Surfactant-induced substrate selectivity in the palladium-nanoparticle- mediated chemoselective hydrogenation of unsaturated aldehydes in water

Article abstract of DOI:10.1002/cctc.201402034

A hundred-fold substrate selectivity was observed in the competitive chemoselective hydrogenation of seven α,β-unsaturated aldehydes from C₄ to C₁₀ in water with palladium nanoparticles stabilized by sodium dodecyl sulfonate as the surfactant. The medium makes the longer lipophilic substrates hundreds of times more reactive than the shorter more hydrophilic ones, whereas in an organic medium almost no difference is observed. Something in the water: A hundred-fold substrate selectivity is observed in the competitive chemoselective hydrogenation of seven α,β-unsaturated aldehydes (C₄-C₁₀) in water with palladium nanoparticles stabilized by sodium dodecyl sulfonate as a surfactant. The medium makes the longer lipophilic substrates hundreds of times more reactive than the shorter more hydrophilic ones, whereas in an organic medium almost no difference is observed.

Full text of DOI:10.1002/cctc.201402034

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DOI: 10.1002/cctc.201402034
Surfactant-Induced Substrate Selectivity in the Palladium-
Nanoparticle-Mediated Chemoselective Hydrogenation of
Unsaturated Aldehydes in Water
Giorgio La Sorella,[a] Patrizia Canton,[b] Giorgio Strukul,[a] and Alessandro Scarso*[a]
A hundred-fold substrate selectivity was observed in the com-
petitive chemoselective hydrogenation of seven a,b-unsaturat-
ed aldehydes from C4 to C10 in water with palladium nanoparti-
cles stabilized by sodium dodecyl sulfonate as the surfactant.
The medium makes the longer lipophilic substrates hundreds
of times more reactive than the shorter more hydrophilic ones,
whereas in an organic medium almost no difference is
observed.
lectivity occurs if the recognition event between the catalyst
and the substrate takes place before the catalytic event, and
a high surface interaction between substrate and catalyst is es-
sential. In this respect, with traditional metal-based catalysts,
the interaction with the substrate occurs on reduced surfaces
(Figure 1), which leads to very poor substrate selection that
indeed remains a challenging task.
Except for the kinetic resolution of racemates,[5] often sub-
strate-selective homogeneous catalysts are studied by compar-
ing different substrates individually and not in direct competi-
tive experiments. Often such substrates, apart from the
common reactive functional group at which the reaction
occurs,[6–8] bear extra functionalities such as hydrogen bonds
or ionic moieties[9–13] that enable secondary interactions with
the catalyst to favor their selective recognition. Examples of
homologous substrates that differ only in small changes in po-
sitions remote from the reactive center and lack extra recogni-
tion sites are indeed rare. In such cases the selection of the
substrate is usually based on steric interactions[14–16] because of
the complete surrounding of the substrate by the catalyst. To
favor the surface interaction between catalyst and substrate,
an alternative approach consists of wrapping the two within
a solvation sphere with a lifetime long enough to impart steric
effects. Impressive examples of this strategy are based on
supramolecular water-soluble metal–ligand coordination cap-
sules as catalysts able to select substrates with size and shape
that match the dimensions of the cavity.[17,18] Recently, our
group disclosed an example of substrate-selective alkyne hy-
dration driven by the encapsulation of a AuI catalyst within
The selectivity of a reaction catalyzed by a synthetic catalyst is
often referred as to as the ability of the given catalyst to pro-
duce one product preferentially over several possible chemo-,
regio-, and stereoisomers. Natural enzyme catalysts are impres-
sive examples of substrate-selective catalysts[1–4] thanks to their
ability to select the substrate that best fits the geometric and
electronic properties of the active site (Figure 1). Substrate se-
a
hydrogen-bonded, self-assembled capsule in organic
media.[19]
Micelles are supramolecular aggregates that form spontane-
ously on addition of surfactants in water. They are able to dis-
solve apolar substrates and catalysts in water and have been
demonstrated to enhance product selectivity at all levels.[20,21]
Rare examples of substrate selectivity imparted by micelles are
known, specifically the synthesis of esters disclosed by Kobaya-
shi,[22] the Diels–Alder reaction mediated by Cr(salen)Cl,[23] and
the Heck coupling reaction mediated by Pd precursors from
our group.[24] The hydrophobic effect is the driving force that
allows substrate selectivity in water under micellar conditions,
and the same effect could also be important in the selection
rules observed in enzymes.
Figure 1. Example of the extended surface interaction between enzyme and
substrate in comparison to the same interaction between a homogeneous
catalyst and the corresponding substrate.
