Highly regioselective and active rhodium/bisphosphite catalytic system for isomerization-hydroformylation of 2-Butene

Article abstract of DOI:10.1007/s10562-011-0751-7

The formation of linear aldehyde from isomerization-hydroformylation of 2-butene represents an important subject and current task in industry. Both high activity and excellent regioselectivity were achieved in the rhodium-catalyzed 2-butene isomerization-hydroformylation with 2,2′- bis(dipyrrolylphosphinooxy)-1,1′-(±)-binaphthyl (1) as ligand. Bulky phosphite with electron-withdrawing pyrrol groups dramatically improved the selectivity of linear product, and a good yield of 90.5% aldehydes was obtained with an excellent linear aldehyde regioselectivity of 95.3% under optimized condition. Graphical Abstract: Bulky phosphite with electron-withdrawing pyrrol groups dramatically improved the selective of linear product, and an excellent yield of 90.5% aldehydes with 95.3% regioselectivity of linear aldehyde was obtained in the Rh-catalyzed isomerization- hydroformylation of 2-butene in the presence of ligand 1.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0751-7

Catal Lett (2012) 142:238–242
DOI 10.1007/s10562-011-0751-7
Highly Regioselective and Active Rhodium/Bisphosphite Catalytic
System for Isomerization–Hydroformylation of 2-Butene
•
Min Mo Tao Yi Cong-Ye Zheng
•
•
•
•
•
Mao-Lin Yuan Hai-Yan Fu Rui-Xiang Li
Hua Chen
Received: 9 November 2011 / Accepted: 28 November 2011 / Published online: 20 December 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The formation of linear aldehyde from isom-
erization–hydroformylation of 2-butene represents an
important subject and current task in industry. Both high
activity and excellent regioselectivity were achieved in the
rhodium-catalyzed 2-butene isomerization–hydroformyla-
tion with 2,20-bis(dipyrrolylphosphinooxy)-1,10-( )-binaph-
thyl (1) as ligand. Bulky phosphite with electron-withdrawing
pyrrol groups dramatically improved the selectivity of linear
product, and a good yield of 90.5% aldehydes was obtained
with an excellent linear aldehyde regioselectivity of 95.3%
under optimized condition.
aldehydes. The development of highly active and selective
isomerization–hydroformylation catalysts for internal
olefins is of great importance from economic and envi-
ronmental points of view. However, only a few progresses
have been reported [4–8]. ‘‘Ligand effect’’, which repre-
sents on both steric and electronic properties of ligand, can
drastically influence the rate and regioselectivity of the
hydroformylation reaction. The steric effect of bisphos-
phine ligand is determined by the natural bite angle in
combination with rhodium [9, 10]. Casey et al. showed
experimentally that a rhodium–bisphosphine complex with
a wide bite angle can easily form ee configuration (phos-
phine atoms occupy two equatorial positions of a trigonal
bipyramid) that played a key role in the high regioselec-
tivity of hydroformylation [11, 12] (Scheme 1). Later, the
same group found that large bite angled rhodium–bis-
phosphine complexes with electron-withdrawing group
would further improve the regioselectivity of internal olefin
hydroformylation [13]. Likewise, Leeuwen and Zhang
have proved the above conclusions [14–17]. Since 2-PH
(2-propylheptanol), the product of condensation and
hydrogenation of aldehyde that was obtained by hydro-
formylation of 2-butene, is superior to the standard 2-EH
(2-ethylhexanol) on physical aspects due to environmental
consideration and its potential value with the attractive
advantage of competition [18], therefore we focused on the
isomerization–hydroformylation of 2-butene to n-pentanal.
In this study, we screened 2,20-bis(dipyrrolylphosphinooxy)-
1,10-( )-binaphthyl (1) [19, 20], 2,20-bis(dipyrrolylphosphi-
nooxy)-1,10-( )-biphenyl (2) [15], and 2,20-bis(diphenyl-
phosphino)methyl)-1,10-biphenyl (BISBI 3) [21] (Fig. 1)
which have different electronic and steric effects to investi-
gate how the reaction conditions as well as the differences of
ligand structure influence the 2-butene hydroformylation
catalytic behavior.
