Boosting the hydrolytic stability of phosphite ligand in hydroformylation by the construction of superhydrophobic porous framework

Article abstract of DOI:10.1016/j.mcat.2019.110408

The development of a catalyst that delivers high activities and selectivities with excellent durability is of great importance. Numerous efficient catalysts suffer from the inherent hydrolysis liabilities, plaguing their practical applications. Herein, we show that this challenge can be tackled by constructing them into superhydrophobic porous frameworks, as exemplified by a water-sensitive phosphite ligand, tris(2-tert-butylphenyl) phosphite. The efficiency and long-term stability of the developed system are remarkably high in the hydroformylation of internal olefins after metalation with Rh species, superior to the corresponding homogeneous analogues. The significantly boosted hydrolytic stability allows for catalytic transformations using water as a green solvent, which not only facilitates the isolation of the products, but also furnishes the aldehydes with higher regioselectivities for the desired linear form in comparison with that operated under benchmark conditions using toluene as a reaction medium. Given these promising results, we anticipate the strategy advanced herein will form the basis for constructive perspectives in the enhancement of the water resistance of catalysts and the development of high performance hydroformylation catalysts.

Full text of DOI:10.1016/j.mcat.2019.110408

MolecularCatalysis474(2019)110408
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Boosting the hydrolytic stability of phosphite ligand in hydroformylation by
the construction of superhydrophobic porous framework
Yongquan Tanga, Ke Donga, Sai Wanga, Qi Sunb,⁎, Xiangju Menga, Feng-Shou Xiaoa,⁎
a Key Laboratory of Applied Chemistry of Zhejiang Province and Department of Chemistry, Zhejiang University, Hangzhou 310028, China
b College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Superhydrophobic
Porous material
Porous organic polymer
Hydroformylation
Heterogeneous catalysis
Phosphite ligand
The development of a catalyst that delivers high activities and selectivities with excellent durability is of great
importance. Numerous efficient catalysts suffer from the inherent hydrolysis liabilities, plaguing their practical
applications. Herein, we show that this challenge can be tackled by constructing them into superhydrophobic
porous frameworks, as exemplified by a water-sensitive phosphite ligand, tris(2-tert-butylphenyl) phosphite. The
efficiency and long-term stability of the developed system are remarkably high in the hydroformylation of in-
ternal olefins after metalation with Rh species, superior to the corresponding homogeneous analogues. The
significantly boosted hydrolytic stability allows for catalytic transformations using water as a green solvent,
which not only facilitates the isolation of the products, but also furnishes the aldehydes with higher regios-
electivities for the desired linear form in comparison with that operated under benchmark conditions using
toluene as a reaction medium. Given these promising results, we anticipate the strategy advanced herein will
form the basis for constructive perspectives in the enhancement of the water resistance of catalysts and the
development of high performance hydroformylation catalysts.
Introduction
hydrolytic stability of the catalysts is crucial to maintain the long-term
stable outputs and to promote their industrialization.
Aldehydes are essential synthetic intermediates because they are
involved in a variety of CeC bond-forming reactions and because their
carbon oxidation level is easily reduced to that of an alcohol or in-
creased to a carboxylic acid. Hydroformylation (oxo synthesis) is a
commodity-scale process that uses synthesis gas and readily available
alkene feedstocks to produce the versatile aldehyde functional group,
often with high chemoselectivity [1–12]. With regards to olefin raw
materials, using mixtures of isomers as feedstock is preferred, given that
single olefins are not available at an economically feasible price, as an
olefin raw material usually is a technical mixture [13–15]. However,
the rate of the hydroformylation greatly falls with increasing steric
hindrance of the substrate and therefore the reactivity of internal C]C
bonds is much more sluggish in relation to the terminal counterpart.
Such that, only a limited number of rhodium catalysts afford acceptable
catalytic rates in the hydroformylation of internal olefins [16–22].
