Selective hydrogenation of α,β-unsaturated carbonyl compounds on silica-supported copper nanoparticles

Article abstract of DOI:10.1039/c8cc08457b

Silica-supported copper nanoparticles prepared via surface organometallic chemistry are highly efficient for the selective hydrogenation of various α,β-unsaturated carbonyl compounds yielding the corresponding saturated esters, ketones, and aldehydes in the absence of additives. High conversions and selectivities (>99%) are obtained for most substrates upon hydrogenation at 100-150 °C and under 25 bar of H₂.

Full text of DOI:10.1039/c8cc08457b

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Selective hydrogenation of a,b-unsaturated
carbonyl compounds on silica-supported copper
nanoparticles†
Cite this: Chem. Commun., 2019,
55, 179
Received 23rd October 2018,
Accepted 28th November 2018
Jorge Mendes-Burak, Behnaz Ghaffari and Christophe Coperet
*
´
DOI: 10.1039/c8cc08457b
rsc.li/chemcomm
Silica-supported copper nanoparticles prepared via surface organo- For instance, Pd nanoparticles supported on carbon in the
metallic chemistry are highly efficient for the selective hydrogenation presence of acetic acid or tri-n-butylamine hydrogenate a,b-
of various a,b-unsaturated carbonyl compounds yielding the corres- unsaturated aldehydes, ketones, and esters with selectivities
ponding saturated esters, ketones, and aldehydes in the absence of 480%.20 Addition of a second metal that acts as a poison
additives. High conversions and selectivities (499%) are obtained generally improves selectivity. In this context, Pb has been
for most substrates upon hydrogenation at 100–150 8C and under largely used as a Pd poison with or without quinoline for
25 bar of H2.
selective hydrogenation in the so-called Lindlar catalyst.21,22
Sn,23 Bi,21 B,24 and Zn12 can also be employed as poisons for Pd
Hydrogenation reactions are key industrial processes to prepare and other noble metal catalysts. Noteworthily, the use of copper
fuels,1 agrochemicals,2 fine chemicals,3 and pharmaceuticals.4 chromite also results in the quantitative conversion of unsaturated
Currently, one-quarter of all synthetic drugs on the market ketones, esters, and carboxylic acids to their corresponding selective
require at least one hydrogenation step in their synthesis.2 a,b-hydrogenated products.25 While acceptable selectivities are
Several organic reaction intermediates are based on compounds reported, the use of toxic additives makes these heterogeneous
with a,b-unsaturated carbonyl groups that constitute an essential catalysts less appealing.
class of chemicals,4 giving access to the corresponding hydrogenated
products, employed in the food, flavor, and fragrance industries,4–6 on supported metal nanoparticles and are prepared by wet
as well as in the pharmaceutical industry.7 impregnation26 or co-precipitation.26 The preparative method
Most heterogeneous hydrogenation catalysts are based
The selective hydrogenation of the carbon–carbon double often leads to broad particle size distribution, associated with
bond in a,b-unsaturated carbonyl compounds still remains numerous active sites and often poorer selectivity. In contrast,
challenging due to the competing hydrogenation of the carbonyl surface organometallic chemistry (SOMC) has emerged as a
groups, over-hydrogenation or even de-oxygenation reactions. powerful method to prepare small and narrowly distributed
Molecular compounds based on noble metals (Ru,8 Rh,9 Ir,10 supported nanoparticles, including copper.27–29 Herein we
Pd,11–13 and Pt14) are mostly used as hydrogenation catalysts with show that silica-supported copper nanoparticles prepared by
selectivities 490% when specific ligands such as bulky phos- SOMC are particularly efficient heterogeneous catalysts for
phines are used.8,9,15 However, these ligands typically sacrifice the selective hydrogenation of carbon–carbon double bonds of
activity for selectivity. Non-noble metals complexes such as a,b-unsaturated carbonyl compounds, without the need for
FeII/Na2EDTA16 or copper hydride complexes, such as [(Ph3P)CuH]6, additives such as ligands or promoter elements.
also known as the Stryker reagent,17 have also been successfully
A Cu/SiO2 catalyst was prepared by grafting [Cu(OtBu)]4 on
used as selective hydrogenation catalysts for a,b-unsaturated silica partially dehydroxylated at 700 1C followed by treatment
carbonyl compounds. While the homogenous systems favor high under H2 at 500 1C,28,30 yielding small and narrowly distributed
conversion and selectivities, catalyst separation and regeneration copper particles of 3.2 Æ 0.8 nm (see ESI† for experimental details).
processes can be problematic and costly.18
With a metal loading of 5.9 wt%, H2 chemisorption data show that
Alternatively, heterogeneous catalysts, mostly based on 0.114 mmol Cu.surface. gÀ1 is the active and accessible metallic
precious metals, have been used for this transformation.3,19 surface corresponding to a dispersion of 15%. We then explored
their activity in the hydrogenation of a,b-unsaturated carbonyl
compounds, which was performed under 25 bar of H2 and at
¨
ETH Zurich, Department of Chemistry and Applied Biosciences, Vladimir-Prelog-Weg 1-5,
150 1C for 15 hours, using (10 mg) of Cu/SiO2 (0.57 mmol of Cu)
¨
CH-8093, Zurich, Switzerland. E-mail: ccoperet@inorg.chem.ethz.ch
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc08457b and 0.5 mmol of substrate in 3 mL of toluene.
