Regioselective rhodium-diphosphine ligand catalyzed hydroformylation of vinyl acetate

Article abstract of DOI:10.1016/S1872-2067(11)60384-7

Rhodium-catalyzed hydroformylation of vinyl acetate with the use of diphosphine ligands was studied. A high regioselectivity (branched:linear of 99:1) and activity (TOF: 4000 h^-1) under optimum conditions were achieved by using a 2,2'-bis(diphenylphosphino methyl)-1,1'-biphenyl ligand. The high turnover number (9200) obtained under mild conditions and stability of the catalyst indicates that it would be useful for industrial vinyl acetate hydroformylation.

Full text of DOI:10.1016/S1872-2067(11)60384-7

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CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 6, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 977–981.
ARTICLE
Regioselective Rhodium-Diphosphine Ligand Catalyzed
Hydroformylation of Vinyl Acetate
LIANG Haoran, ZHANG Lin, ZHENG Xueli, FU Haiyan, YUAN Maolin, LI Ruixiang, CHEN Hua*
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, Sichuan, China
Abstract: Rhodium-catalyzed hydroformylation of vinyl acetate with the use of diphosphine ligands was studied. A high regioselectivity
(branched:linear of 99:1) and activity (TOF: 4000 hꢀ1) under optimum conditions were achieved by using a 2,2’-bis(diphenylphosphino
methyl)-1,1’-biphenyl ligand. The high turnover number (9200) obtained under mild conditions and stability of the catalyst indicates that it
would be useful for industrial vinyl acetate hydroformylation.
Key words: vinyl acetate; hydroformylation; regioselectivity; diphosphine ligand; 2-acetoxypropanal
Rhodium-catalyzed hydroformylation is an important reac-
tion in academic research and industry [1]. Much effort has
been devoted to the hydroformylation of functional olefins to
give bifunctional products with greater added values [2ꢀ5]. For
example, the hydroformylation of industrial vinyl acetate gives
1,2- and 1,3-bifunctional products with wide applications
[6ꢀ8] (Scheme 1). However, due to the chelating effect of the
ester carbonyl [9], there has been limited progress in this area,
and there are few results with acceptable selectivity and rates.
Williams et al. [10] reported that the hydroformylation of vinyl
acetate in ionic liquids gave satisfactory results, but the com-
plex procedure needed for the preparation of the ionic liquid is
a limitation [11]. Recently, Dabbawala et al. [12] reported good
catalytic activity with the use of a bulky phosphite,
tri-1-naphthylphosphite (P(ONp)3), as a ligand for the
Rh-catalyzed hydroformylation of vinyl acetate, and they
demonstrated that the ligand strongly influenced the catalytic
performance of the rhodium complex. An important drawback
of phosphite ligands is their instability, which limits their
practical application [13].
the use of sulfonated 1,1'-bis(diphenylphosphinomethyl)-2,2'-
biphenyl (BISBIS) as a ligand in Rh-catalyzed biphasic hy-
droformylation of high olefins gave good results [19ꢀ21],
which suggested that we can use the commercially available
diphosphine 1,1'-bis(diphenylphosphinomethyl)-2,2'-biphenyl
(BISBI) [22] in Rh-catalyzed homogeneous hydroformylation
of vinyl acetate. The satisfactory catalytic activity, excellent
regioselectivity for 2-acetoxypropanal (branched aldehyde),
and high turnover number (TON) that were obtained showed
the practical potential of diphosphine ligands in this reaction.
1 Experimental
All hydroformylation reactions were carried out in a
stainless steel autoclave of 60 ml stirred with a magnetic stirrer.
A typical procedure was as follows. A toluene solution of
Rh(CO)2(acac) [23], ligand, and vinyl acetate was added to the
autoclave, which was subsequently evacuated and purged with
synthesis gas three times. The autoclave was then pressurized
with synthesis gas and stirred at the reaction conditions. After
the reaction was completed, the autoclave was cooled quickly
to room temperature in an ice-water bath, and then vented
slowly. The reaction mixture was immediately analyzed on a
HP 9710 gas chromatograph equipped with an FID detector
and a capillary column (25 m ꢀ 0.53 mm) CP-SIL 5cb. The
We have previously reported on the hydroformylation of
high added value functional olefins [14]. The results showed
that bidentate phosphine ligands such as bis-3,4-diazaphos-
pholanes gave good regio- and enantioselectivities in the
asymmetric hydroformylation of vinyl acetate [15ꢀ18]. And
Received 30 December 2011. Accepted 7 March 2012.
*Corresponding author. Tel/Fax: +86-28-85412904; E-mail: scuhchen@163.com
English edition available online at Elsevier ScienceDirect (http://www.sciencedirect.com/science/journal/18722067).
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60384-7

