Polymer-Anchored Bifunctional Pincer Catalysts for Chemoselective Transfer Hydrogenation and Related Reactions

Article abstract of DOI:10.1002/cssc.201901728

A series of polymer-supported cooperative PC(sp³)P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer-tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.

Full text of DOI:10.1002/cssc.201901728

Accepted Article
Title: Efficient, Recoverable, and Durable Polymer-anchored
Bifunctional Pincer Catalysts for Chemoselective Transfer
Hydrogenation and Related Reactions
Authors: Luigi Vaccaro, Dmitri Gelman, shrouk mujahedd, Federica
Valentini, and Shirel D Cohen
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To be cited as: ChemSusChem 10.1002/cssc.201901728
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10.1002/cssc.201901728
ChemSusChem
FULL PAPER
Efficient, Recoverable, and Durable Polymer-anchored
Bifunctional Pincer Catalysts for Chemoselective Transfer
Hydrogenation and Related Reactions
Shrouq Mujahed,[a]‡ Federica Valentini,[c]‡ Shirel Cohen,[a] Luigi Vaccaro[c]*, and Dmitri Gelman[a],[b]
*
Abstract: A series of polymer-supported cooperative PC(sp3)P pincer
catalysts was synthesized and characterized. Their catalytic activity in
the acceptorless dehydrogenative coupling of alcohols and the
transfer hydrogenation of aldehydes with formic acid as a hydrogen
source was investigated. This comparative study, examining
homogeneous and polymer-tethered species, proved that carefully
designing a link between the support and the catalytic moiety, which
takes into consideration the mechanism underlying the target
transformation, might lead to superior heterogeneous catalysis.
substrate activation and product formation.[5] The concept of
metal–ligand cooperativity has been exploited to develop a new
generation of bifunctional catalysts (Figure 1). Each of these
systems operates via
a unique ligand-metal cooperation
mechanism; however, it is inapplicable for large-scale synthesis
due to the high direct costs of the transition metals and the even
higher costs of the sophisticated ligands.[6] Therefore, replacing
these current homogeneous catalysts with new, efficient
heterogenized equivalents has attracted much interest.
H
X
H
X
Introduction
X
H
H
N
N
H
M
M
Ru Ru
CO
M
H
O
The grand challenge in modern organic chemistry is to achieve
an ideal synthesis in terms of selectivity, atom-, and step-
efficiency.[1] This goal led us to develop new catalytic
methodologies allowing us to achieve clean and sustainable
production of chemicals in such a way that the price of the
catalysts, the use of toxic reagents and stoichiometric promoters,
the investment in time and energy, as well as the production of
wastes are well balanced. Among such methodologies, a one-pot
operation using heterogeneous catalysis is important, because it
saves time and energy owing to the straightforward separation
and purification procedures, as well as the easy recycling of the
catalyst.[2] From the perspective of environmentally friendly
organic synthesis, alternative reaction pathways, based on C-H
activation, and hydrogen transfer reactions, which occur via either
H-X or H2 evolution, rather than via production of high molecular
weight by-products, have recently attracted much attention, since
they are both waste- and atom-economical.[3] [4]
CO
CO
CO
X = O, N
Bera et al.6n
R2
P
O
O
Ph
Ph
Ph
H
H
H
Ru
Ru
Ph
Ru
Ph
N
Cl
PR2
N
N
Ph
Ph
HN
Ir
Tf
Ru
NTs
Ph
Cl
Cl
Fujita et al.5l
OH
OC
Ph
CO
CO
OC
Ph
Shvo et al.16a
Milstein et al.5a
Ikariya et al.5m
H
OH
H
N
N
P(i-Pr)2
O
Ru
HN
i-Pr
Ir
H2N
N
N
Ir
Ph2P
P(i-Pr)2
N
PPh2
Cl
Ph
N
Gelman et al.14
Morris et al.5k
Kempe et al.5i
Figure 1. Ligand-metal cooperating systems.