Significant progress has been made by using bulky phosphites as
modifying ligands, but they generally suffer from an inherent liability
towards hydrolysis as well as low thermal stability, retarding their use
in industry [23–29]. Considering that water is inevitably formed via
side-reaction of aldol condensation of aldehyde products, improving the
To tackle this challenge, we provide an effective strategy for pro-
tecting phosphite ligands from hydrolysis issues by constructing them
into superhydrophobic porous frameworks [30]. Given that the super-
hydrophobicity is stemmed mainly from the hierarchically porous
structure, the resultant catalysts fully retained the intrinsic reactivity of
the corresponding molecular catalysts while being imparted with ad-
ditional benefits, such as enhanced durability as well as facile recovery
and reuse, as demonstrated in the context of catalytic hydroformylation
of olefins and in the dehydrative allylation of allyl alcohol. We have
proven that due to the excellent water repellency of the super-
hydrophobic porous organic polymer scaffolds, they not only restrict
water molecules in the reaction medium to reach the active sites but
also quickly eliminate the in-situ generated water molecules from the
catalytic processes. These results highlight a major breakthrough for
improving the durability of water-sensitive ligands against hydrolysis
while simultaneously addressing the tedious and often expensive cata-
lyst recovery and recycling. In view of these promising results, we were
interested in investigating the impact of Rh species loading amounts on
the reactivity and stability with the aim to achieve the optimal catalyst
and to establish correct comparison with the state of the art materials.
⁎ Corresponding authors.
E-mail addresses: sunqichs@zju.edu.cn (Q. Sun), fsxiao@zju.edu.cn (F.-S. Xiao).
https://doi.org/10.1016/j.mcat.2019.110408
Received 19 March 2019; Received in revised form 14 May 2019; Accepted 16 May 2019
2468-8231/©2019PublishedbyElsevierB.V.

Y. Tang, et al.
MolecularCatalysis474(2019)110408
In addition, we also extend their use in the hydroformylation reactions
of practically relevant substrates under solvent-free conditions and in
aqueous reaction systems to facilitate the products’ isolation and make
the reaction greener.
chromatography equipped with a flame ionization detector and a
Supelco γ-DEX 225 capillary column.
Characterization
Experimental
Liquid NMR spectra were recorded on a Bruker Avance-400
(400 MHz) spectrometer in which the chemical shifts are expressed in
ppm downfield from TMS at δ = 0 ppm and J values are given in Hz.
Nitrogen sorption isotherms were measured using Micromeritics ASAP
2020 M and Tristar system and before the measurements, the samples
were degassed at 100 °C for 10 h. Inductively coupled plasma optical
emission spectroscopy (ICP-OES) analysis was measured with a Perkin-
Elmer plasma 40 emission spectrometer. Contact angles of various li-
quids were measured on a contact angle measuring system SL200KB
(USA KNO Industry Co.) with a CCD camera. Sessile drop mode was
used for the static contact angles measurements. Water vapor adsorp-
tion and desorption isotherms were collected via SMS Instruments DVS
Advantage equipped with a balance which has a sensitivity of 0.1 μg.
These isotherms were measured at 25 °C by monitoring the weight
change of the sample as a function of relative humidity of water. The
relative humidity of water was stepped up from 0 to 98% with an in-
crement of 10% in each step and then was stepped down to 0. 31P
(161.8 MHz) MAS NMR experiments were recorded on a Bruker Avance
500 spectrometer equipped with a magic-angle spin probe in a 1.9-mm
Materials
Solvents were purified according to standard laboratory methods.
Tetrahydrofuran (THF) was distilled over LiAlH4. Dichloromethane
(DCM) and toluene were distilled over CaH2. All other chemicals were
purchased in high quality and used without further treatment, unless
otherwise indicated.
Catalyst preparation
Synthesis of phosphite-POP
As a typical procedure, tris(2-tert-butyl-4-vinylphenyl) phosphite
(2.0 g) was dissolved in THF (20 mL), followed by the addition of
azobisisobutyronitrile (AIBN, 50 mg). The resulting solution was
transferred into an autoclave and maintained at 100 °C for 24 h,
yielding a monolithic solid. The title product was achieved after
washing with CH2Cl2 and drying under vacuum. Tris(2-tert-butyl-4-vi-
nylphenyl) phosphite was synthesized as follows: 2-tert-butyl-4-vinyl-
phenol (5.0 g, 28.5 mmol) and triethylamine (5.75 g, 125.4 mmol) were
dissolved in dry THF (200 mL) at 0 °C under N2 atmosphere and then
PCl3 (1.3 g, 9.5 mmol) diluted by THF (50 mL) was added dropwise.
