Mild and efficient rhodium-catalyzed deoxygenation of ketones to alkanes

Article abstract of DOI:10.1039/c9nj02954k

A new and simple method for the deoxygenation of ketones to alkanes is presented. Most substrates are reduced under mild conditions by triethylsilane in the presence of catalytic amounts of [Rh(μ-Cl)(CO)₂]₂. This system selectively provides the methylene hydrocarbons in good to excellent yields starting from acetophenones and diaryl ketones. A rapid examination of the reaction pathway suggests that the ketone is first converted into an alcohol, which then undergoes hydrogenolysis to give the alkane.

Full text of DOI:10.1039/c9nj02954k

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Mild and efficient rhodium-catalyzed deoxygenation of ketones to
alkanes
Gilles Argouarch*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A new and simple method for the deoxygenation of ketones to molybdenum complexes were used as well and the
alkanes is presented. Most substrates are reduced under mild deoxygenation of ketones was achieved by PhSiH3 with catalytic
conditions by triethylsilane in the presence of catalytic amounts of amounts of [MoCl2O2(H2O)2],8 and [ReOCl3(SMe2)(OPPh3)] or
[Rh(-Cl)(CO)2]2. This system selectively provides the methylene [ReOCl3(PPh3)2].9 Finally, an oxazolinylborate-coordinated
hydrocarbons in good to excellent yields starting from rhodium silylene, highly active in the deoxygenation of esters to
acetophenones and diaryl ketones. A rapid examination of the ethers, was briefly tested on ketones but led to the formation
reaction pathway suggests that the ketone is first converted in of alkanes with modest yields.10
alcohol which then undergoes hydrogenolysis to give the alkane.
Herein we disclose that the commercially available rhodium(I)
dimeric complex [Rh( -Cl)(CO)2]2 constitutes a simple and

Over a century ago the reactions of Clemmensen and Wolff-
Kishner opened the way to the transformation of ketones to
alkanes through carbonyl-to-methylene complete reduction.
Unfortunately, these methodologies were suffering from harsh
reaction conditions making them incompatible to a large panel
of functional groups. In the search for mild and less toxic
reactions that reduce ketones into methylenes, later advances
were made by the use of metal-hydride reagents or silanes with
Lewis or Brønsted acids, even if some limitations remained
regarding chemoselectivity.1
More recently, deoxygenation of ketones was also
accomplished by transition-metal catalysis. In this field, a
limited number of catalytic systems based on the use of
molecular hydrogen as the reducing agent were reported and
proved to be enough selective to overcome the hydrogenation
competitive pathway in the course of the reaction.2 Even more
rare are the metal-based catalysts employing a hydrosilane as
the hydride source that allowed this deoxygenation reaction, in
place of the more classical hydrosilylation reduction. Palladium
species were first introduced in 2008 with the combination
[PdCl2]/HSiEt3,3 and subsequent improvements were made by
using [Pd(OAc)2],4 [PdCl2],5 or even Pd/C6 in the presence of
polymethylhydrosiloxane (PMHS). Another method relying on
the system [FeCl3]/PMHS and which necessitated a microwave
irradiation was also reported.7 High-valent rhenium and
efficient catalyst for the mild deoxygenation of aromatic
ketones under hydrosilylation conditions. Although this well-
known neutral complex has been used previously in very
important catalytic transformations,11 to the best of our
knowledge, this represents its first application in the reduction
of carbonyl compounds.
Optimization experiments were carried out with 4-
acetylbiphenyl (1a) as the model substrate (Scheme 1). Various
solvents and silanes were tested with a catalyst loading of 1
mol%. After reaction and acidic hydrolysis, the conversion of 1a
and the proportions of alcohol 2a to hydrocarbon 3a were
determined by 1H NMR (Table 1).
Scheme 1 Reduction of 1a under various conditions.
When the reduction of 1a was carried out with Ph2SiH2 and
[Rh(-Cl)(CO)2]2, good conversions ranging from 89% to 95%
Univ Rennes, CNRS, ISCR - UMR 6226, F-35000 Rennes, France.
E-mail: gilles.argouarch@univ-rennes1.fr
† Electronic Supplementary Information (ESI) available: experimental details and
were achieved in acetonitrile, toluene, THF, and DCM, and
alcohol 2a was obtained as the major product (Table 1, entries
1-4). At that stage, the deoxygenated product 3a was formed
analytical data of the products. See DOI: 10.1039/x0xx00000x
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only in negligible amounts (1-5%). By using PMHS in DCM, the not restricted to one silane but is rather dependent on the two
reactivity was maintained with an increase in the amount of 3a reactants in presence. Similarly, in the case of benzoylferrocene
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DOI: 10.1039/C9NJ02954K
corresponding to 8% of the product (Table 1, entry 5). The more
(1k), no reaction occured with HSiEt3 whereas PhSiH3 gave 3k in
reactive silane PhSiH3 gave full conversion of the ketone and a 94% isolated yield.
significant change in the outcome of the reaction which gave
Table
2
Scope and limitations of the Rh-catalyzed
28% of 3a versus 72% of 2a (Table 1, entry 6). Finally, a switch
of the selectivity in favor of the deoxygenation pathway was
fully reached with a combination between the Rh(I) complex
and the monohydrosilane HSiEt3 in DCM as 3a was virtually the
only product arising from the complete reduction of 1a by this
way (Table 1, entry 7). Under these conditions, attempts to
decrease the catalyst loading turned out to be detrimental to
deoxygenation of ketonesa
Entry Ketone
1
Silane
Product
Conv. (Yield)b
>99 (84)
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HSiEt3
the conversion of ketone 1a
.
2
3
HSiEt3
HSiEt3
>99 (83)
>99
Table 1 Optimization experiments of the Rh-catalyzed
deoxygenation of 4-acetylbiphenyl (1a
)
Entry
Silane
Solvent
Conv. (%)a 2a/3a (%)a
1
2
3
4
5
6
7
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PMHS
MeCN
toluene
THF
89
99/1
99/1
95/5
99/1
92/8
72/28
1/99
94
95
4
5
6
7
HSiEt3
HSiEt3
HSiEt3
HSiEt3
>99
DCM
DCM
DCM
DCM
94
96
>99 (63)
>99 (67)
>99 (62)
PhSiH3
HSiEt3
>99
>99
a Determined by 1H NMR of the crude products.
The optimized reaction conditions were next presented to a
variety of substrates (Table 2). Ketones 1a were thus treated
-
r
with 1 mol% catalyst and a two-fold amount of HSiEt3 in DCM
for one day. Overall, 1H NMR analysis of the crude samples after
reaction revealed that no hydrosilylation adduct (alkoxysilane)
was present or only in trace amounts. Consequently work-up
became useless and the samples could be simply evaporated
and subjected to chromatography. For electron-neutral ketones
8
9
HSiEt3
HSiEt3
>99
78
1a
-
d,
the corresponding arenes 3a
-
d
were formed
quantitatively and high isolated yields were obtained for 3a and
3b (Table 2, entries 1-4). The para and ortho isomers of
methylacetophenone 1e and 1f gave also good rates of
formation of products 3e and 3f which were isolated in 63% and
67% yields, respectively (Table 2, entries 5 and 6). These
moderate yields were in part attributable to the low polarity of
such compounds making their separation from the by-products
of the silane sometimes delicate. This method was also tested
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13
HSiEt3
PhSiH3
PMHS
HSiEt3
85 (71)
>99 (88)
>99
0
14
15
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PhSiH3
HSiEt3
PhSiH3
>99 (94)
>99
on more encumbered substrates such as ketones 1g
entries 7-9). Although a complete conversion was observed
with -tetralone (1g) and cyclohexyl phenyl ketone (1h),
isobutyrophenone (1i) only produced 78% of hydrocarbon 3i
-i (Table 2,