The most spectacular examples of homogeneous catalysts
capable of promoting such processes belong to an emerging
class of catalysts in which the ligand backbone actively
participates in the bond-breaking and bond-making processes via
a reversible structural transformation of the catalyst during
The strategies for sustainable catalysis are numerous and include
covalent anchoring to inorganic and organic supports, physical
immobilization in inorganic or hybrid matrices,[7] liquid/liquid or
solid/liquid biphasic conditions,[8] and others.[9] All these strategies
are successful in recycling transition metal catalysts as well as
removing metal complexes from waste products. However,
unfortunately, when compared with their homogeneous
counterparts, the heterogenized catalysts often suffer from lower
performance and diminished selectivities due to poorely defined
catalytic sites, thus nullifying all the benefits.
‡ Both authors contributed equally to this work
[a]
S. Mujahed, S. Cohen, Prof. Dr. D. Gelman
Institute of Chemistry, Edmond J. Safra Campus, The Hebrew
University of Jerusalem, Jerusalem, 91904 Israel
E-mail: dmitri.gelman@mail.huji.ac.il;
https://scholars.huji.ac.il/dmitrigelman/home
Organic polymers are arguably among the most versatile catalyst
supports available because of the tunability of various parameters
such as molecular weight, polydispersity, support flexibility,
solubility, and others.[10] However, careful design of the polymer
support alone does not guarantee the desired catalytic activity
and selectivity. Clearly, fine-tuning the link between the support
and the catalytic moiety, while taking into consideration the
mechanism underlying the target catalysis as well as possible
catalyst decomposition pathways, could achieve this goal. A
[b]
[c]
Prof. Dr. D. Gelman
Peoples’ Friendship University of Russia (RUDN University),
Miklukho-Maklay St., 6, 117198 Moscow, Russia
F. Valentini, Prof. Dr. L. Vaccaro
Dipartimento di Chimica, Biologia e Biotecnologie, Università degli
Studi di Perugia, Via Elce di Sotto, 8, 06124 Perugia, Italy
E-mail: luigi.vaccaro@unipg.it; http://www.dcbb.unipg.it/greensoc
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cssc.201901728
ChemSusChem
FULL PAPER
detailed understanding of the support/catalyst interface is even
more important when the aim is to immobilize transition metal
catalysts bearing so-called non-innocent (actor) ligands, because
even a slight structural modification toward their anchoring may
have pronounced deleterous effects on the performance.
for 24 hours at 60 °C, affording PS-L with a loading of 0.33 mmol/g
(Scheme 1). The high regioselectivity of this step is usually
dictated by a diminished steric demand of the backside group.
H
H
OH
OH
We recently disclosed a series of Ir- and Ru-based PC(sp3)P
H
H
HO
H
HO
Cl
pincer complexes that can be efficiently prepared in
a
combinatorial fashion via Diels-Alder cycloaddition of
a
anthracene-based pincer ligands or their corresponding
Ir
Ru
11]
complexes to a variety of symmetrical dienophiles.[5j,
The
Ph2P
Ph2P
Cl
PPh2
PPh2
OC
complexes proved themselves as active ligand-metal cooperating
catalysts for efficient acceptorless dehydrogenative processes
and other reactions.[12]
CO
2
1
Figure 3. Complexes used in this work.
actor
spectator
H
The 31P NMR of the free ligand L possessed a pair of well-
resolved doublets at -18.5 and -19.8 ppm in CDCl3. 31P CP-MAS
of the supported ligand exhibits a single broad signal centered at
-15 ppm, indicating the successful anchoring of the ligand to the
support (Figure 4).
