LETTER
Reductive Deoxygenation of Aryl Ketones and sec-Benzylic Alcohols
183
The aforementioned procedure was found to be readily
adaptable to the corresponding benzylic alcohols such as
benzhydrol and 1-phenyl-1-ethanol (scheme 1). To the
CH2Cl2 solution of Me2SiClH (2.7 equiv) and 5 mol% of
InCl3, sec-benzylic alcohol was slowly added. This reac-
tion also completed at 25 °C within 2 h to afford over 80%
yields of reduction products.
Although, the reaction mechanism has not been verified
yet, hydrosilylation, the successive generation of carbo-
cation by desiloxylation, and reduction of the cation may
be involved. The fast and selective reduction can explain
the depression of the Friedel-Crafts alkylation of prod-
ucts. In addition, the course of reduction of benzylic chlo-
ride could not be excluded because a benzylic chloride
was isolated in the reaction of p-cyanoacetophenone (Ta-
ble 2, entry 13). This reaction mechanism is an analogue
of our reported one (ref. 5).
Thus, we found the novel catalytic ability of indium com-
pounds. The deoxygenation of aryl ketones and the sec-
benzylic alcohols proceeded with sufficient yields. It is
advantage of this indium catalyst system that such substi-
tuents halide, ester, ether, and nitro group tolerated in this
reduction conditions.
Acknowledgement
This work was supported by the Grant-in-Aid for Science Research
from the Ministry of Education, Science, Sports and Culture of Ja-
pan.
AlCl3 with trialkylsilane has been reported.8 3-Benzoyl-
propionic acid methyl ester furnished 1-phenylbutyric
acid methyl ester in 77% yield and no reduction of ester
group took place (entry 9). In contrast, in the cases of alu-
minum hydride or boron hydride with Lewis or Brønsted
acids, the reduction and hydrolysis of ester groups were
frequently observed as side reactions.2,3 It was a little sur-
prising that no Friedel-Crafts alkylation of the resulting
alkylated aromatics was detected in almost runs, because
we revealed the alkylation of aromatics by the similar
system.5 Acetophenone exceptionally gave 19% yield of
a Friedel-Crafts adduct (entry 2). However, p-Methoxy-
acetophenone gave p-methoxyethylbenzene in 61% yield
with no Friedel-Crafts product, and the rest of 2j was un-
changed (entry 10). Although, the reason why successive
Friedel-Crafts alkylation did not proceed is not clear, the
reaction with 2j was very sluggish. It is interesting that the
reaction of p-nitrobenzophenone afforded 3l in 80% yield
and did not lead to the reduction of nitro group (entry 12),
because indium metal promoted selective reduction of
nitro group was reported lately.6d This catalyst system
could reduce even alkyl ketone 2n though a low yield (en-
try 14).
References and Notes
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Scheme 1
Synlett 1999, No. 2, 182–184 ISSN 0936-5214 © Thieme Stuttgart · New York