After stirring overnight, the reaction was quenched by the addition of
saturated NH4Cl aqueous solution and extracted with ethyl acetate
(EtOAc, 3*100 mL). The combined organic phase was washed with
brine, dried over MgSO4, and concentrated to yield the crude product,
which was purified by column chromatography over silica gel (EtOAc/
hexane = 1/20) afforded tris(2-tert-butyl-4-vinylphenyl) phosphite as a
white solid (4.26 g, 86%). 1H NMR (400 MHz, CDCl3, 298 K, TMS): δ
7.38 (d, 1H, J = 2.0 Hz), 7.27–7.29 (m, 1 H), 7.16–7.19 (m, 1 H),
6.62–6.69 (m, 1 H), 5.63 (d, 1H, J = 17.6 Hz), 5.16 (d, 1H,
J = 10.8 Hz), 1.42 (s, 9 H) ppm. 13C NMR (100 MHz, CDCl3) δ 151.1,
139.9, 136.5, 132.7, 125.8, 124.5, 119.5, 112.7, 34.9, 30.0 ppm. 31P
NMR (162 MHz): δ 130.8 (s, 1 P) ppm.
Synthesis of x wt.%Rh/phosphite-POP catalyst (x stands for the
mass weight of Rh species in the catalyst). As a typical procedure,
the phosphite-POP (500 mg) was swollen in toluene (80 mL) for 30 min,
followed by the addition of Rh(CO)2(acac) (25.5 mg). After stirring at
room temperature under N2 atmosphere for 24 h, the mixture was fil-
tered and washed with an excess of toluene and dried under vacuum.
The resulting light yellow solid with a Rh loading amount of 2.0 wt.%
was denoted as 2.0 wt.% Rh/phosphite-POP, wherein the phosphite to
Rh species ratio is 9 (hereafter abbreviated as phosphite-POP). Rh, P, C
found: Rh, 2.0; P, 5.5; C, 77.8.%.
(
31P) ZrO2 rotor. IR spectra were recorded on a Nicolet Impact 410 FTIR
spectrometer. ICP-OES analysis was measured with a Perkin-Elmer
plasma 40 emission spectrometer.
Results and discussion
Material synthesis
The synthesis of the superhydrophobic porous organic polymer
scaffold turned out to be straightforward. Reaction of 2-tert-butyl-4-
vinylphenol with PCl3 in the presence of trimethylamine yielded vinyl-
functionalized tris(2-tert-butylphenyl) phosphite, which was quantita-
tively converted to the corresponding porous framework (phosphite-
POP) with the assistance of azobisisobutyronitrile (AIBN) using THF as
a porogenic solvent in an autoclave at 100 °C for 24 h (Fig. 1a). Derived
from the N2 sorption isotherms collected at −196 °C, the Brunauer-
Emmett-Teller (BET) surface area of the resultant polymer was calcu-
lated to be 643 m2 g−1 with the pore sizes mainly distributed in the
range of 0.5–1.4 and 2–10 nm (Fig. 1b, inset). The superhydrophobicity
of phosphite-POP was supported by the following evidences that this
material yielded a contact angle of water of 152° (Fig. 1c, inset)
[31–36] and showed negligible water vapor adsorption capability even
at a high relative pressure of 0.98 (Fig. 1c). Metalation with Rh species
was carried out by the addition of Rh(CO)2(acac) (acac = acet-
ylacetonate) to the phosphite-POP suspension in toluene. The formation
of a chelate complex was inferred from the
For the synthesis of the catalysts with other Rh species loading
amounts, similar synthetic recipe was used, except that different
amounts of Rh precursor were introduced. The ligand to Rh species
ratio in the resulting catalyst was calculated by the following equation:
Atom weight of Rh/(molecular weight of tris(2-tert-butyl-4-vinyl-
phenyl) phosphite x weight percent of the loaded Rh).