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LIANG Haoran et al. / Chinese Journal of Catalysis, 2012, 33: 977–981
H
O
O
O
O
O
Rh(CO)2(acac)/Ligand
O
+
+
+
CO H2
Toluene
O
H
O
2-acetoxypropanal
branched (major)
3-acetoxypropanal
linear (minor)
OCH3
N
Ligands:
PPh2
PPh2
CH3O
CH3O
PPh2
PPh2
PPh2
PPh2
N
OCH3
P-Phos
BISBI
BINAP
Scheme 1.
Hydroformylation of vinyl acetate. BINAP: 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl; P-Phos: 2,2',6,6'-tetramethoxy-4,4'-
bis(diphenylphosphino)-3,3'-bipyridine.
1
products were identified by GC-MS and H NMR spectros-
copy.
tion rate. However, the chemoselectivity decreased from 95%
to 81%, with propanal as the main byproduct. This was ex-
plained by that a high temperature favored the direct reaction of
vinyl acetate with the rhodium hydride complex to ethylene
and acetic acid [9], and ethylene was converted to propanal
under the hydroformylation conditions.
To have a high reaction rate and acceptable chemoselectiv-
ity, 110 °C was chosen as the reaction temperature for the
following experiments.
2 Results and discussion
BISBI was chosen as the ligand to investigate the effect of
the reaction parameters on the Rh-catalyzed hydroformylation
of vinyl acetate. The branched aldehyde was the main product,
with propanal, acetic acid, and a marginal amount of
2-acetoxypropanol as side products. To our surprise, no linear
aldehyde (3-acetoxypropanal) was detected in all the experi-
ments.
2.2 Effect of pressure
The total pressure of CO/H2 is also a key element in the
hydroformylation of vinyl acetate. As shown in Table 2, under
low pressure, both reaction rate and chemoselectivity were low.
Increasing the pressure from 2 to 6 MPa greatly improved the
reaction rate (no substrate was detected at 6 MPa), and in-
creased the chemoselectivity. The carbonyl group of vinyl
acetate hindered the insertion of CO into the Rh–R bond of the
rhodium complex to give the active intermediate Rh–COR,
which slowed down the reaction rate. A higher pressure
weakens the hindrance to CO insertion and improves the reac-
tion rate. In addition, more CO and H2 inhibited the direct
reaction of vinyl acetate with the rhodium complex to produce
ethylene, and suppressed the formation of the propanal by-
2.1 Effect of reaction temperature
Previous studies on the kinetics demonstrated that the tem-
perature plays a key role in the hydroformylation of vinyl
acetate [24], thus the influence of temperature on the activity
and selectivity of the Rh/BISBI catalyzed vinyl acetate hy-
droformylation was first investigated in the range of 80ꢀ120
°C. As shown in Table 1, the reaction rate was low at 80 °C
although a high regioselectivity was observed. The increase in
temperature from 80 to 120 °C led to a large increase in reac-
Table 1 Effect of temperature on Rh-catalyzed hydroformylation of
vinyl acetate with BISBI as ligand
Temperature Conversiona Chemoselectivityb
Table 2 Effect of pressure on Rh-catalyzed hydroformylation of vinyl
acetate with BISBI as ligand
Entry
Regioselectivityc
(°C)
80
90
100
110
120
(%)
25
52
76
96
96
(%)
95
92
90
90
81
Pressure Conversion Chemoselectivity
1
2
3
4
5
> 99
> 99
> 99
> 99
> 99
Entry
Regioselectivity
(MPa)
(%)
72
(%)
80
87
90
91
93
1
2
3
4
5
2
3
4
5
6
> 99
> 99
> 99
> 99
> 99
90
96
99
100
Reaction conditions: vinyl acetate = 5.4 mmol, [Rh] = 1.4 mmol/L, S:C =
2000, 30 min, 4 MPa (CO:H2 = 1), [BISBI]:[Rh] = 1.
aConversion of vinyl acetate. bSelectivity for 2-acetoxypropanal and
3-acetoxypropanal. cMolar ratio of branched to linear aldehyde.
Reaction conditions: vinyl acetate = 5.4 mmol, [Rh] = 1.4 mmol/L, S:C =
2000, 30 min, 110 °C, [BISBI]:[Rh] = 1.