FG
H
PR2
PR2
FG
L
M
R2P
PR2
L
M = Pd, Ni, Pt, Rh, Ru, Ir
o
Metalation of PS-L with [IrCl(COE)]2 proceeds at 60 C in a i-
PrOH/acetonitrile mixture to afford PS-1 as a light-cream powder
(Scheme 2). The 31P CP-MAS spectrum shows the presence of
some unreacted ligand in addition to two overlapping signals
centered at 30 and 12 ppm (Figure 4). The major signal (30 ppm)
presumably belongs to PS-1 - the unsupported 1 that appears in
the 31P NMR as a virtual double doublet centered at 28 ppm. The
second peak (12 ppm) may be assigned to a PS-1-derived iridium
alkoxide species that formed as a result of the intramolecular
proton-hydride interaction described in Scheme 2. The chemical
shift of this minor peak correlates well with the previously reported
1’ that features a pair of doublets at 15 and -1 ppm.[14] MP-AES
analysis shows 0.15 wt % of iridium in PS-1. The species PS-1
H
H
OH
OH
H
H
HO
-H2
O
H
Ir
+H2
Ir
Ph2P
Ph2P
PPh2
PPh2
Cl
Cl
1
1'
Figure 2. Catalysts developed by our groups.
and PS-1‘ are interconvertible and are both catalytically active as
Mechanistic studies revealed that these catalysts can
activate/form chemical bonds via an intramolecular interaction
between the metal center and the ligand’s functional frontside arm
of the catalyst, whereas the backside is a spectator, although it
can be used as an anchor (Figure 2).[13] In this manner,
immobilization is achieved with minimal perturbation of the
catalyst, so that both the catalytic site and the support/catalyst
interface remain well defined.
[14]
was demonstrated by us previously,
purification is required.
thus no isolation or
PS
H
OH
H
H
O
NaH, DMF
Cl
HO
H
HO
PS
We wish to report the results of our comparative studies on the
catalytic activity of Merrifield resin-anchored catalysts in several
acceptorless dehydrogenative coupling reactions.
Ph2P
PPh2
L
Ph2P
PS-L
PPh2
Scheme 1. The immobilization step of L on Merrifield resin.
Results and Discussion
H
O
H
Ir
O
H
PS
H
PS
HO
H
[IrCl(COE)]2
Immobilization. The previously described iridium-based 1 and
ruthenium-based 2 materials were chosen as catalysts (Figure 3)
and the standard commercially available Merrifield resin (200-400
mesh, 3.5-4.5 mmol/g of Cl-loading, 1% cross-linked) as a support
for this study. The less sterically demanding backside hydroxyl
group has been used for the immobilization of ligand on Merrifield.
L has been tethered to microporous polystyrene via nucleophilic
substitution of the chlorine atoms of a Merrifield resin (PS) in DMF
O
PS iPrOH/MeCN
Ir
H
O
Ph2P
Ph2P
Cl
PPh2
Cl
PS-1'
PPh2
H
PS-1
HO
H
O
H
PS
HO
Cl
[RuCl2(CO)3]2
Ph2P
PS-L
PPh2
3 stereoisomers
CHCl3
Ru
Ph2P
PPh2
OC
CO
PS-2
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10.1002/cssc.201901728
ChemSusChem
FULL PAPER
Scheme 2. Synthesis of PS-1 and PS-2.
was less efficient than 1 so that complete conversion could only
be achieved at a higher catalyst loading (2 mol% versus 1 mol%
for 1) after heating in boiling xylene for 24 hours (12 h for 1) in the
presence of equimolar KOH. Applying these reaction conditions
for the cross-coupling of 1-phenylethanol and 4-methylbenzyl
alcohol, catalyzed by the Merrifield-supported PS-1 and PS-2 in
amounts that corresponded to 1 and 2 mol%, respectively,
showed full conversion of the starting material to 1-phenyl-3-(p-
tolyl)propan-1-one in both cases, implying that catalyst loading
may be reduced (Scheme 4). Indeed, a brief optimization revealed
that full conversion of the model substrates could be achieved
using only 0.02 mol% of PS-1 and 0.2 mol% of PS-2, indicating
TONs of 5000 and 500, respectively. These high TON values are
especially remarkable because they are at least one order of
magnitude higher than for the corresponding non-supported
complexes and comparable to the most effective heterogeneous
systems described in the recent years and featuring high
efficiency in terms of recoverability, recycle and durability.[16]
Experimentation with different bases such as KOt-Bu, Cs2CO3,
DBU, or Et3N, as well as different reaction media such as DMF or
DME led to no further improvement. Thus, a stoichiometric
mixture of 4-methylbenzyl alcohol and 1-phenyl ethanol resulted
in the formation of 1-phenyl-3-(p-tolyl)propan-1-one in 92 and
86% yield, after being catalyzed by 0.02 mol% of PS-1 and 0.2
mol% of PS-2, respectively (Table 1, entry 1).