corresponding solid-state 31P NMR and X-ray photoelectron spec-
troscopy (XPS) spectra. In the NMR spectrum of Rh/phosphite-POP (the
ratio of phosphite moiety to Rh species is 9), in addition to the doublet
peak of the free ligand at 140.7 and 124.1 ppm, it also exhibited a signal
at 107.9 ppm, which can be assigned to the metalized phosphite ligand
(Fig. 2a) [37]. The binding energies of the rhodium 3d electrons in Rh/
phosphite-POP were found to be 308.5 and 313.2 eV for Rh3d5/2 and
Rh3d3/2, respectively, indicating that the chemical state of Rh species
was maintained as +1. These values were lower than those of the
precursor Rh(CO)2(acac) (309.9 and 314.6 eV); Whereas, the P2p
binding energy of Rh/phosphite-POP (133.5 eV) was greater than that
of the parent phosphite-POP (133.1 eV), confirming the coordination
between the Rh and P species in phosphite-POP (Fig. 2b). The IR
spectrum indicates that Rh/phosphite-POP contains the CO group
(2012 cm−1) and acetylacetone (1580 cm−1, Fig. 2c). Given that Rh+ is
a square-planar low-spin d8 metal ion, the corresponding complex is an
Catalytic tests
As
a typical prodecure, Rh-based catalyst (0.1 mol%), olefin
(3 mmol), and solvent (10 mL) were added into a stainless steel auto-
clave (100 mL) with a magnetic stir bar. After sealing and purging with
syngas (CO/H2 = 1:1) for 3 times, the pressure of syngas was adjusted
to the desired value and the autoclave was put into a preheated oil bath
and stirred with a speed of 1000 rpm. After the reaction, the catalysts
were taken out from the system by centrifugation and analyzed by gas
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Y. Tang, et al.
MolecularCatalysis474(2019)110408
Fig. 1. (a) Chemical structure of 2-tert-butyl-4-
vinylphenol and synthetic scheme of phos-
phite-POP. (b) N2 sorption isotherms of phos-
phite-POP collected at −196 °C, inset: corre-
sponding pore size distribution based on the
nonlocal density functional theory method. (c)
Water vapor sorption isotherms collected at
25 °C, inset: photograph of a water droplet on
the pellet disk made from phosphite-POP.
exception to the 18-electron rule, being a 16-electron complex. Ac-
cordingly, we can formulate the structure of Rh species in Rh/phos-
phite-POP that each Rh ion is coordinated with one CO group, one
phosphite ligand, and one acac compound.
varying the amount of the introduced Rh precursor. To achieve the
optimal ligand to Rh ratio, catalytic performances of the resulting
catalysts with the P/Rh ratios in the range of 2–36 were evaluated in
the hydroformylation of 2-octene at 100 °C, 2 MPa of syngas (CO/
H2 = 1/1), and a substrate to Rh mole ratio of 3000. Comparison of
these results showed the impact of this parameter was not as significant
as that for phosphine-based catalytic systems [38–40] and all the cat-
alysts afforded an aldehyde yield of around 50% after 6 h with very
similar chemo- and regioselectivity (Fig. 3a). Indeed, theoretical and
spectroscopic results revealed that either for electronic or steric rea-
sons, bulky phosphite ligands gave rise to complexes with low co-
ordination numbers and thereby ready formation of unsaturated
Catalytic evaluation
Since it is recognized that the ligand (amount and type) constitutes
a crucial element in hydroformylation, we therefore investigated the
influence of the Rh loading amount on the reactivity of the resulting
catalyst. Given the homogeneity of the phosphite moieties in phosphite-
POP, the ratio of ligand to Rh (P/Rh) can be continuously adjusted by
Fig. 2. (a) Solid-state 31P NMR spectra. (b) Rh3d and P2p XPS spectra. (c) IR spectra.
