Highly regioselective and active Rh-2,2′-bis(dipyrrolylphosphinooxy)- 1,1′-(±)-binaphthyl catalyst for hydroformylation of 2-octene

Article abstract of DOI:10.1246/cl.2009.596

Rhodium catalyst bearing 2,2′-bis(dipyrrolyphosphinooxy)-1,1′- (±)-binaphthyl (ligand 1) shows high regioselectivity and activity for the hydroformylation of 2-octene. The introduction of a pyrrolyl group in the ligand greatly improves the yield of the linear aldehyde. The regioselectivity is up to 97.5% under mild conditions (100 °C, 0.7 MPa H₂/CO). Copyright

Full text of DOI:10.1246/cl.2009.596

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596
Chemistry Letters Vol.38, No.6 (2009)
Highly Regioselective and Active Rh–2,20-Bis(dipyrrolylphosphinooxy)-1,10-(ꢀ)-binaphthyl
Catalyst for Hydroformylation of 2-Octene
Wenjing Liu, Maolin Yuan, Haiyan Fu, Hua Chen,ꢀ Ruixiang Li, and Xianjun Li
Key Laboratory of Green Chemistry and Technology of Ministry of Education, Institute of Homogeneous Catalysis,
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
(Received March 6, 2009; CL-090232; E-mail: scuhchen@scu.edu.cn)
Rhodium catalyst bearing 2,20-bis(dipyrrolyphosphinooxy)-
1,10-(ꢁ)-binaphthyl (ligand 1) shows high regioselectivity and
activity for the hydroformylation of 2-octene. The introduction
of a pyrrolyl group in the ligand greatly improves the yield of
the linear aldehyde. The regioselectivity is up to 97.5% under
mild conditions (100 ꢂC, 0.7 MPa H2/CO).
A 50-mL round-bottom flask was charged with 0.66 g
(2.30 mmol) of (ꢁ)-binaphthyl, 0.82 mL (5.88 mmol) of Et3N,
and 5 mL of THF. A solution of 1.13 g (5.70 mmol) of chlorodi-
pyrrolylphosphine in 10 mL of THF was then added dropwise
with stirring over a period of 30 min at 0 ꢂC. The reaction mix-
.
ture was stirred overnight at room temperature. The Et3N HCl
salts were then filtered off, and the solvent was removed under
vacuum. The crude product was purified by recrystallization
from ethanol to yield 0.90 g (64%) of ligand 1 as a white solid
with a melting point of 88–90 ꢂC.15 1H NMR (400 Hz, CDCl3):
ꢀ 7.9 (d, J ¼ 8:4 Hz, 4H), 7.4 (t, J ¼ 7:5 Hz, 2H), 7.3 (t, J ¼
7:6 Hz, 2H), 7.2 (d, J ¼ 9:9 Hz, 2H), 7.1 (d, J ¼ 8:7 Hz, 2H),
6.4 (m, 8H), 6.1 (dd, J ¼ 15:6, 1.8 Hz, 8H); 31P NMR
(162.0 Hz, CDCl3): ꢀ 108.2 (s); 13C NMR (100.6 Hz, CDCl3):
ꢀ 133.7, 130.7 (d, J ¼ 37:2 Hz), 128.1, 127.1, 125.9, 125.3,
121.0 (q, J ¼ 37:2 Hz), 119.2, 112.1.
The rhodium-catalyzed hydroformylation of olefins has
attracted much attention as a potential tool for preparing alde-
hydes,1–3 which are important precursors for synthesizing vari-
ous pharmaceuticals, agrochemicals, commodity, and fine chem-
icals. This great demand has initiated tremendous efforts in the
development of effective phosphorus ligands to achieve both
high regioselectivity and activity in hydroformylation. The
majority of these catalysts have been rhodium(I) complexes
with chelating bis(phosphine), bis(phosphite), bis(phospholane),
or mixed bis(phosphine/phosphite) ligands.4 These ligands give
reasonably high regioselectivity and conversion. After first being
reported by Billig and co-workers, diphosphites were recognized
as a new generation of promising ligands in rhodium-catalyzed
hydroformylation of internal alkenes.5,6 Zhang et al. used a
pyrrole-based biphenyl bisphosphorus for hydroformylation of
2-octene and got better results.7