[a] G. La Sorella, Prof. G. Strukul, Dr. A. Scarso
Dipartimento di Scienze Molecolari e Nanosistemi
Universitꢀ Ca’ Foscari di Venezia
Calle Larga S. Marta 2137, 30123, Venezia (Italy)
E-mail: alesca@unive.it
[b] Dr. P. Canton
Dipartimento di Scienze Molecolari e Nanosistemi
Universitꢀ Ca’ Foscari di Venezia
Via Torino 155/b, 30123,Venezia-Mestre (Italy)
Herein we report the hundred-fold substrate selectivity im-
parted by the use of sodium dodecyl sulfonate in water in the
selective hydrogenation of a,b-unsaturated aldehydes mediat-
ed by Pd nanoparticles (PdNPs) in water (Scheme 1). Longer
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402034.
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the steric hindrance of the C=C
bond. Notably, in the course of
these experiments in an organic
solvent, the addition of H2 led to
a color change of the solution
from pale orange to grey, indica-
tive of Pd metal formation,
which did not precipitate as
a black element probably thanks
to the solubilizing effect impart-
ed by the large excess of sub-
strates present.
Subsequently, we switched to
the employment of water as the
solvent in the presence of differ-
ent surfactants characterized by
different charge, lipophilicity,
and critical micellar concentra-
tion (cmc). Examples of the sta-
bilization of PdNPs by micellar
Scheme 1. Chemoselective hydrogenation of a,b-unsaturated C4ÀC10 aldehydes 2a–2g to the corresponding satu-
rated aldehydes 3a–3g. Comparison of the substrate selectivity displayed in organic and aqueous media with sur-
factant.
and more lipophilic substrates reacted hundreds of times
faster than the shorter ones because of their partition between
bulk water and surfactant aggregates in which the PdNPs are
stabilized.
media in aqueous media have been proposed recently for the
Matsuda–Heck[25] and Mizoroki–Heck[26] coupling reactions in
water with the observation of the beneficial effect of the use
of cationic surfactants in combination with palladium acetate
as the metal precursor. In the present work, in all cases
Pd(OAc)2 (0.6 mol%) was dissolved in water in the presence of
a series of surfactants for 1 h followed by activation of the
metal under 1 bar of H2 until the color change, which was indi-
cative of the formation of PdNPs. Initially, we investigated the
use of the cationic surfactant cetyl trimethylammonium bro-
mide (CTAB, 80 mm) that required a long activation with H2
before the color change was observed. Under these experi-
mental conditions, the competitive hydrogenation reaction
showed a low activity and only 0.5–9% conversion of the dif-
ferent unsaturated aldehydes 2a–2g was observed after 1 h.
Similar experiments were performed with Triton-X100 and
Triton-X114 as neutral surfactants (8 mm) but in these cases
the activation process with H2 and the subsequent addition of
the substrates caused the precipitation of Pd metal. If the
Triton concentration was increased to 80 mm, a stable black
suspension was obtained after 20 min activation with H2 in
both cases, but the resulting mixture separated into a lower
aqueous and higher surfactant phase if the mixing was
stopped. This is indicative of a biphasic system rather than
a microemulsion typical of micellar conditions. The reaction led
to the selective formation of the corresponding saturated alde-
hyde with quantitative yield after approximately 40 and 50 min
using Triton-X114 and Triton-X100, respectively. The competi-
tive hydrogenation reaction for aldehydes 2a–2g showed an
increase of activity from 2a to 2d followed by a small decrease
up to 2g (Supporting Information). The presence of two differ-
ent phases could explain the activity trend observed with
shorter substrates based on their hydrophobicity. In contrast,
longer substrates that reside all the time in the organic phase,
in which the Pd catalyst is stabilized, react as they do in THF
and show a decrease of activity with the increasing length of
the aldehyde.
Initially, the hydrogenation reaction was tested in THF as the
organic medium to ascertain the intrinsic relative activity of
the series of unsaturated aldehydes. The reaction was per-
formed with 0.6 mol% of Pd(OAc)2 in the presence of 59.4 mm
of each substrate from trans-2-butenal (2a) to trans-2-decenal
(2g) at room temperature under 1 bar of H2. For all substrates,
the reaction led to the selective formation of the correspond-
ing saturated aldehyde that was formed quantitatively in ap-
proximately 75 min. Following the decrease of the starting re-
agent over time, it was possible to determine the initial rate of
the reaction for each substrate. The experiment was repeated
three times, and normalization of the initial reaction rates with
respect to the longer substrate 2g showed a gradual increase
of selectivity of the reaction in favor of 2a, which reacted 3.6
times faster than the longer 2g (Figure 2).
The slight decrease in activity on increasing the length of
the alkyl chain is likely to be imputable to the partial folding
of the longer substrates with a consequent small increase in
Figure 2. Normalized reaction rates for the hydrogenation of unsaturated al-
dehydes 2a–2g with Pd(OAc)2 in THF at RT.
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Finally, anionic surfactants were the most suitable to both
stabilize the PdNPs and impart substrate selectivity to the cata-
lytic system. If sodium dodecyl sulfate (SDS, 80 mm) was em-
ployed as the surfactant, the activation of the metal with H2
flushing was fast and required only 5 min. The competitive hy-
drogenation tests with equimolar amounts of unsaturated al-
dehydes 2a–2g showed the short 2a as the less reactive sub-
strate, whereas 2g was 3.7 times more active. The use of
sodium dodecylbenzenesulfonate (SDBS, 80 mm) as an alterna-
tive anionic surfactant led to a less efficient catalytic system,
which showed only 18–38% conversion of the substrates after
1 h (Supporting Information).
