismuth(III) triflate
catalyzed allylation of acetals and aldehydes (Ref. 14).
Bismuth compounds are environmentally friendly cata-
lysts because of their remarkably low toxicity.
18. Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisberger, A.;
Dunbar, K. R. In Inorganic Syntheses; Coucouvanis, D.,
Ed.; Wiley-Interscience: New York, NY, 2002; Vol. 33,
Chapter 2, p 75.
In summary, the synthesis of a range of homoallyl ethers
from acetals and aldehydes has been accomplished using
a commercially available, inexpensive and relatively
non-corrosive catalyst, iron(III) tosylate.
Acknowledgement
19. Typical procedure for allylation of acetals: A solution of
p-anisaldehyde dimethyl acetal (1.128 g, 6.190 mmol) in
CH3CN (20.0 mL) was stirred at rt as allyltrimethylsilane
(1.060 g, 1.48 mL, 9.277 mmol, 1.50 equiv) and Fe(OTs)3Æ
6H2O (0.0838 g, 0.1237 mmol, 2.0 mol %) were added.
The heterogeneous mixture was stirred at rt and the
reaction progress was followed by GC. After 1 h 30 min,
the reaction mixture was concentrated on a rotary
evaporator and the residue was partitioned between ether
(20.0 mL) and aqueous 10% Na2CO3 (15.0 mL). The
aqueous layer was extracted again with ether (20.0 mL)
and the combined organic extracts were washed with
aqueous saturated NaCl (15.0 mL), dried (Na2SO4) and
concentrated on a rotary evaporator to yield 1.124 g of a
yellow oil. The crude product was purified by flash
chromatography on 50 g of silica gel (EtOAc/hexanes, 5/
95) to yield 0.899 g (76%) of the homoallyl methyl ether
product as a colorless liquid. The product was deter-
mined to be >98% pure by GC, 1H and 13C NMR
spectroscopy.
The authors gratefully acknowledge funding from the
National Science Foundation for a RUI (Research
in Undergraduate Institutions) Grant (# 0650682)
awarded to R.S.M.
Supplementary data
1
General experimental section and copies of H NMR
and 13C NMR spectra of all products. Supplementary
data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2007.10.026.
References and notes
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23. Typical procedure for the one-pot conversion of aldehydes to
homoallyl ethers: A solution of p-bromobenzaldehyde
(2.0320 g, 10.982 mmol) in CH3CN (40.0 mL) was stirred
at rt as allyltrimethylsilane (2.509 g, 3.49 mL, 21.959
mmol, 2.0 equiv), benzyloxytrimethylsilane (3.961 g,
4.32 mL, 21.967 mmol, 2.0 equiv), and Fe(OTs)3Æ6H2O
(0.7441 g, 1.0983 mmol, 10.0 mol %) were added sequen-
tially. The heterogeneous mixture was stirred at rt and the
reaction progress was followed by GC. After 15 h 15 min,
the reaction mixture was concentrated on a rotary
evaporator and the residue was partitioned between ether
(25.0 mL) and aqueous 10% Na2CO3 (20.0 mL). The
aqueous layer was extracted again with ether (25.0 mL)
and the combined organic extracts were washed with
aqueous saturated NaCl (20.0 mL), dried (Na2SO4) and
concentrated on a rotary evaporator to yield 4.4105 g of a
brownish liquid. The crude product was purified by flash
chromatography on 250 g of silica gel (EtOAc/hexanes, 3/
97) to yield 2.670 g (77%) of the homoallyl benzyl ether
product as a colorless liquid. The product was determined
1
to be >97% pure by GC, H and 13C NMR spectroscopy.
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