A mixture of 2-octene (1.25 mmol, 0.2 mL), rhodium cata-
lyst (0.66 mmol), ligand 1 (0.66 mmol if used) was dissolved
in 1.5 mL of toluene and placed in a 100-mL autoclave with a
Teflon liner and mechanical stirring. The autoclave was flushed
thoroughly three times with a 1:1 syngas of H2 to CO and pres-
surized to the desired pressure, and then heated to the desired
temperature. The reaction was maintained under these condi-
tions for 1 h. The autoclave was then cooled to room temperature
and the gas was carefully released.
Many bisphosphine ligands have a natural bite angle,
defined by Casey et al.8,9 and ligand backbone chains from
two to five carbons in length are reasonably accessible. Longer
chains have increased natural bite angles, but are also more flex-
ible than shorter chains. Later, Casey et al.10–12 showed experi-
mentally that the bite angle influences the nature of intermediate,
which then influence product regioselectivity and a rhodium
complex bearing a large bite angle bisphosphine gives a high
n=i ratio. At the same time, it was found that the formation of
an ee complex, in which biphosphine occupies two equatorial
positions of a trigonal bipyramid plays a key role in the high
regioselectivity. So a wide bite angle biphosphine forms easily
an ee complex and promotes the formation of linear aldehydes.
Casey et al. also found that introduction of electron-withdrawing
groups in large bite angle biphosphines would further increase
the n:i ratio.13 NMR studies indicated that an electron-poor
phosphine preferred occupying the equatorial position of metal
center. The above results are also suppported by the work of
van Leeuwen et al.14
The products were identified with NMR and GC. NMR
spectra were obtained on a Bruker AV-400 and AMX 360
1
(400 MHz for H, 162.0 MHz for 31P, and 100.6 MHz for 13C).
Melting points were determined with a melting point apparatus
(Yanaco MP-500). Gas chromatography was performed with a
Hewlett-Packard 960 instrument with an Alltech ECTM-1 column
(30 m ꢃ 0:25 mm, 0.25-mm film).
The catalyst system showed very high activity and regiose-
lectivity in the temperature range of 80 to 120 ꢂC. Compared
with known results, our catalytic system shows a higher regiose-
lectivity (Table 1). At 80 ꢂC, a conversion of 53.3% was achiev-
ed with a n=i ratio of 21.9. The n:i of 21.9 is much higher than
10.1 reported by Zhang and colleagues for the hydroformylation
of 2-octene.7 Interestingly, if the reaction temperature was in-
creased to 100 ꢂC, under similar conditions the n:i ratio was up
to 39.5.
To optimize reaction conditions, we examined the effect of
syngas pressures and found that the syngas pressure of 0.7 MPa
gave the best results with conversion of 85.0% and n=i ratio of
39.5. van Leeuwen16 and his co-workers concluded that the con-
centrations of HRh(P^P)(P^P0)(CO) and HRh(P^P)(CO)2 in
solution depend on the ratio of ligand to rhodium as well as
the carbon monoxide pressure (Figure 1). For ligand 1 a higher
ligand-to-rhodium ratio than 1 is required to reach complete
In this contribution, we synthesized a diphosphine 2,20-bis-
(dipyrrolylphosphinooxy)-1,10-(ꢁ)-binaphthyl with a large bite
angle and electron-withdrawing pyrrolyl groups. We used a
RhI complex bearing this ligand for the hydroformylation of 2-
octene and investigated its activity and regioselectivity to linear
aldehyde.
Copyright ꢀ 2009 The Chemical Society of Japan