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Table 1 Selective a,b-hydrogenation of cinnamate derivatives
Table 2 Selective a,b-hydrogenation of cinnamyl substrates
Subst.
E
T (1C)
Conv. (%)
Sel. (%)
Subst.
R1
R2
R3
T (1C)
Conv. (%)
Sel. (%)
8
9
10
11
12
13
–COCH3
–CHO
–COOH
–CONH2
–CON(CH2CH3)2
–CH2OH
100
100
150
150
150
150
499
499
499
499
499
499
83
90
499
499
499
499
1
2
3
4
5
6
7
7
–H
–CH3
–H
–H
–H
–H
–H
–H
–H
–H
–CH3
–H
–H
–H
–H
–H
–H
–H
–H
–Cl
–CH3
–OCH3
–NO2
–NO2
150
150
150
150
150
150
150
100
499
499
o1
20
36
86
499
499
—
499
499
499a
499b
499c
499
499
GC-MS. Decreasing the reaction temperature to 100 1C provides
high selectivity (499%) for the desired product(I) at complete
conversion.
Besides a,b-hydrogenation: a Aromatic ring is fully hydrogenated. b Nitro
group reduced to amine and ester to alcohol. c Nitro group reduced to amine.
The hydrogenation of acrylate derivatives (15–19) was also
investigated; they all exhibited high conversions and selectivities
(499%), independent of the methyl substituent position
(Table 3). Highly encumbered derivatives bearing methylenecyclo-
hexane (18) or longer alkyl chains like (19) are also selectively
hydrogenated with good conversions. Note that hydrogenation
also occurs selectively when the unsaturation is more remote from
the ester groups as in (20), but the conversion is lower (88%)
under similar reaction conditions. This shows that copper prefers
to hydrogenate electron-deficient olefins. For ketones, whether
conjugated or not such as (21) and (22), the carbonyl is partially
hydrogenated to the corresponding alcohol at 150 1C. The
selectivity is hardly improved at 100 1C (21), as the ketone group
to alcohol transformation also occurs at lower temperatures. The
a-angelica lactone (23), a relevant intermediate of biomass, is
only hydrogenated with low selectivity, due to competitive oligo-
merization and decarboxylation pathways, which also take place
at 100 1C.34 Note that in a mixture of unfunctionalized olefins,
which contains 1-nonene (80%), 2-nonene (10%), and 4-nonene
(10%) (24) – only the terminal olefin was hydrogenated at 100 1C,
while no conversion was observed for internal olefins.
First, the hydrogenation of cinnamate derivatives was evaluated
(1–7) with various substituents on the olefin and aromatic moieties
(Table 1). The carbon–carbon double bond of the parent cinna-
mate (1) is selectively hydrogenated (499%).31 This reactivity is
in sharp contrast to what is observed for (i) the corresponding
silica-supported Pt nanoparticles that yield the hydrogenated
alcohol or (ii) silica-supported nickel nanoparticles and Cu/ZnO/
Al2O3 that show no significant conversion under the same
reaction conditions. It is noteworthy that having an additional
methyl substituent in the a-position (2) does not affect catalysis
and provides the hydrogenated product with 99% selectivity on
complete conversion.32 Having a methyl substituent in the b-position
(3) is however detrimental, with a conversion of o1%.33
Addition of functional groups to the para-position of the
aromatic ring (4–7) affects the conversion, while selectivity
remains unchanged (499%): the addition of an electron with-
drawing group (i.e., chloro) significantly decreases the conversion
(20% – 4), but conversions seem less influenced when functional
groups are electron donating, such as methyl (36% – 5) or in
particular methoxy (86% – 6). If a nitro group is present in the
para position as in (7), complete conversion is reached; although
the nitro and the ester groups are both reduced to their corres-
ponding amine and alcohol. However, decreasing the reaction
temperature to 100 1C leaves the ester group untouched; still, the
nitro group is reduced.33
Finally, recycling tests using ethyl cinnamate have been
carried out: no deactivation was observed during the first 3 cycles,
while additional recycling tests show a decrease of conversion by
ca. 5% in the two subsequent cycles. Catalytic tests performed after
hot filtration on cycles 1, 3, and 5 show no activity in the filtered
solution, consistent with active supported nanoparticles and not
Besides cinnamates, the hydrogenation of unsaturated ketone,
aldehyde, carboxylic acid, and amide derivatives, as well as allylic
alcohols, was also investigated (8–13, Table 2). At 150 1C, all the
substrates were fully converted. While a high selectivity (99%) for
a,b-hydrogenation was observed for (10–12), the substrates with
ketone and aldehyde functionalities, (8) and (9), were converted to
the hydrogenated alcohol. Nevertheless, at 100 1C exclusively high
selectivities of 83% and 90% in a,b-hydrogenation for (8) and (9)
were achieved, respectively. The corresponding allylic alcohol 13
was also selectively hydrogenated.
The hydrogenation of coumarin (14), an important class of
lactones, was also explored. However, at 150 1C, hydrogenation
yielded allylbenzene(III), probably via formation of the corresponding
saturated ester and pyran derivatives (Scheme 1), evidenced by
Scheme 1 Possible reaction path of (14).
180 | Chem. Commun., 2019, 55, 179--181
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Table 3 Selective a,b-hydrogenation of carbonyl compounds
Conflicts of interest
Subst.
T (1C)
Conv. (%)
Sel. (%)
83
There are no conflicts to declare.
15
150
499
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Chem. Commun., 2019, 55, 179--181 | 181