Highly active rhodium/phosphorus catalytic system for the hydroformylation of α-methylstyrene

Article abstract of DOI:10.1016/j.cclet.2016.01.028

Rhodium catalyzed hydroformylation of α-methylstyrene was investigated in the presence of monodentate phosphine ligands L1-L6. We found that the phosphine with good π-acceptability could efficiently improve the activity of the α-methylstyrene hydroformylation. The big steric hindrance of α-C in α-methylstyrene enhanced the regioselectivity towards the linear aldehyde, which resulted in 3-phenylbutanal as the predominant product (>99.0%). When tris(N-pyrrolyl)phosphine (L1) modified Rh(acac)(CO)₂ was employed as the catalyst, the TOF could reach up to 5786 h^-1 in the α-methylstyrene hydroformylation at relatively mild conditions (110 °C, 6 MPa).

Full text of DOI:10.1016/j.cclet.2016.01.028

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CCLET-3555; No. of Pages 3
Chinese Chemical Letters xxx (2016) xxx–xxx
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Chinese Chemical Letters
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Original article
Highly active rhodium/phosphorus catalytic system for the
hydroformylation of
a-methylstyrene
a
a
a
Xue-Li Zheng a, , Cong-Ye Zheng , Fan-Ding Zhou , Hai-Yan Fu ,
*
Mao-Lin Yuan a, Rui-Xiang Li a, Bin Xu b, Hua Chen a,
*
a Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China
b Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, Zigong 643000, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Rhodium catalyzed hydroformylation of
a
-methylstyrene was investigated in the presence of
-acceptability could
-methylstyrene hydroformylation. The big steric hindrance of -C
-methylstyrene enhanced the regioselectivity towards the linear aldehyde, which resulted in
Received 13 October 2015
Received in revised form 22 December 2015
Accepted 4 January 2016
Available online xxx
monodentate phosphine ligands L1–L6. We found that the phosphine with good
efficiently improve the activity of the
in
p
a
a
a
3-phenylbutanal as the predominant product (>99.0%). When tris(N-pyrrolyl)phosphine (L1) modified
Rh(acac)(CO)2 was employed as the catalyst, the TOF could reach up to 5786 hÀ1 in the
a-methylstyrene
hydroformylation at relatively mild conditions (110 8C, 6 MPa).
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Keywords:
a-Methylstyrene
Hydroformylation
Rh complex
Phosphine
Published by Elsevier B.V. All rights reserved.
Pyrrole
1. Introduction
steric and electronic properties, tris(N-pyrrolyl)phosphine (L1),
tris(9-carbazolyl)phosphine (L2), naphthalenyldipyrrolylphosphor-
The rhodium-catalyzed hydroformylation of
a
-methylstyrene
usamidite (L3), triphenylphosphine (L4), tris(4-trifluoromethylphe-
nyl)phosphine (L5), and tris(4-methoxyphenyl) phosphine (L6)
were investigated with Rh(acac)(CO)2 as the catalyst precursor
(Fig. 1), and the reaction parameters were also optimized.
is of great interest, because the corresponding aldehydes are
important intermediates for the production of spices. Due to the
bulky phenyl and the adjacent methyl groups, the hydroformyla-
tion proceeds quite slowly. Therefore, the development of an
effective catalyst is a necessary challenge. Keoken and co-workers
reported that tri(2,4-di-tert-butylphenyl) phosphite modified
2. Experimental
rhodium could catalyze the hydroformylation of a-methylstyrene
a
-Methylstyrene (>99%) was redistilled prior to use and all
with a TOF of 130 hÀ1 in supercritical CO2 [1]. A cationic rhodium
complex containing a bis(dioxaphospholane) ligand exhibited high
other reagents were of analytical grade and used as received.
Rh(acac)(CO)2 and ligands L1, L2, L3, L5, and L6 were prepared
according to the literature [3–6], and their structures were
confirmed by 1H NMR, 13C NMR and 31P NMR measurements.
All the hydroformylation reactions were carried out in a stainless
steel autoclave of 60 mL with a magnetic stirrer. A toluene solution
regioselectivity in the hydroformylation of
a-methylstyrene;
however, the TOF value was less than 2.5 hÀ1 [2]. The above
results are not satisfactory; the main problem may lie in the steric
hindrance of
groups on C-1. It is well known that the ligand plays a vital role in
the performance of catalyst via its steric and electronic
properties. In this study, in order to find a highly active catalyst
a-methylstyrene caused by the phenyl and methyl
of Rh(acac)(CO)2, ligand, and
a-methylstyrene were added to the
a
autoclave, which was subsequently evacuated and purged three
times with 1 MPa of syngas (H2/CO = 1:1). The autoclave was then
pressurized with syngas and stirred under the specific reaction
conditions. After the reaction was completed, the vessel was
cooled to room temperature before excess syngas was carefully
released. The products were then analyzed on Agilent 6890 N gas
chromatograph with a capillary column SE-30 (30 m  0.25 mm)
and identified by GC-MS and NMR.
for
a-methylstyrene hydroformylation, six ligands with different
*
Corresponding authors.
E-mail addresses: zhengxueli@scu.edu.cn (X.-L. Zheng), scuhchen@scu.edu.cn
(H. Chen).
http://dx.doi.org/10.1016/j.cclet.2016.01.028
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: X.-L. Zheng, et al., Highly active rhodium/phosphorus catalytic system for the hydroformylation of
methylstyrene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.01.028
a-