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Chemistry Letters Vol.38, No.6 (2009)
597
Table 1. Hydroformylation of 2-octene with ligand 1a
Table 2. Effect of different rhodium-precursors on hydrofor-
mylation with ligand 1a
Conversion Regioselectivity
/%c
p/MPa T/ꢂC P/Rh n=ib
/%d
Rhodium
precursors
Conversion
/%c
n=ib
Regioselectivity/%d
0.7
0.7
0.7
0.7
0.7
0.7
0.5
1.0
80
90
1.5 21.9
1.5 23.0
1.5 39.5
2.0 18.5
1.0 12.4
1.5 16.2
1.5 25.5
1.5 22.1
53.3
67.3
85.0
53.1
45.6
20.2
47.0
29.0
95.6
98.5
97.5
94.9
92.5
94.1
96.2
95.7
RhCl(CO)(PPh3)2 25.8
20.5
85.0
11.3
44.9
96.3
97.5
87.3
97.0
Rh(acac)(CO)2
[Rh(COD)Cl]2
39.5
6.9
100
100
100
110
100
100
HRh(CO)(PPh3)3 31.8
aCondition: [Rh] = 0.66 mM, P:Rh:2-octene = 5:1:1325, tol-
uene (1.0 mL) as solvent, time: 1 h. bLinear/branched
ratio, determined on the basis of GC analysis. Percentage of
c
linear and branched aldehydes in all aldehydes. dPercentage
of linear aldehyde in linear and branched aldehydes.
aCondition: [Rh] = 0.66 mM, P:Rh:2-octene = 5:1:1325,
toluene (1.0 mL) as solvent, time: 1 h. bLinear/branched
ratio, determined on the basis of GC analysis. cPercentage of
ably difficult to hydrogenate and dissociates from the metal cen-
ter under low syngas pressure, so it gave the lowest activity and
selectivity.
d
linear and branched aldehydes in all aldehydes. Percentage
of linear aldehyde in linear and branched aldehydes.
In summary, compound 1 with a bulky binaphthyl backbone
and electron-withdrawing pyrrolyl groups is an excellent ligand
for the hydroformylation of 2-octene. A low ligand to Rh molar
ratio of 1.5/1 gave the highest n=i ratio of 39.5 with an olefin
conversion of 85.0%.
H
H
P
P
P
P
P
P
Rh
P
P'
Rh CO
CO
CO
CO
HRh(P^P)(P^P')(CO)
HRh(P^P)(CO)2
CO
R
CO
CO
P
P
We thank the National Natural Science Foundation (No.
29792074) and the National Basic Key Research Project of
China (G2000048008) for the financial support.
Rh
Linear product
CO
R
H
R
H2
P
P
Rh
CO
CO
P
P
Rh
Branched product
References
R
CO
1
P. W. N. M. van Leeuwen, C. Claver, Rhodium Catalysed Hydro-
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Figure 1. Catalytic procedure for hydroformylation of 2-
octene.
2
conversion to the rhodium–hydride complex [HRh(P^P)(CO)2]
and L/Rh = 1.5 gives the best results. When the syngas pressure
is high in the case of L/Rh > 1.5, the rhodium–hydride complex
HRh(P^P)(CO)2 is converted to hydride complex HRh(P^P)-
(P^P0)(CO). One of the bidentate ligands, denoted P^P0, coordi-
nates in a monodentate fashion.17 It was reported that linear
Rh–alkyl intermediate forms faster than the branched isomer
intermediate.18 At 100 ꢂC and 0.7 MPa, linear rhodium–alkyl
complex undergoes carbon monoxide insertion to form linear
aldehydes.16
3
4
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6
7
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The hydroformylation of 2-octene was also studied by vary-
ing the rhodium precursors in toluene (Table 2). Rh(acac)(CO)2
shows a high conversion of 85.0% with a n=i ratio of 39.5. Ac-
cording to reported results, the active intermediate RhH(L)2(CO)
is easily formed in a system composed of Rh(acac)(CO)2 and the
diphosphine even at low temperature19 and gives a high n=i ratio.
When HRh(CO)(PPh3)3 was used as a catalyst precursor, it gave
a conversion of 44.9% with a regioselectivity of 97.0%, which is
lower than Rh(acac)(CO)2. The probable reason is the stronger
base PPh3 in the rhodium precursor is not easily substituted by
the weak base ligand 1, so that the partial existence of PPh3 in
active intermediate does not efficiently stabilize the ee coordina-
tion model active species that can give high n=i ratio aldehyde.
Similarly, RhCl(CO)(PPh3)2 as the catalytic precursor gave a
close regioselectivity to RhH(CO)(PPh3)3, but a lower activity
was obtained as the transformation of of RhCl(CO)(PPh3)2 into
the active species under the reaction conditions is slower than
RhH(CO)(PPh3)3.20 For precursor [Rh(COD)Cl]2, COD is prob-
´
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Molchanova, K. Lyssenko, T. Mastryukova, G.-V. Roschenthaler,
¨
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´
20 R. P. J. Bronger, J. P. Bermon, J. Herwig, P. C. J. Kamer,
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Published on the web (Advance View) May 23, 2009; doi:10.1246/cl.2009.596