Keywords Isomerization–hydroformylation ꢀ
Homogeneous catalysis ꢀ 2-Butene ꢀ Rhodium complex
1 Introduction
Hydroformylation of olefins is among the most important
industrial homogeneous catalytic processes for the prepa-
ration of aldehydes and corresponding alcohols which are
important precursors for various pharmaceuticals, agro-
chemicals, and plasticizers [1–3]. As internal olefins are
much cheaper and more readily available feedstock than
terminal olefins, recently, researches are focused on
the internal olefins hydroformylation to produce linear
M. Mo ꢀ T. Yi ꢀ C.-Y. Zheng ꢀ M.-L. Yuan ꢀ H.-Y. Fu ꢀ
R.-X. Li ꢀ H. Chen (&)
Key Laboratory of Green Chemistry and Technology, Ministry
of Education; College of Chemistry, Sichuan University,
Chengdu 610064, China
e-mail: scuhchen@scu.edu.cn
123

Highly Regioselective and Active Rhodium/Bisphosphite Catalytic System
239
2 Experimental
2.1 Reagents
H
H
OC
P
P
P
Rh CO
Rh CO
P
CO
ee
The solvent toluene was purified before use. The ratio of
hydrogen (99.99%) and carbon monoxide (99.9%) were
1:1. 2-Butene (97%, mixture of trans and cis), 1-butene
(98%), and other reagents were of analytical grade.
Rh(acac)(CO)2 [22], ligand 1 [19, 20], 2 [15], and BISBI 3
[21] were prepared according to literatures and their
structures were confirmed by IR and NMR spectrum.
ae
Scheme 1 Equilibrium of ee–ae configurations in rhodium trigonal
bipyramidal complexes
2.2 General Procedure of Hydroformylation
Experiments
N
P
N
P
N
N
N
N
O
O
O
O
P
P
P
P
N
N
The hydroformylation reaction was carried out in a 60 mL
stainless steel autoclave with magnetic stirring. Ligand and
Rh(acac)(CO)2 were dissolved in toluene at certain ratio,
and 2-butene was transferred into autoclave using Schlenk-
technique at low temperature. After filling with syngas to
3
1
2
Fig. 1 Structure of ligands 1, 2, and 3
Scheme 2 Proposed
mechanism for the
isomerization–hydroformylation
of internal olefin to linear
aldehyde
H
H
H
P
P
P P
CO
CO
CO
P
P
P P
CO
Rh P P'
CO
Rh CO
CO
Rh P P'
CO
HRh(P^P)(CO)2
HRh(P^P')(CO)3
HRh(P^P)(P^P')(CO)
III
I
II
O
+CO
-CO
CHO
H
O
P
P
P
H
Rh
Rh
+H2
P
+H2
CO
CO
IV
O
P
P
P
P
Rh
Rh CO
CO
OC
H
P
P
Rh
CO
+CO
CO
A
B
P
Rh
P
CO
P
P
P
P
Rh
Rh CO
CO
H
P
P
P
P
H
Rh
V
Rh
+CO
CO
CO
(A) hydroformylation of internal olefin, (B) isomerization-hydroformylation ofinternal olefin
123

240
M. Mo et al.
desired pressure, the autoclave was heated to the reaction
temperature and vigorously stirred. At the end of the
reaction, the vessel was cooled before excess syngas
was carefully released. Using n-heptane as internal stan-
dard, the products were analyzed on Agilent 6890N gas
chromatography with a capillary column SE-30 (30 m 9
0.25 mm).
played a key role in the reaction. At low temperature,
though high regioselectivity was observed, the activity was
unsatisfactory. High temperature (120 °C) that could
accelerate the isomerization of 2-butene to 1-butene was
more active for the isomerization–hydroformylation of
2-butene, and a good yield of 63.2% aldehydes was
achieved with an excellent regioselectivity of 96%
(Table 1, entries 1–4). And the results excelled the previ-
ous report (yield of 27.3% aldehydes with regioselectivity
of 86.3% for the hydroformylation of 2-butene using 2,20-
bis(diindolylphosphinooxy)-1,10-( )-biphenyl as ligand)
[23]. Further increasing the temperature the thermody-
namic equilibrium between b-hydride elimination of
branched rhodium–alkyl complex (Scheme 2, V) to
1-butene and 2-butene was broken (2-butene is more stable
than 1-butene on thermodynamics), resulting in lower
activity and regioselectivity (Table 1, entry 5). Compared
with isomerization–hydroformylation of 2-butene under
high temperature (120 °C), hydroformylation of 1-butene
was carried out at 60 °C, giving excellent activity and
regioselectivity (Table 1, entry 6). The results indicated
that high temperature at certain range benefits isomeriza-
tion process.