Synthesis of the ruthenium-based PS-2 proceeded smoothly as
well. Thus, exposing PS-L to [RuCl2(CO)3]2 in chloroform at room
temperature for 24 hours (Scheme 2) resulted in the formation of
three stereoisomers of the target compound, different from the
location of the chloride ligand, as was described by us previously
(50, 16, and 0 ppm, respectively). The 31P CP-MAS spectrum
shows the presence of nonmetalated sites in addition to the latter.
MP-AES analysis revealed 0.5 wt % ruthenium in PS-2.
Figure 4. 31P CP-MAS spectrum of PS-1 (red line), PS-2 (green line) in
comparison with PS-L (blue line).
OH
OH
O
PS-1
PS-2
or
+
KOH, Na2SO4
toluene reflux
Catalysis. The functional resins PS-1 and PS-2 were then tested
in the acceptorless dehydrogenative cross-coupling of primary
and secondary alcohols to form β-alkylated ketones.[13c] This one-
pot dehydrogenation of alcohols to the corresponding carbonyl
compounds was followed by aldol condensation and subsequent
partial double bond hydrogenation under the same reaction
conditions using the unsupported 1 and 2, which were described
by us in the past and, therefore, they appear to be a good
benchmark now for comparison purposes (Scheme 3 and Table
1).
3a
4a
5aa
Full conversion
PS-1
: 0.02 mol% 16h
PS-2: 0.2 mol% 24h
Scheme 4. The optimized conditions in the acceptorless dehydrogenative
cross-coupling of primary and secondary alcohols with PS-1 and PS-2.
The optimized reaction conditions are applicable to the cross-
coupling of a variety of primary and secondary alcohols according
to the same scheme. Excellent isolated yields (89-97%) of the
corresponding cross-coupled products were obtained using 1-
phenyl ethanol and electron-rich to electron-deficient benzyl
alcohols as coupling partners and 0.02 mol% of PS-1 as a catalyst
(Table 1, Entries 1-4). The reactions catalyzed by 0.2 mol% of PS-
2 exhibited generally lower selectivity toward the cross-coupled
product (63-86% isolated yield) because of the completive homo-
coupling of the primary alcohol to the corresponding Tischenko
product, benzyl benzoate (Table 1, Entries 1-4).
O
OH
OH
Cat
+
H2
+
Base
Cat
Cat
- 2 H2
+ H2
O
O
O
Base
The same reactivity trend was observed when benzyl alcohol was
reacted with a series of 1-phenylethanols possessing electron
withdrawing and electron-donating groups – complete conversion
to the cross-coupled products in excellent selectivity and isolated
yield were obtained using the iridium-based PS-1 (Table 1,
Entries 5-7), whereas only moderate to poor yields were observed
using the ruthenium-based PS-2. We also found that aliphatic
primary alcohols are suitable coupling partners despite that upon
dehydrogenation they acquire an enolizable nature.
+
Scheme 3. Acceptorless dehydrogenative cross-coupling of primary and
secondary alcohols forming β-alkylated ketones.
