TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6391–6394
The catalytic Friedel–Crafts alkylation reaction of aromatic
compounds with benzyl or allyl silyl ethers using
Cl2Si(OTf)2 or Hf(OTf)4
Isamu Shiina* and Masahiko Suzuki
Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601,
Japan
Received 30 May 2002; revised 1 July 2002; accepted 5 July 2002
Abstract—The Friedel–Crafts alkylation reaction of various aromatic compounds with benzyl or allyl silyl ethers is effectively
promoted under mild reaction conditions using Lewis acid catalysts. A mixture of the desired phenyltolylmethanes is obtained in
80% yield from toluene with benzyl dimethylsilyl or trimethylsilyl ether at 50°C in the presence of a catalytic amount of
Cl2Si(OTf)2 or Hf(OTf)4. © 2002 Elsevier Science Ltd. All rights reserved.
The Friedel–Crafts reaction is one of the most funda-
mental and useful reactions in synthetic organic chem-
istry and is widely applied to the synthesis of
substituted arenes via the electrophilic addition of
cationic species to aromatic compounds.1 In the
Friedel–Crafts alkylation reaction, alkyl halides have
been employed as one of the suitable electrophiles for
the synthesis of various aromatic compounds due to
their appropriate stability and reactivity. Although ben-
zyl or allyl alcohols have also been used for the synthe-
sis of the corresponding alkylated compounds by the
Friedel–Crafts alkylation reaction of aromatic com-
pounds, there is an obvious limitation because the
reaction employing alcohols usually requires a high
temperature even if it is accelerated by acid catalysts
such as TeCl4,2 Nafion-H,3 and Sc(OTf)3.4,5 Further-
more, it is necessary to use an excess amount of ordi-
nary Lewis acids except Sc(OTf)3 to produce the
alkylated compounds in high yields since free hydroxyl
groups in the electrophiles sometimes deactivate the
catalysts.
ing alkyl silyl ethers as electrophiles because the
catalysts are not deactivated by the electrophiles in this
case. From such a viewpoint, our laboratory developed
an efficient esterification reaction of silyl carboxylates
with alkyl silyl ethers using a catalytic amount of
Cl2Si(OTf)2 in 1992.6 In 1997, Mikami et al. reported
the Friedel–Crafts alkylation reaction of anisole with
three kinds of alkyl silyl ethers using TMSNTf2 as a
Lewis acid catalyst.7 Though the benzylation of anisole
with benzyl trimethylsilyl ethers and allylation of
anisole with an allyl trimethylsilyl ether gave satisfac-
tory yields, stoichiometric amounts of TMSNTf2 were
required in these cases.
Furthermore, Baba et al. developed the reductive
Friedel–Crafts alkylation of aromatics using carbonyl
compounds by the promotion of InCl3 in 1998.8 In this
reaction, intermediary alkyl chlorodimethylsilyl ethers
were generated in situ and the successive Friedel–Crafts
reaction of the formed silyl ethers was carried out.
However, it was mentioned in their reports that tri-
ethylsilyl ethers reacted with only very active nucle-
ophiles such as anisole since the activity of the
On the other hand, there have not been many trials for
the development of the Friedel–Crafts alkylation reac-
tion using alkyl silyl ethers as electrophiles since it is
not predicted that siloxy groups work as effective leav-
ing groups for the substitution reaction. However, it is
expected that Lewis acid catalysts more efficiently pro-
mote the reaction under milder conditions by employ-
trialkylsilyl
ethers
is
lower
than
that
of
chlorodimethylsilyl ethers when using InCl3 as a
catalyst.
As far as we know, there has not been a systematic
study of the Friedel–Crafts alkylation reaction of aro-
matic compounds with alkyl silyl ethers. Therefore, we
now report the development of an effective Friedel–
Crafts alkylation utilizing benzyl or allyl silyl ethers
* Corresponding author. Fax: (+81)-3-3260-5609; e-mail: shiina@
ch.kagu.sut.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01376-X