.
>99 (59)
Interestingly, this method could also be applied to ferrocene
derivatives (Table 2, entries 10-14). With acetylferrocene (1j),
an incomplete conversion of 85% was obtained when HSiEt3
was used as the hydride source but fortunately PhSiH3 and
PMHS afforded ethylferrocene (3j) in quantitative form. This
contrasts from the results gathered with the model substrate 1a
for which HSiEt3 was the most efficient silane to achieve
deoxygenation and implies that the selectivity of this protocol is
17
18
HSiEt3
HSiEt3
>99 (69)
0
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likely the result of a 1,4-hydrosilylation of the stVaierwtinArgticleenOonlninee
followed by decomposition of the silyl enDoOl eI:t1h0e.1r03in9t/Cer9mNJe0d2i9a5t4eK,
as reported elsewhere with rhodium,13 rather than a
hydrogenation of the C=C bond with H2 arising from silane
dehydrodimerization or hydrolysis.14 Similarly, flavone 1u was
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HSiEt3
HSiEt3
HSiEt3
>99
>99
20
nicely converted into flavanone
yield.
7 which was isolated in 53%
Some experiments on various aldehydes and aliphatic ketones
were also run. After hydrolysis, only alcohols were obtained. For
instance, the reduction of 1,3-diphenylacetone (1v) led to the
clean formation of 1,3-diphenylpropan-2-ol (2v) (Scheme 3).
Keto-esters followed the same trend according to which ethyl
phenylglyoxalate (1w) was fully transformed into ethyl
mandelate (2w).
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HSiEt3
>99 (77)
a
Typical conditions: ketone (1 mmol), silane (2 mmol), [Rh(-
Cl)(CO)2]2 (1 mol%), DCM (3 mL), 0 °C to RT, 24 h. b Conversions
(%) determined according to 1H NMR spectroscopy of the crude
products, isolated yields (%) are given into brackets.
In the screening of ketones with significant electronic effects,
some limitations were apparent (Table 2, entries 15-21). While
methoxy groups on compounds 1l and 1m had no effect on the
reactions affording alkylated anisoles 3l and 3m with good
yields, the diethylamino moiety within the electron-rich ketone
1n completely inhibited the deoxygenation. Likewise,
halogenated ketones 1o and 1p participated well in the reaction
but a severe limitation were encountered in the case of the
more challenging electron-poor 2,2,2-trifluoroacetophenone
1q) which gave a low conversion of 20%. In addition, as a typical
diaryl ketone, benzophenone (1r) was also nicely converted to
arene 3r in 77% isolated yield (Table 2, entry 22).
The Rh/HSiEt3 system was further extended to miscellaneous
substrates (Scheme 2). The phenolic ketone 1s was engaged in
the reaction, and despite its full consumption, 4-ethylphenol
Scheme 3 Rh-catalyzed hydrosilylation of compounds 1v and
1w
.
(
In the literature,4-9 although the mechanistic details of the
transition-metal catalyzed deoxygenation of ketones in the
presence of silanes are not yet clear, it was established that this
reductive process proceeds in two steps, where the ketone is
first reduced to an alcohol which in turn undergoes
hydrogenolysis to form the alkylated product. To assess this
with rhodium, control experiments were conducted (Scheme 4).
(
3s) was isolated only in 45% yield because triethylsilyl ethers
and were formed concomitantly. This competition between
deoxygenation and O-silylation was not surprising since
rhodium complexes are known to catalyze the dehydrogenative
silylation of alcohols and phenols with hydrosilanes.12
4
5
Scheme 2 Rh-catalyzed reduction of compounds 1s-u
.
Scheme 4 Control experiments on the deoxygenation process
and generalization of the Rh-catalyzed hydrogenolysis of
benzylic alcohols.
The reaction of the
completion and gave product
,