The optimal reaction conditions derived for the unsupported
catalysts 1 and 2 were used as the starting point for our initial
experimentation. As we found previously, the ruthenium-based 2
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10.1002/cssc.201901728
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Here, no self-coupling was observed under iridium or ruthenium
catalysis (Table 1, entry 8-9). Finally, homo-coupling of the
secondary 1-phenyl ethanol was carried out, also with moderate
selectivity (ca. 70% isolated yield of the coupled product
accompanied by acetophenone as a major by-product). It is
remarkable that at least for the iridium-based supported catalyst,
PS-1, the performance is comparable to that of the unsupported
1, whereas its stability is significantly higher (5000 TONs for PS-
1 versus 830 for 1).
terms of stability and reusability, high TON values have been
reached, from 24200 to 20200 (Table 2, entries 1-3) when PS-1
was employed and 1735 and 1935 (Table 2, entries 2-3) using
PS-2. High efficiency and TONs values of heterogeneous
supported catalysts PS-1 and PS-2 are expected due to a higher
concentration induced on the catalyst surface as a results of the
immobilization of the active catalyst. This effect also suggests that
the after the immobilization, the combination polymeric
support/reaction medium used are effective for regulating the
swelling phenomenon and as a results, the catalytic sites are
easily accessible active.
Table 1. Representative coupling of the primary and secondary alcohols
catalyzed by 1-2 and PS-1-2.a-b
Table 2. Recycle of PS-1-2 in homo- and cross-coupling of alcohols a-b
[a] PS-1 (0.02 mol%) or PS-2 (0.2 mol%), R1OH/R2OH/KOH = 1/1/1, toluene.
[b] Yield of the isolated compounds as an average of two runs in entries 1-
3.
Encouraged by the excellent performance of the supported
catalyst, we decided to apply it to another transformation in order
to demonstrate the broad applicability of the approach. Based on
the efficient decomposition of formic acid by some of our
complexes, we decided to focus on some generic transformations
utilizing formic acid as a hydrogen source, such as the selective
transfer hydrogenation of carbonyl compounds.[12b, 13a] Although
numerous examples of transfer hydrogenation from formic acid
(FA) to aldehydes are known, many of them suffer from low
chemoselectivity.[17] The most frequent problem in these reactions
is aldol-type side-reactivity under basic reaction conditions, which
are typically required in transfer hydrogenation. In addition,
Tishchenko-type by-products may form under these conditions.[18]
Another possible side-reaction is the decarbonylation of
aldehydes, which is known to proceed with rhodium and iridium
complexes. Finally, uncontrollable hydrogenation of unsaturated
aldehydes often accompanies the hydrogenation of the carbonyl
group.[19] Therefore, to develop highly selective protocols, new
catalysts operating via alternative mechanisms may be employed.
Initially, the reaction conditions were optimized using unsupported
1. In the initial experiment, benzaldehyde as a model substrate
was hydrogenated with equimolar formic acid in DME.
Interestingly, only incomplete conversion of the model substrate
was obtained under these conditions with or without inorganic
basic additives. We suspected that premature decomposition of
[a] PS-1 (0.02 mol%) or PS-2 (0.2 mol%), R1OH/R2OH/KOH = 1/1/1, toluene.
[b] Yield of the isolated compounds as an average of two runs in entries 1-
10.
Recovery and reusability of the supported catalysts were
demonstrated. Thus, homo- and cross-coupling of alcohols was
performed up to five consecutive runs without a notable decline in
the performance of the catalyst PS-1 (Table 2, runs 1-3), whereas
the catalyst PS-2 (Table 2, entries 2-3), as expected, exhibited
poorer durability. Owing to the good properties of the catalysts, in
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10.1002/cssc.201901728
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FULL PAPER
FA to the corresponding H2 and CO2 causes an incomplete
reaction. We found, however, that addition of 10 mol% of
triethylamine impedes FA decomposition, thus, achieving a
complete reduction of the model substrate by 1. Replacing
triethylamine with 5% K2CO3 or 20% of sodium formate
dramatically lowered the conversion. We also investigated the
influence of different solvents on the benchmark reaction.