3

Y. Tang, et al.
MolecularCatalysis474(2019)110408
Fig. 3. (a) The influence of the ratio of phosphite ligand to Rh species in the catalyst on the aldehyde yield. All the catalysts tested gave an aldehyde selectivity of
higher than 99.5% and a linear to branched aldehyde ratio of 0.32. Reaction conditions: 2-octene (3.0 mmol), toluene (10 mL), 100 °C, 6 h, catalyst based on Rh
(0.1 mol%), and CO/H2 = 1:1 (2.0 MPa). (b) Recycling tests of the x wt.%Rh/phosphite-POP (blue, orange, grey, yellow, and purple correspond to the catalyst with a
P/Rh ratio of 36, 18, 9, 4, and 2, respectively). Reaction conditions: 2-octene (3.0 mmol), toluene (10 mL), 100 °C, 12 h, catalyst based on Rh (0.1 mol%), and CO/
H2 = 1:1 (2.0 MPa). (c) The plot of the ratio of phosphite ligand to Rh species in the catalyst on the total Rh leaching amount after 5 cycles. Dashed lines are
guidelines for the eyes.
species. Moreover, previous NMR spectroscopic investigations showed
that only one tris(2-tert-butylphenyl) phosphite ligand was bound to the
Rh center during the catalytic cycles, lending support to our catalytic
results [41].
reaction systems facilitate the product isolation processes [43–46]. The
kinetic profiles revealed that in solvent-free conditions, a comparable
initial rate and selectivity were observed as those using toluene as a
reaction medium in the hydroformylation of 2-octene; however, along
with the reaction proceeding, the aldehyde productivity was decreased
(Fig. 4). A steady decline in productivity from 0.295 mmol h−1 during
the initial 2 h to 0.2 mmol h−1 during the period of 6 to 8 h was ob-
served. Surprisingly, when water was used as a reaction solvent, sig-
nificant improvements in both parameters, namely, the productivity of
aldehydes and the selectivity to nonanal were achieved, from 0.32 to
0.61. Taken together, the key finding was that the use of the rather
unusual solvent H2O, which was considered to be detrimental to cata-
lysis involved with phosphite ligands, led to significantly higher se-
lectivity for this industrially important reaction relative to the bench-
mark conditions. To gain more insights, the performance of the
corresponding homogeneous analogous (tris(2-tert-butylphenyl) phos-
phite/Rh = 9) was evaluated under otherwise identical conditions. A
marked increase in regioselectivity (l/b) from 0.4 to 0.62 was also
observed. However, such an outcome was not stable with steady re-
duction in activity and selectivity, likely as a result of the decomposi-
tion of the ligand. To provide evidence for this assumption, 31P NMR
analysis was carried out, showing that nearly half of the phosphite li-
gand was hydrolyzed after 12 h. The aforementioned results verified
that the construction of the superhydrophobic framework can protect
phosphite moieties against hydrolysis. This was further supported by
the following experimental evidences. We were able to perform this
transformation on a 20-mmol scale while simultaneously lowering the
Next, we turned our attention to investigate their stability, as for
solid-supported homogeneous catalysts to be implemented in industry,
they must be recyclable for long-term use. The reuse of these catalysts
was evaluated by performing a series of consecutive runs. We observed
no deterioration of the catalytic performance in terms of both activity
and selectivity for at least ten cycles for the catalysts with ligand to Rh
ratios higher than nine. However, decreases in rate in successive cata-
lytic cycles were observed for those with a higher Rh loading (Fig. 3b),
suggestive of the occurrence of Rh leaching, which was verified by Rh
analysis of the product by means of inductively coupled plasma-optical
emission spectroscopy (ICP-OES). No rhodium traces were detected in
any of the experiments using the catalyst with a P/Rh ratio higher than
9, indicating that phosphite-POP retained the rhodium quantitatively
during catalysis. However, an appreciable fraction of Rh species was
leached into the solution, with increased amount along with decreased
P/Rh ratio (Fig. 3c). After 5 cycles, a total Rh loss of 6.8% and 16.9%
was seen for the catalyst with a P/Rh ratio of 4 and 2, respectively.
These phenomena can be explained as follows: during the catalytic
cycles, it is accompanied by the breaking and reformation of the bonds
between the metal and ligand. Such that, the catalytic elements may
break away from the support, thereby resulting in the loss of activity
after recovery by filtration. Whereas, a high density of free ligands in
close proximity to the active sites facilitate the reconstitution of the
damaged catalyst, thus preventing them from decomposing and main-
taining the reactivity [42]. In this context, our heterogenization
strategy could be a promising solution to address the insurmountable
challenge encountered by the traditional immobilization as a result of
the continuous loss of the metal.
