Indium compound-catalyzed deoxygenative allylation of aromatic ketones by a hydrosilane-allylsilane system

Article abstract of DOI:10.1016/S0040-4039(00)00178-7

A combination of chlorodimethylsilane and allyltrimethylsilane effectively promoted the deoxygenative allylation of aromatic ketones in the presence of a catalytic amount of an indium compound to give the terminal alkenes. Indium trihalide or metallic indium showed high catalytic activity. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00178-7

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TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2425–2428
Indium compound-catalyzed deoxygenative allylation of aromatic
ketones by a hydrosilane–allylsilane system
Makoto Yasuda, Yoshiyuki Onishi, Takeshi Ito and Akio Baba ∗
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 5 January 2000; revised 20 January 2000; accepted 21 January 2000
Abstract
A combination of chlorodimethylsilane and allyltrimethylsilane effectively promoted the deoxygenative allyl-
ation of aromatic ketones in the presence of a catalytic amount of an indium compound to give the terminal alkenes.
Indium trihalide or metallic indium showed high catalytic activity. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: indium and compounds; silicon and compounds; allylation; deoxygenation; reduction.
Synthetic applications of metallic indium have attracted much attention during the last decade1 due
to its low ionic potential2 and stability under aqueous conditions.3 The synthetic potential of indium(III)
halides is also increasing.4 However, they are used in an analogous manner to that of conventional Lewis
acids such as aluminum halides and boron halides. There are few synthetic applications peculiar to indium
compounds. We have recently reported the InCl3-catalyzed reductive Friedel–Crafts alkylation5,6 where
InCl3 promotes the hydrosilylation of carbonyl compounds with chlorodimethylsilane and the successive
deoxygenation led to the carbocationic intermediate which reacts with aromatics. When an excess
amount of chlorodimethylsilane was added, the cation was reduced to the corresponding hydrocarbons
as previously reported.7 If the resulting carbocation could be trapped by carbon nucleophiles other than
aromatics, a novel protocol of deoxygenative functionalization of ketones could be developed.
In this communication, we report a case of trapping the cation by an allyl group, where indium
trichloride characteristically catalyzed the deoxygenative allylation of ketones by using allylsilanes and
chlorodimethylsilane.
(1)
At first, we examined the catalysts in the reaction of acetophenone, chlorodimethylsilane and allyltri-
methylsilane (Table 1). Indium trichloride effectively promoted the deoxygenative allylation to furnish
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00178-7
tetl 16458

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4-phenyl-1-pentene (1) in 86% yield for 2 h (entry 1). It is interesting that even metallic indium gave a
moderate yield (41%, entry 4) which could be improved up to 62% under a longer reaction time (entry
5). In contrast, neither AlCl3, a representative Friedel–Crafts catalyst, nor Sc(OTf)3, a characteristic
promoter for reductive Friedel–Crafts reaction,6c–e gave the desired allylated product (entries 6 and 7).
Of all the Lewis acids examined, ZnCl2, BF3·OEt2, and CuCl2 did not show any catalytic effect at all
under similar conditions. These results indicate that indium trichloride is an indispensable catalyst for
this type of allylation.
(2)
Table 1
The choice of solvent was also found to be definitely significant in this deoxygenative allylation
as shown in Table 1 (entries 1 and 8–14). The reaction with acetophenone proceeded only when
dichloromethane or 1,2-dichloroethane was used as a solvent. In contrast, the use of cyclohexane, DMF
or THF completely suppressed the reaction, where the starting ketone was recovered (entries 9–11). Even
other haloalkanes such as carbon tetrachloride and chloroform were unsuitable solvents (entries 13 and
14).
(3)
Table 2

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2427
We then explored the generality of the InCl3-catalyzed deoxgenative allylation of aromatic ketones
and the results are summarized in Table 2. The allylation of various types of aromatic ketones proceeded
in moderate to excellent yields. Benzophenone, particularly, gave a quantitative yield at ambient tempe-
rature. The functionalities such as nitro, ester or chlorine were tolerated under these reducing conditions
even at high temperature to give the corresponding olefins selectively (entries 3–6). The reaction with
alkylketone gave a complicated mixture including a trace amount of a deoxygenative allylation product
(entry 7). A harshly exothermic reaction took place with the use of benzaldehyde to give a complicated
mixture (entry 8), in which side reactions such as direct allylation of the aldehyde, hydrosilylation and
reductive Friedel–Crafts alkylation would simultaneously occur.5
A general procedure is as follows: acetophenone (2.0 mmol) was added to a mixture of indium
trichloride (0.1 mmol), chlorodimethylsilane (2.2 mmol) and allyltrimethylsilane (2.2 mmol) in dichlo-
romethane (10 mL) at ambient temperature. The reaction mixture was stirred for 2 h, quenched by
water (20 mL), and extracted with ether (20 mL×3). After the organic layer was dried over MgSO4
and concentrated in vacuo, the residue was purified by chromatography (eluted by hexane) on silica gel
to give the product 1.
We propose a plausible reaction path as illustrated in Scheme 1. At first, Lewis acid-promoted
hydrosilylation of ketone takes place to form a silyl ether 8. Then desiloxylation is accelerated by
InCl3 (via 8a or 8b) to generate the cation 9 (or a cationic species),8 which is readily trapped by
allyltrimethysilane. It is pointed out that the reaction (Table 1, entry 1) was accompanied with only a
small amount (8%) of ethylbenzene formed by the trapping of the cation 9 with chlorodimethylsilane
instead of allyltrimethylsilane. The stability of the cation intermediate should be an important factor,
because p-nitrobenzophenone, which is facilely susceptible to hydrosilylation showed more sluggish
reaction than benzophenone. This hypothesis is also supported by the fact that aromatic ketones gave
higher yields than alkylketones or aldehydes. The diphenylmethyl cation derived from benzophenone, in
particular, is very stable to accomplish this reaction course.
Scheme 1. Plausible reaction paths of the deoxygenative allylation
Another possible path in which the allylation is followed by deoxygenative reduction (path 2) can
be considered. However, no direct allylation of ketones with allyltrimethylsilane was observed in the
presence of an InCl3 catalyst. In addition, no formation of double allylated compounds, which should be
facilely formed by the second allylation,9 was observed. These results rule out path 2.
In conclusion, a novel deoxygenative allylation of aromatic ketones, namely, a selective transfunc-
tionalization from carbonyl oxygen into hydrogen and an allyl group, was achieved by using allyltri-
methylsilane and chlorodimethylsilane in the presence of a catalytic amount of an indium catalyst. The

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2428
disclosure of the detailed role of indium trichloride and further extension of this method are now in
progress.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education,
Science, Sports, and Culture, Japan.
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