Whereas a low hydrogenation rate was observed in toluene,
affording low to moderate conversions, in THF, dioxane, and DME
the desired product formed equally well. The reaction temperature
had a major effect on the reactivity of the system, for example, an
increase from 60oC to 80oC raised the conversion rate from 19%
to 96%.
Next, we investigated the general applicability of our catalyst
system using both supported and unsupported catalysts. To
explore the functional group tolerance in more detail, we started
to investigate the reactions of different substituted benzaldehydes
(Table 3). First, unsubstituted benzaldehydes were fully
converted to benzyl alcohols (Table 3, entries 1 and 8) using both
stressing again the fact that heterogenization of the molecular
catalysts can be performed without a significant loss of their
activity.
Finally, recovery and reusability of the catalyst was tested by
repeatedly employing PS-1 as a catalyst in the transfer
hydrogenation
of
benzaldehyde,
p-bromo-,
and
p-
nitrobenzaldehyde. In all the reactions, some decline in the
reactivity was observed only after 5 consecutive runs.
Conclusions
In conclusion, we showed that careful design of the link between
the support and the catalytic moiety, which takes into
consideration the mechanism underlying the target transformation,
may lead to very efficient and stable heterogenized catalysts that
maintain their high performance in direct comparison with the
homogeneous ones and may even exceed them. In particular, we
synthesized and characterized a series of polymer-supported
cooperative PC(sp3)P pincer catalysts and demonstrated their
excellent catalytic activity in the acceptorless dehydrogenative
coupling of alcohols and the transfer hydrogenation of aldehydes
with FA as a hydrogen source. Remarkably high TONs were
PS-1 and
1 in nearly equal selectivity and efficiency.
Benzaldehydes bearing electron-donating (alkyl, p-, and o-
methoxy groups) and electron-withdrawing (bromo- and nitro-)
groups yielded the desired products in nearly quantitative yields
(Table 3, entries 2-7). Remarkably, benzaldehydes with sensitive
functional groups such as nitro were reduced with excellent
chemoselectivity under these conditions (Table 3, entries 6).
achieved
using
these
sophisticated
catalysts
after
heterogenization.
Experimental Section
Table 3. Representative transfer hydrogenation of aldehydes catalyzed by
1 and PS-1.a-b
All manipulations were performed using standard Schlenk techniques
under dry N2 or Ar. All reagents were purchased from the usual suppliers
and used without further purification. All reagents were weighed and
handled in air. Flash column chromatography was performed with Merck
ultra-pure silica gel (230-400 mesh). All catalytic reactions were carried out
under N2. Yields refer to isolated compounds with greater than 95% purity,
as determined by proton Nuclear Magnetic Resonance spectroscopy (1H-
NMR) analysis. The CAS numbers of the known compound were listed.
Spectroscopy data of the known compounds match the data reported in
the corresponding reference. 1H-, 13C-, and 31P-NMR spectra were
recorded on Bruker 400 or 500 MHz instruments with chemical shifts
reported in ppm relative to the residual deuterated solvent or the internal
standard tetramethylsilane. Elemental analysis of phosphorous for loading
calculations was carried out using PE analyzer 2400 Series II. Metal
loading was measured using a MP-AES 4210 instrument.
Synthesis of ligand PS-L. The 25 mL round bottom flask equipped with a
double surface condenser was charged with L1 (0.997 g, 1.57 mmol) in 10
mL of anhydrous DMF under N2 flow. To clear the solution, NaH (251.2
mg, 60% oil dispersion, 6.28 mmol, 4 equiv.) was added portionwise and
then the reaction mixture was heated at 50 °C for 30 minutes. After having
been cooled to RT, the Merrifield resin (1.57 g, 5.52 mmol, 3.5 equiv.) was
added and heating to 70 °C was resumed for another 36 hours. Filtration
of the product afforded PS-L as a light-beige solid that was washed with 3
mL of milli-Q water, 3 mL of MeOH, 3 mL of THF, and 3 mL of hexane and
dried under vacuum for 24 h. The loading was determined by Elemental
Analysis on phosphorous (P: 2.05 %, 0.33 mmol/g).