Given the high stability, superior catalytic performance, and po-
tential high volumetric productivity, Rh/phosphite-POP with a ligand
and Rh ratio of 9 was thus chosen as a representative sample for further
studies. Previously, we have shown that this catalyst is remarkably
stable under catalytic conditions, even in the presence of water, as
evidenced by the fact that in the mimic continuous hydroformylation
processes with consecutive introduction of substrate and gases every
12 h using wet toluene as the reaction medium (5.0 wt.% H2O), we
were able to use this catalyst for at least one week without loss of ac-
tivity and selectivity [30]. This long-term stability indicates that very
high turnover numbers can be achieved with this system. On the other
hand, as the products’ concentrations increased along with the suc-
cessive runs, the stable outputs implied that toluene is not a necessity
for the observed reactivity. Given this, the question arose, whether this
catalytic system is able to catalyze the hydroformylation reaction in a
solvent-free condition or using water as a solvent, as both of these
Fig. 4. Plots of aldehyde yields versus time of 2-octene hydroformylation.
Reaction conditions: 2-octene (1.0 g, 9 mmol), 100 °C, solvent (10 mL), if ap-
plicable, catalyst based on Rh (0.001 mmol), and CO/H2 = 1:1 (2.0 MPa) under
various reaction media, water (dark blue), toluene (dark green), and a solvent-
free condition (wine).
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Y. Tang, et al.
MolecularCatalysis474(2019)110408
catalyst loading to 0.0005 mmol of Rh without any detrimental effect
on the aldehyde yield or the regioselectivity, demonstrating the scal-
ability and practicality of this method. Specifically, after 120 h, a 93%
yield of aldehyde products and a TON value as high as 37,200 was
achieved. Remarkably, similar values (91% and TON at 36,400) could
also be yielded by the recycled catalyst, indicative of the robustness of
the catalyst. To rationalize the observed changes, interactions of water
with the hydride ligand or with coordinated olefin could be considered.
Water facilitates the formation of rhodium hydride (catalytically active
species) and enhances the hydride transfer step, thereby leading to a
higher activity [47]. The observed changes in regioselectivity can be
explained according to the well-accepted mechanism of hydro-
formylation. The insertion of an olefin into the Rh catalytic inter-
mediate is the critical step for hydroformylation regioselectivity. Thus,
stabilization of the transition state leading to the linear Rh–alkyl in-
termediate results in an increase in the linear aldehyde amount and,
consequently, a higher n/iso ratio. This is observed in the presence of
water in our system. It could be therefore proposed that nonbonding
interactions of water molecules present in the coordination sphere of
rhodium decrease the activation energy of the transition state as a result
of the interfacial interactions, leading to the linear Rh–alkyl inter-
mediate. In this way, the reaction pathway that leads to the linear al-
dehyde is favored. Detailed mechanistic studies to probe how water
molecules participate in the catalytic cycles and thus perturb the re-
action could be necessary, and research along this line is being con-
ducted [48,49].
For economic reasons, mixtures of internal and terminal olefins are
the starting materials of choice as feedstocks [16–22], for example di-n-
butene from fraction II of the refining process, offering a very finan-
cially attractive reactant. Given this, to further demonstrate the pro-
mising potential of our catalyst for practical utilization, we evaluated
its performance in the hydroformylation of the mixtures of octane
isomers. It was found that all octene isomers, including 1-octene, 2-
octene and 4-octene (m/m/m = 1/1/1) can be smoothly converted into
aldehyde products by Rh/phosphite-POP using water as a solvent under
standard conditions. Impressively, Rh/phosphite-POP afforded an al-
dehyde yield of 98.5% after 10 h, clearly outperforming its homo-
geneous counterpart (82.7%), and the results underscore the viability of
this catalytic system for the direct transformation of olefin isomer
mixtures into aldehydes.
It is worth to mention that Rh/phosphite-POP is the most efficient
immobilized hydroformylation catalyst to date [1–12]. Moreover, this
system is one of the few examples of a heterogeneous hydroformylation
catalyst that is free of catalytic element leaching, which sufficiently
addresses the persistent, unresolved challenge faced by traditional im-
mobilized catalysts. Furthermore, waste streams are expected to be
relatively small because: i) no side product, octane, was detected, ii)
water was used as a solvent, allowing for the ready separation of the
products, and iii) the catalyst was stable for a long time and showed no
metal leaching. Therefore, it can be concluded that Rh/phosphite-POP
is an interesting environmentally benign catalyst, giving great potential
for application in hydroformylation. We anticipate that the distinct
reactivity of hydroformylation using water as a solvent revealed in the
current system will inspire further advances in a range of fine chemical
transformation processes.