3 Results and Discussion
A goal of ligand screening is to achieve high activity and
regioselectivity in the isomerization–hydroformylation of
internal olefins. The catalyst must perform higher isomer-
ization rate of internal olefin to terminal olefin (Scheme 2,
B) than direct hydroformylation rate of internal olefin
(Scheme 2, A), so that the system can maintain a small
amount of terminal olefin in the olefin mixture under the
thermodynamic equilibrium, as a result of which, once
terminal olefin was formed, it would be immediately
transformed to aldehyde in the presence of syngas due to
the more quick hydroformylation rate of terminal olefin.
Meanwhile, if only the activity and regioselectivity for the
hydroformylation of the terminal olefin are high enough,
the linear aldehyde would be the main product [1, 16, 18].
The isomerization–hydroformylation of 2-butene was
first investigated using Rh(acac)(CO)2/1 as catalyst, and
the effects of temperature and molar ratio of ligand to
rhodium were summarized in Table 1. The temperature
The ligand/[Rh] molar ratio has a dramatic effect on the
isomerization–hydroformylation (Table 1, entries 4, 7–9).
At low ratios, low regioselectivity was obtained. Ligand/
[Rh] ratio of five is essential to give the best result. Further
increasing the ligand/[Rh] ratio does not significantly
influence the regioselectivity, however, resulted in lower
activity. In contrast to the data (Table 1, entry 4), ligand/
[Rh] ratio of 1.5 gave the best result in hydroformylation of
2-octene in our previous work (Rh: 2-octene = 1:1325,
T = 100 °C, p = 0.7 MPa) [20]. Due to the different
concentrations of catalyst in both systems, the best results
might be obtained under respective L/[Rh] ratios. Leeuwen
et al. [15] concluded that the ligand/[Rh] ratio as well as
the CO pressure determined the concentration of the active
species HRh(P^P)(CO) (Scheme 2, IV) in solution. Under
the appropriate ligand/[Rh] ratio, high concentration of
species II that could transform into active species IV gave
excellent regioselectivity.
Table 1 Isomerization–hydroformylation of 2-butene with ligand 1
under different conditionsa
Entry
L/Rh
T (°C)
n:ib
38.0
Linear (%)c
Yield (%)d
1
2
3
4
5
6e
7
8
9
a
5
5
5
5
5
5
1
3
8
90
100
110
120
130
60
97.4
96.6
96.4
96.0
92.8
99.3
88.8
94.8
96.3
24.8
50.2
59.4
63.2
57.1
63.8
37.8
57.8
46.5
28.1
27.1
23.9
12.9
128.0
7.9
120
120
120
Due to the rapid pressure–dropping during the reaction,
the effect of pressure was investigated at constant pressure
range from 1.5 to 3.5 MPa (Table 2). The increasing of
pressure benefitted the activity of 2-butene hydroformyla-
tion. However, the regioselectivity of linear aldehyde
improved to 95.7% at 2.5 MPa then decreased later with
the increasing of the pressure, especially at 3.5 MPa the
regioselectivity dropped to 84.4%. As previously men-
tioned (Scheme 2), regardless of high or low pressure, the
concentration of II decreased, followed by the reduction of
active species IV, leading to low regioselectivity of linear
aldehyde.