-unsaturated ketone 1t went to
in good yield (76%). This was
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When the benzylic alcohol 2a was engaged in the
deoxygenation reaction, it was successfully converted to arene
3a which was isolated in 88% yield. On the contrary, no reaction
occured starting from the trimethylsilyl ether 8. Thus, it can be
postulated that the mechanism of this reaction also involves the
release of an alcohol and that this latter probably doesn’t
originate from a hydrosilylation adduct of type
inert under the reaction conditions.
Finally, the applicability of the hydrogenolysis of benzylic
alcohols by the system [Rh( -Cl)(CO)2]2/HSiEt3 was briefly
extended (Scheme 4). Besides the model compound 2a, full
conversion of alcohols 2l and 2r also happened, and products 3l
and 3r were isolated in 74% and 70% yields, respectively.
In conclusion, this work has provided an original and
straightforward method for the deoxygenation of ketones and
has drawn light on the catalytic properties of the readily
View Article Online
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9
10 S. Xu, J. S. Boschen, A. Biswas, T. Kobayashi, M. Pruski, T. L.
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11 For recent examples see: a) W. Song, N. Zheng, M. Li, K. Ullah
and Y. Zheng, Adv. Synth. Catal., 2018, 360, 2429; b) N. Zheng,
W. Song, T. Zhang, M. Li, Y. Zheng and L. Chen, J. Org. Chem.,
2018, 83, 6210; c) Y. Kawaguchi, K. Yabushita and C. Mukai,
Angew. Chem. Int. Ed., 2018, 57, 4707; d) Y. Kawaguchi, A.
Nagata, K. Kurokawa, H. Yokosawa and C. Mukai, Chem. Eur.
J., 2018, 24, 6538; e) C.-H. Liu and Z.-X. Yu, Angew. Chem. Int.
Ed., 2017, 56, 8667; f) X. Li, J. Pan, H. Wu and N. Jiao, Chem.
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
Sci., 2017,
8
, 6266; g) Y. Liao, Q. Lu, G. Chen, Y. Yu, C. Li and X.
, 7529.
Huang, ACS Catal., 2017,
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available rhodium complex [Rh(-Cl)(CO)2]2 in the reduction of
12 a) I. Ojima, T. Kogure, M. Nihonyanagi, H. Kono and S. Inaba,
Chem. Lett., 1973, 501; b) R. J. P. Corriu and J. J. E. Moreau, J.
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Noguchi, H. Miura, T. Hashimoto and R. Takeuchi,
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carbonyl compounds. Herein, this precatalyst was able without
the aid of any additives to convert the C=O group of a series of
ketones into methylenes in the presence of weak reducing
agents such as HSiEt3. This system operating under mild
conditions and in non protic media was also efficient in the
hydrogenolysis of benzylic alcohols. Other interesting uses of
this catalyst in reductive reactions will be reported in due
course.
14 Several attempts to reduce styrene derivatives with the
present system were unsuccessful.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Université de Rennes 1 and CNRS are acknowledged for financial
supports
.
Notes and references
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Ueba and A. Baba, Synlett, 1999, 182.
2
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a) H. Wang, L. Li, X.-F. Bai, J.-Y. Shang, K.-F. Yang and L.-W. Xu,
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Mild and efficient rhodium-catalyzed deoxygenation of ketones to alkanes
New Journal of Chemistry
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Table of contents entry
Chlorodicarbonylrhodium(I) dimer: a simple catalyst for the reduction of carbonyl compounds
also.
OC
OC
Cl
Cl
CO
CO
Rh
Rh
O
H
H
1 mol%
R2
R2
HSiEt3
R1
R1
DCM, 0 °C to RT, 24 h