[a] PS-1 or 1 (0.01 mol%), RCHO/FA/TEA = 1/2/1, DME, 80 oC. [b] Yield of
the isolated compounds as an average of two runs in entries 1-9.
Similarly, the hydrogenation aliphatic aldehydes were efficiently
accomplished (Table 3, entry 9). Remarkably, decarbonylation,
Cannizzaro, and aldol condensation reactions were not observed
in this run using both supported PS-1 and unsupported 1,
Synthesis of PS-1. The supported ligand PS-L (800 mg) and [IrCl(COE)2]2
(122mg, 0.14 mmol, 0.5 equiv.) in an acetonitrile/isopropanol mixture was
stirred for 24 hours at 60 °C under a constant flow of N2. Filtration provided
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10.1002/cssc.201901728
ChemSusChem
FULL PAPER
the product as a light-cream solid and then it was washed with 3 mL of
THF and 3 mL of hexane.
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Synthesis of PS-2. The supported ligand PS-L (800 mg) and [RuCl2(CO)3]2
(70 mg, 0.14 mmol, 0.5 equiv.) in CHCl3 was stirred for 24 hours at RT
under a constant flow of N2. Filtration afforded the product as a yellow-
orange solid and then it was washed with 3 mL of THF and 3 mL of hexane.
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General reaction conditions for coupling of the primary and secondary
alcohols. A flame-dried Schlenk tube was charged with PS-1 or PS-2
(0.02-0.2 mol%, 25-40 mg, respectively), KOH (1 equiv.), and Na2SO4 (200
mg) in toluene (1.5 mL) under Nitrogen. Alcohols (1 mmol of each) were
added and the mixture was heated to 150 °C with stirring for the specified
time. Filtration was performed and then the filtrate was evaporated under
reduced pressure to obtain the product. The product was purified by Combi
Flash Chromatography.
General reaction conditions for the transfer hydrogenation of aldehydes. A
flame-dried Schlenk was charged with PS-1 (0.01 mol%, 13 mg) and
FA/TEA (2 equiv.) in DME (1 mL) under Nitrogen. The aldehyde (1 mmol)
was added and the mixture was heated at 80°C with stirring for the
specified time, followed by filtration. Then the filtrate was evaporated under
reduced pressure to obtain the product, which was purified by Combi Flash
Chromatography.
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Acknowledgements
This research was supported by the ISF (Israel Science
Foundation) Grant Nos. 917/15 and 1384/13. The authors declare
no competing financial interests. The Università degli Studi di
Perugia and MIUR are also acknowledged for financial support to
project AMIS, through the program “Dipartimenti di Eccellenza -
2018-2022”. This work was supported by the RUDN University
Program “5-100”. The Lady Davis Fellowship Trust is gratefully
acknowledged for the support and fellowship awarded to L.V. for
his stay at The Hebrew University of Jerusalem.
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Keywords: cooperative catalysis • heterogenized • pincer
complexes • acceptorless dehydrogenation • transfer
hydrogenation • formic acid
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Entry for the Table of Contents
Layout 1:
FULL PAPER
Durable and recyclable polymer-
supported bifunctional cooperative
PC(sp3)P pincer catalysts for
acceptorless dehydrogenative
coupling of alcohols and transfer
hydrogenation of aldehydes.
Shuruq Mujahed, Federica Valentini,
Shirel Cohen, Luigi Vaccaro*, Dmitri
Gelman*
Page No. – Page No.
Efficient, Recoverable, and Durable
Polymer-anchored Bifunctional
Pincer Catalysts for Chemoselective
Transfer Hydrogenation and Related
Reactions.
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