With the optimal conditions in hand, next, the scope and limitations
of the Rh/phosphite-POP catalyst for the hydroformylation of various
olefins were examined on 7 examples of industrially relevant transfor-
mations and found that the Rh/phosphite-POP catalytic system is
broadly applicable. Various aliphatic, aromatic, as well as functiona-
lized internal olefins were hydroformylated in high yields. Applying
other linear olefins such as 2-hexene, similarly to 2-octene, a con-
siderable reaction rate was also achieved, providing a mixture of hep-
tanals at the linear/branched ratio of 0.62. In addition, cyclic olefins,
such as cyclohexene and cyclooctene, reacted well: corresponding al-
dehyde was obtained in almost quantitative yield, although a low
Table 1
Scope of Rh/phosphite-POP catalyzed hydroformylation of olefinsa.
Entry Substrate Product
Time (h) Yield (%)
l/b or m/
ob
1
2
3
4
5
6
7
12
12
30
36
36
48
36
> 99.5
(88.3)
> 99.5
(84.5)
> 99.5
(74.1)
> 99.5
(80.2)
> 99.5
(90.1)
> 99.5
(65.2)
> 99.5
(85.4)
0.62 (0.63)
–
–
2.03 (1.98)
13.3 (12.1)
–
–
a
Reaction conditions: olefin (3.0 mmol), water (10 mL), 100 °C, Rh/phos-
phite-POP catalyst (0.1 mol%), and CO/H2 = 1:1 (2.0 MPa).
b
The ratio of linear/branched or meta-substituted/ortho-substituted alde-
hyde. The data in parenthesis were afforded by the homogeneous catalyst with
the mole ratio of tris(2-tert-butylphenyl) phosphite to Rh of 9 under otherwise
identical conditions.
reactivity was observed. In the case of heteroatom-substituted cyclo-
pentenes, such as 2,5-dihydrofuran and N-Boc-2,3-dihydro-1H-pyrrole,
the hydroformylation process also proceeded smoothly, and a mixture
of two different aldehydes was obtained with ortho- to meta- sub-
stituted ratio of aldehyde at 33/67 and 7/93, respectively. The hydro-
formylation of aromatic olefins proceeded to give the branched alde-
hyde preferentially. However, a reaction time of 48 h was needed to
achieve a complete conversion. When limonene was used, the internal
double bond remained intact and only the double bond in the side chain
was selectively hydroformylated to the corresponding aldehyde with an
excellent result.
To further highlight the superiority of Rh/phosphite-POP, we
evaluated the performance of the homogeneous counterpart in the
hydroformylation of the substrates listed in Table 1, under otherwise
identical conditions. It gave rise to inferior results in terms of both
activity and selectivity. We ascribe this to the hydrolysis of the tris(2-
tert-butylphenyl) phosphite ligand during the reaction as evidenced by
the 31P NMR analysis.
Conclusion
In summary, we have shown that resistance against hydrolytic de-
gradation of a phosphite ligand can be greatly improved after con-
struction into a superhydrophobic porous framework with retained
intrinsic reactivity. After metalation with Rh species, the resulting
catalysts were applied in the hydroformylation of challenging internal
olefins showing excellent results in terms of activity and selectivity.
More importantly, these catalysts afforded unprecedented durability
and unique performance using water as a green solvent. These results
highlight a major breakthrough in the sustainable hydroformylation of
challenging internal alkenes using readily available phosphites as
modification ligands. The full scope of the concept of stabilizing water-
sensitive homogeneous catalysts by the construction of super-
hydrophobic frameworks is presently under investigation for a range of
ligands and metal-catalyzed transformations. We anticipate that this
work will draw attention from the synthetic community for further
development of current technologies and contribute to the development
of next-generation catalysts for the industrially relevant hydro-
formylation of olefins.
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MolecularCatalysis474(2019)110408
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