Angewandte
Chemie
DOI: 10.1002/anie.201411059
Heterogeneous Catalysis
Mild Deoxygenation of Aromatic Ketones and Aldehydes over Pd/C
Using Polymethylhydrosiloxane as the Reducing Agent**
Alexey Volkov, Karl P. J. Gustafson, Cheuk-Wai Tai, Oscar Verho,* Jan-E. Bꢀckvall,* and
Hans Adolfsson*
Abstract: Herein, a practical and mild method for the
deoxygenation of a wide range of benzylic aldehydes and
ketones is described, which utilizes heterogeneous Pd/C as the
catalyst together with the green hydride source, polymethylhy-
drosiloxane. The developed catalytic protocol is scalable and
robust, as exemplified by the deoxygenation of ethyl vanillin,
which was performed on a 30 mmol scale in an open-to-air
setup using only 0.085 mol% Pd/C catalyst to furnish the
corresponding deoxygenated product in 93% yield within
3 hours at room temperature. Furthermore, the Pd/C catalyst
was shown to be recyclable up to 6 times without any
observable decrease in efficiency and it exhibited low metal
leaching under the reaction conditions.
and green reducing agent, its use is generally associated with
high pressure, special equipment, and safety precautions to
minimize the explosion risk.[8] Among the many hydride
donors used in catalytic procedures,[9] silanes can be consid-
ered as one of the most attractive donors since they are
generally cheap and easy to handle. Moreover, silanes usually
allow the reduction to occur under milder reaction conditions
and with higher chemoselectivity in comparison to other
methods.[10] Of the many available silanes, polymethylhydro-
siloxane (PMHS) is an attractive option as it demonstrates
high air and moisture stability. PMHS is cost-effective and
readily available, since it is generated in large quantities as
a byproduct in the silicon industry.[11] To date, several
transition-metal-based catalytic protocols employing PMHS
as the reducing agent have been developed for the reduction
of amides to amines,[12] as well as for the conversion of
ketones, aldehydes, and esters either into alcohols[13] or to
saturated alkanes.[14]
Although homogeneous systems for the deoxygenation of
aromatic alcohols, aldehydes, and ketones with PMHS have
been developed,[15,16] there exists no heterogeneous catalytic
system for the deoxygenation of aromatic ketones and
aldehydes under hydrosilylation conditions. Heterogeneous
protocols are generally associated with several practical
advantages, such as improved catalyst stability, simpler
separation and purification procedures, possibility of catalyst
recycling, and lower levels of metal impurities in the final
product. Herein, we report on the first heterogeneous system
for the deoxygenation of a wide range of aromatic carbonyl
compounds with good chemoselectivity by utilizing low
catalyst loading of commercially available palladium on
carbon (Pd/C) and PMHS under mild reaction conditions.
For preliminary screening, the deoxygenation of 4-
methoxyacetophenone (1) into 4-ethylanisole (3) was inves-
tigated in different solvents (see Table S1 in the Supporting
Information). We identified that commercially available Pd/C
(5 wt% according to commercial provider)[17] could function
as an efficient catalyst for this deoxygenation reaction when
PMHS is used as the hydride source. The solvent screening
was carried out at 658C for over 16 hours, and this revealed
that the reaction exhibited the highest efficiencies in alcohol
solvents, while nonprotic solvents resulted in no or low
conversions. The best results were observed in alcohols
bearing long alkyl chains, and were ascribed to a better
solubility of the polymeric PMHS in these solvents
(Scheme 1). However, the reaction still exhibited satisfactory
activity in MeOH, thus resulting in full conversion of the
starting ketone 1 to alcohol 2 and deoxygenated product 3 in
a ratio of 2:3. Therefore, to allow simple isolation procedures
R
eductive deoxygenation of aldehydes and ketones to the
corresponding saturated compounds has attracted consider-
able attention given its many applications in fine-chemical
synthesis[1] and biofuel production.[2] Unfortunately, classical
methods for the deoxygenation of carbonyl compounds, such
as those based on either the Barton–McCombie,[3] Clemmen-
sen,[4] or Wolff–Kishner[5] methodologies, are generally asso-
ciated with harsh reaction conditions, the use of stoichiomet-
ric amounts of toxic reagents, and poor functional-group
tolerance. Later developments led to mild stoichiometric
protocols for the deoxygenation of carbonyl compounds,
where different metal hydrides are used as reagents.[6]
However, these methods generally suffer from unsatisfactory
chemoselectivity, poor atom economy, and substantial
byproduct formation, which complicate the workup and
purification procedures.
Currently, there exist a number of catalytic protocols for
the deoxygenation of carbonyls employing molecular hydro-
gen (H2).[7] Although, H2 constitutes the most atom-efficient
[*] A. Volkov, K. P. J. Gustafson, Dr. O. Verho, Prof. Dr. J.-E. Bꢀckvall,
Prof. Dr. H. Adolfsson
Department of Organic Chemistry, Stockholm University
SE-106 91, Stockholm (Sweden)
E-mail: oscar@organ.su.se
Dr. C.-W. Tai
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
SE-106 91, Stockholm (Sweden)
[**] The Swedish Research Council, The Berzelii Center EXSELENT,
European Research Council (ERC AdG 247014) and The Knut and
Alice Wallenberg Foundations are gratefully acknowledged.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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