In the case of sodium dodecyl sulfonate (SDSU), we ob-
served interesting results in terms of the catalytic activity and
stability of the PdNPs that required a more detailed analysis of
the effect of the concentration of the surfactant. Experiments
were initially performed at 20, 40, 60, and 80 mm of SDSU in
water at room temperature using trans-2-hexenal (2c) as
a model substrate at a ratio of 100:1 with respect to the
Pd(OAc)2 (Table 1). We observed that a higher concentration of
Figure 3. Normalized reaction rates for the hydrogenation of unsaturated al-
dehydes 2a–2g with Pd(OAc)2 in water with SDSU 80 mm as the surfactant
at RT.
of the length and hydrophobicity of the substrates caused
a linear increase of activity up to 2d followed by a second
linear increasing trend with a lower slope up to 2g. The latter
substrate was completely converted within 40 min, which
speaks clearly for the high catalytic activity of the PdNPs ob-
tained under these experimental conditions. Notably, between
the most and the least active substrate, a ratio of more than
330 was observed, which is remarkable if we consider that the
selection occurs mainly because of the partition properties of
the substrates between the bulk water and the apolar support
provided by the surfactant in which the NPs are stabilized by
the hydrophobic effect.
Table 1. Catalytic hydrogenation of 2c by PdNPs in water with SDSU as
the surfactant at different concentrations in water.[a]
t [min]
Yield [%]
20 mm
40 mm
60 mm
80 mm
SDSU
SDSU
SDSU
SDSU
10
30
30
70
21
71
35
92
29
>98
It has been reported in some cases that surfactants can be
used to control the geometry and the stability of PdNPs.[28,29]
To shed light on the level of aggregation of the Pd, TEM[30]
was used to visualize the presence of PdNPs with an average
size of 4.6 nm and a size distribution between 2.0 and 7.5 nm.
[a] Experimental conditions: [2c] 120 mm, [Pd(OAc)2] (2%) 2.4 mm, H2
1 atm, water 3 mL, RT.
surfactant caused an increase of the time required for the for-
mation of PdNPs, probably because of a better dispersion of
the metal in the medium. Although at lower concentrations of
SDSU the PdNPs were not stable and we observed the forma-
tion of Pd black after approximately 30 min (20 mm SDSU,
30% yield after 10 min and 70% yield after 30 min), at higher
surfactant concentration the PdNPs were more stable. This led
to high catalytic activity with well-dispersed NPs and no forma-
tion of Pd black at the end of the reaction (80 mm SDSU, 29%
yield after 10 min and >98% yield after 30 min).
The latter concentration of surfactant was selected for the
following catalytic tests as it was able to ensure the high sta-
bility of the PdNPs. Notably, SDSU is characterized by a very
low cmc (0.9 mm),[27] and at 80 mm in water the system is pres-
ent as a sort of aqueous colloidal system without complete
dissolution of the surfactant. It is likely that the catalytic
system does not operate under micellar conditions but rather
the PdNPs are stabilized by interaction with the organic por-
tion of the medium that stabilizes the NPs and avoids their ag-
gregation to Pd black.
We then studied the substrate selectivity of the catalytic
system developed in SDSU. The short substrates 2a and 2b
were characterized by a very low rate of conversion into the
corresponding saturated aldehydes (Figure 3), and an increase
Figure 4. a) TEM image of PdNPs prepared at RT by H2 flushing of a solution
of Pd(OAc)2 (2.4 mm) in the presence of SDSU (80 mm) as the surfactant in
water (3 mL). b) Size distribution determined for the PdNPs.
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Acknowledgements
The authors thank MIUR, Universitꢀ Ca’ Foscari di Venezia, and
“Consorzio Interuniversitario Nazionale per la Scienza e Tecnolo-
gia dei Materiali” for financial support. Mrs. Laura Sperni is grate-
fully acknowledged for GC–MS analysis.
Keywords: hydrogenation
palladium · surfactants
·
micelles
·
nanoparticles
·
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Published online on && &&, 0000
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Something in the water: A hundred-
fold substrate selectivity is observed in
the competitive chemoselective hydro-
genation of seven a,b-unsaturated alde-
hydes (C4–C10) in water with palladium
nanoparticles stabilized by sodium do-
decyl sulfonate as a surfactant. The
medium makes the longer lipophilic
substrates hundreds of times more reac-
tive than the shorter more hydrophilic
ones, whereas in an organic medium
almost no difference is observed.
G. La Sorella, P. Canton, G. Strukul,
A. Scarso*
&& – &&
Surfactant-Induced Substrate
Selectivity in the Palladium-
Nanoparticle-Mediated
Chemoselective Hydrogenation of
Unsaturated Aldehydes in Water
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 4
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These are not the final page numbers!