18.3
26.2
[Rh] = 6.0 9 10-3 mmol, 2-butene (2.4 g), S/C = 7,143,
p = 2.5 MPa, t = 2 h, toluene (2.0 mL) as solvent, heptanes as
internal standard
b
Molar ratio of linear to branched aldehydes, determined on the basis
of GC
c
Percentage of linear aldehyde in all aldehydes
d
The yield of all aldehydes, determined on the basis of GC
e
Hydroformylation of 1-butene, [Rh] = 4.0 9 10-3 mmol, 1-butene
(2.4 g), S/C = 10,714, p = 2.5 MPa, t = 1 h, toluene (2.0 mL) as
solvent, heptanes as internal standard
123

Highly Regioselective and Active Rhodium/Bisphosphite Catalytic System
241
Table 2 Isomerization–hydroformylation of 2-butene with ligand 1
under constant pressurea
Entry
p/MPa
n:ib
Linear (%)c
Yield (%)d
1
2
3
4
5
a
1.5
2.0
2.5
3.0
3.5
14.3
19.5
22.7
20.4
5.4
93.5
95.1
95.7
95.3
84.4
63.5
71.4
73.2
76.0
79.8
[Rh] = 6.0 9 10-3 mmol, 2-butene (2.4 g), S/C = 7,143, L/[Rh]
ratio = 5, T = 120 °C, t = 2 h, toluene (2.0 mL) as solvent, hep-
tanes as internal standard, maintain constant pressure during the
reaction
Fig. 2 Recycle of Rh/1 catalytic system for 2-butene isomerization–
hydroformylation [Rh] = 5.0 9 10-2 mmol, 2-butene (20 g),
S/C = 7,143, L/[Rh] ratio = 5, T = 120 °C, t = 2 h, toluene
(20 mL) as solvent, heptanes as internal standard, maintain constant
pressure (2.5 MPa) during the reaction
b–d
see Table 1
Table 3 Comparison of different ligands in isomerization–hydro-
formylation of 2-butenea
Entry
L
t (h)
n:ib
Linear (%)c
Yield (%)d
Under optimized reaction condition (120 °C, p = 2.5 MPa,
ligand/[Rh] ratio = 5), high regioselectivity have been
achieved with ligand 1 (96.5% of linear aldehyde in all
aldehyde products after 0.5 h) (Table 3, entry 2). Though
elongating the reaction time to 6 h resulted in slightly lower
n/i ratio, and the linear aldehyde percentage remained high
up to 95.3% with a high yield of 90.5% aldehyde (Table 3,
entry 4). We then recycled the Rh/1 catalytic system; it
maintained excellent activity and regioselectivity until the
fifth run of the catalyst (Fig. 2). As for ligand 2 and 3, the
regioselectivity decreased obviously when reaction time
elongated (Table 3, entries 6, 8). In summary, ligand 1
showed outstanding catalytic performance and stability on
isomerization–hydroformylation of 2-butene.
1
2
3
4
5
6
7
8
a
None
2
0.8
27.4
22.9
20.3
9.0
44.4
96.5
95.8
95.3
90.0
76.7
78.7
47.4
17.9
35.7
53.0
90.5
46.7
85.7
24.2
59.8
1
1
1
2
2
3
3
0.5
1
6
1
6
3.3
1
3.7
6
0.9
[Rh] = 6.0 9 10-3 mmol, 2-butene (2.4 g), S/C = 7,143,
p = 2.5 MPa, L/[Rh] ratio = 5, T = 120 °C, toluene (2.0 mL) as
solvent, heptanes as internal standard, maintain constant pressure
during the reaction
b–d
see Table 1
It demonstrated that the isomerization–hydroformylation
of 2-butene was sensitive to the reaction conditions
according to the data above. And then we subsequently
investigated the effect of ligand and the results were sum-
marized in Table 3. Being free of ligand, neither activity of
catalyst nor regioselectivity of linear product was satisfac-
tory (Table 3, entry 1); it is essential for the existence of
ligand in isomerization–hydroformylation of 2-butene. In
addition, the nature of ligands also influences the isomeri-
zation–hydroformylation of 2-butene. Ligand 1 with a
bulky binaphthyl backbone and electron-withdrawing pyr-
rol groups obviously improves the selectivity of linear
aldehyde (Table 3, entries 2–4), while ligand 2 and 3
showed less advantage either on activity or regioselectivity
(Table 3, entries 5–8). According to the comparison of
ligand 2 and 3, both of which have similar backbone, ligand
2 showed better activity and regioselectivity than ligand 3,
and electronic effect played a decisive role.
4 Conclusions
Inconclusion, the bisphosphiteligand with bulky skeletonand
electron-withdrawing groups can effectively promote the
isomerization–hydroformylation of internal olefins regardless
of the reaction activity and regioselectivity. What is different
from terminal olefins is that internal olefins require higher
temperature for isomerization–hydroformylation. To the best
of knowledge, the catalyst system provides high activity,
excellent regioselectivity, and stability in isomerization–
hydroformylation of 2-butene with ligand 1. More applica-
tions of this ligand and mechanism studies are now under
further investigation and will be reported in due course.
Acknowledgment We thanked the National Natural Science
Foundation of China (No. 21072138) for the financial support.
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