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LIANG Haoran et al. / Chinese Journal of Catalysis, 2012, 33: 977–981
Table 3 Effect of molar ratio of BISBI to Rh on Rh-catalyzed hydro-
Table 5 Effect of phosphine ligand on Rh-catalyzed hydroformylation
formylation of vinyl acetate with BISBI as ligand
of vinyl acetate
BISBI/Rh Conversion Chemoselectivity
Conversion Chemoselectivity
Entry
Regioselectivity
Entry
Ligand
Regioselectivity
(mol/mol)
(%)
99
(%)
91
(%)
99
(%)
91
1
2
3
1
2
3
> 99
> 99
> 99
1
2
3
BISBI
BINAP
P-Phos
> 99
> 99
> 99
51
81
90
94
40
68
83
95
Reaction conditions: vinyl acetate = 5.4 mmol, [Rh] = 1.4 mmol/L, S:C =
2000, 30 min, 110 °C, 5 MPa (CO:H2 = 1).
Reaction conditions: vinyl acetate = 5.4 mmol, [Rh] = 1.8 mmol/L, S:C =
2000, 30 min, 110 °C, 5 MPa (CO:H2 = 1), ligand:Rh = 1.
tivity may be due to that the higher concentration of rhodium
gave more of the direct reaction of vinyl acetate with the rho-
dium hydride complex to ethylene, which gave the byproduct
propanal under the hydroformylation conditions.
product.
2.3 Effect of molar ratio of ligand to Rh
The effect of the concentration of ligand on the conversion
and selectivity of vinyl acetate hydroformylation was studied
by varying the molar ratio of BISBI to Rh from 1 to 3. The
results are shown in Table 3. A higher concentration of the
ligand decreased the conversion and chemoselectivity, and
increased the formation of the propanal byproduct. An excess
of BISBI prevents the catalyst from disassociating the catalytic
active species, and so the reaction rate declined. In addition,
increased stereo resistance of the catalytic active species due to
an excess of BISBI was also detrimental for the conversion of
coordinated vinyl acetate to an alkyl-Rh complex in the cata-
lytic cycle and slowed down the reaction rate. At the same
time, this would boost the direct reaction of vinyl acetate with
the rhodium hydride complex to form ethylene and acetic acid
[9] and increased the formation of the propanal byproduct. A
similar observation had been reported in the iridium-catalyzed
hydroformylation of olefins [25].
2.5 Effect of diphosphine ligands
The high regioselectivity and high activity when BISBI was
used prompted us to test other available diphosphine ligands in
the Rh-catalyzed hydroformylation of vinyl acetate. As shown
in Table 5, when BINAP and P-Phos were used as the ligand,
better selectivities were obtained as compared with when
monodentate ligands were used [7,12]. However, both BINAP
and P-Phos gave lower activities than BISBI under the same
reaction conditions. On comparing the 7-membered P-Rh-P
ring of BINAP or P-Phos with the 9-membered P-Rh-P ring of
BISBI, it is probable that the latter was more efficient in in-
hibiting the coordination of another carbonyl onto Rh to form
the intermediate (7) (see Section 2.6, Scheme 2). And with the
latter, the rate determining step, which is the oxidative addition
of H2 to the intermediate (8), would be more rapid and it gave a
higher conversion.
2.4 Effect of catalyst concentration
2.6 Reaction mechanism
The effect of the catalyst concentration was studied. The
results are shown in Table 4. When the concentration of rho-
dium was increased from 0.9 to 1.4 mmol/L, the reaction rate
was also increased. Further increase in the concentration of
rhodium from 1.4 to 2.2 mmol/L gave lower reaction rates.
Since the concentration of vinyl acetate was increased (sub-
strate to catalyst ratio was kept constant), this was probably due
to that the inhibition effect on the reaction rate from the ester
carbonyl group was enhanced. The decrease of chemoselec-
In previous studies, diphosphines with chelate bite angles
close to 120°, such as BISBI (bite angle of 112°), were shown
to give a high regioselectivity to linear aldehydes from terminal
olefins [26]. However,
a
reversed regioselectivity
(branched:linear > 99) was found in vinyl acetate hydrofor-
mylation with diphosphine ligands. Therefore, it can be sup-
posed that the chelating effect from the ester carbonyl of vinyl
acetate dominated the regioselectivity, where a more stable
intermediate with
a five-membered ring (5) via the
anti-Markovnikov addition of Rh-H to vinyl acetate is prefer-
entially formed to give the branched aldehyde,
2-acetoxypropanal (Scheme 2). By considering the better re-
gioselectivity that was obtained as compared to that obtained
with monodentate ligands [7], it can be inferred that the di-
phosphine ligand has more steric bulk in the rhodium complex,
which encouraged the formation of Rh-(branched alkyl) (5)
and is responsible for forming the branched aldehyde. The
mechanism for the formation of propanal and acetic acid is still
Table 4 Effect of concentration of catalyst on Rh-catalyzed hydrofor-
mylation of vinyl acetate with BISBI as ligand
[Rh]/
(mmol/L)
0.8
Conversion Chemoselectivity
Entry
Regioselectivity
(%)
90
(%)
89
1
2
3
> 99
> 99
> 99
1.4
2.2
99
91
94
83
Reaction conditions: toluene = 1.5 ml, S:C = 2000, 30 min, 110 °C, 5
MPa (CO:H2 = 1), [BISBI]:[Rh] = 1.

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LIANG Haoran et al. / Chinese Journal of Catalysis, 2012, 33: 977–981
propanal
hydroformylation
H2C CH2
vinyl acetate
H
Rh
CO
P Rh
P
P
CO
O
CO/H2
CO
P
acetic acid
1
O
O
_CO
O
+CO
H
O
H
2-acetoxypropanal
P Rh CO
P
2
H
Rh
O
H
Rh
CO
P
P
O
O
O
P
P
H
O
CO
O
9
3
H2
O
O
O
P
P
O
O
P Rh CO
Rh
O
P Rh CO
CO
P
4
5
P
8
O
O
O
CO
P
P
P
P
Rh
CO
O
O
Rh CO
CO
7
6
Scheme 2. Mechanism for vinyl acetate hydroformylation.
unclear. They could be formed by the decomposition of
3-acetoxypropanal or by the direct reaction of vinyl acetate
with the rhodium hydride complex 1 [9]. Because no
3-acetoxypropanal was observed in all the experiments, the
possibility of the decomposition of 3-acetoxypropanal was
excluded. Thus it is likely that the byproducts (propanal and
acetic acid) were formed by the direct reaction of vinyl acetate
with the rhodium hydride complex 1, as reported in the litera-
ture [9].
2.7 Catalyst stability
In order to investigate the industrial potential of the
Rh(CO)2(acac)/BISBI catalyst, vinyl acetate hydroformylation
was carried out under mild conditions (80 °C) for a prolonged
reaction time. The results are summarized in Fig. 1. The TON
increased to more than 9200 (conversion > 90%) and the high
selectivity was still maintained (up to 90%) after the prolonged
time (12 h), whereas the TON reported previously with the
rhodium complex of monodentate phosphite P(ONp)3 catalyst
was less than 2000 [12]. Therefore, catalyst Rh(CO)2(acac)/
BISBI showed better stability.
10000
8000
6000
4000
2000
0
3 Conclusions
0
2
4
6
8
10
12
Reaction time (h)
The use of a diphosphine ligand (BISBI, BINAP, P-Phos) in
the Rh-catalyzed hydroformylation of vinyl acetate gave ex-
cellent catalytic activity and high regioselectivity. A catalytic
mechanism was proposed. The high stability of the Rh/BISBI
Fig. 1. Hydroformylation of vinyl acetate for a prolonged reaction time.
Reaction conditions: vinyl acetate = 27 mmol, [Rh] = 1.4 mmol/L, S:C =
10000, 80 °C, 5 MPa (CO:H2 = 1), [BISBI]:[Rh] = 1.