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X.-L. Zheng et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Fig. 1. Structures of ligands L1–L6.
3. Results and discussion
afford 3-phenylbutanal as the dominant product. 1H NMR
(400 MHz, CDCl3): 9.73 (s, 1H), 7.42–7.20 (m, 5H), 3.38 (s, 1H),
2.79 (d, 2H, J = 14.9 Hz), 1.34 (s, 3H). 13C NMR (100 MHz, CDCl3):
d
From the outset, six different ligands (Fig. 1) were studied in the
d
Rh-catalyzed hydroformylation of
a-methylstyrene under 5 MPa
201.90, 145.50, 128.72, 126.81, 51.76, 34.30, 22.22 (see Supporting
information).
of syngas at 100 8C. As demonstrated in Table 1, all the ligands
exhibited excellent selectivity for aldehydes and regioselectivity
for linear aldehyde (3-phenylbutanal, >97.0%). L1 and L3, which
contained a di(N-pyrrolyl)phosphino group, resulted in much
higher activity. In contrast, L2, bearing the N-carbazolyl group with
a larger steric hindrance, resulted in relatively lower activity. L4–
L6, which contained triphenylphosphine and its derivatives, also
gave lower activity, and the one with an electron-withdrawing
substituent –CF3 (L5) demonstrated higher activity compared to
the one with the electron-donating substituent –OCH3 (L6)
(Table 1, entries 4 and 5). Of the tested ligands, L1, which
contained an electron-withdrawing N-pyrrolyl group and caused
less steric hindrance, is the best ligand in terms of activity and
regioselectivity. This is because the electron-withdrawing group
substituted on the phosphorus weakens the Rh-carbonyl bond and
thus favors the insertion of CO and the formation of Rh-acyl active
species. In addition, a smaller ligand might somehow facilitate the
The hydroformylation is highly dependent on the reaction
conditions; therefore, the optimization of L1/Rh molar ratio, initial
pressure, and reaction temperature was performed in the presence
of Rh(acac)(CO)2/L1 as catalyst, and the results were shown in
Table 2. When L1/Rh molar ratio of 5:10 was used, high activity
(Table 2, entries 1 and 2) could be achieved at 100 8C. Further
increasing the L1/Rh molar ratio decreases the activity (Table 2,
entry 3). Considering the stability of the catalyst, a L1/Rh molar
ratio of 10 is selected for the sequent experiments. A clear impact of
the reaction temperature on the activity was observed (Table 2,
entries 4–8). For instance, the reaction rate was unsatisfactory at
90 8C, while at 110 8C a high activity was achieved. From entries 4 to
8, it is not difficult to find that the chemoselectivity of the aldehyde
decreased slightly when the temperature increased, because high
temperatures were likely to benefit the hydrogenation of
a
-methylstyrene, which was confirmed by the observation of
coordination of
a-methylstyrene to the Rh center [7], and thus
2-phenylpropane via GC. The total pressure of syngas (CO/H2: 1/1)
Table 1
Effect of ligand on Rh-catalyzed hydroformylation of
a
-methylstyrene.a
e
Entry
Ligand
Con. (%)b
Sel. of Aldehydes (%)c
Regiosel. of linear aldehyde (%)d
TOF (hÀ1
)
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
97.2
67.5
72.9
42.6
48.6
46.7
98.2
94.3
98.3
98.6
98.4
99.1
99.2
97.7
99.0
97.0
98.8
97.1
194
135
146
85
97
93
a
[Rh(acac)(CO)2] =1.5 mmol/L,
a
-methylstyrene (1.0 mL), S/C = 1000, L/Rh = 10, 100 8C, 5 MPa (CO/H2 = 1, pressure ratio), 5 h, toluene 4.0 mL.
b
c
Conversion of
a-methylstyrene was determined on the basis of GC.
Selectivity for aldehydes, determined on the basis of GC.
d
e
Regioselectivity for linear aldehyde, determined on the basis of GC.
Turnover frequency: moles of aldehydes per mole of Rh per hour.
Table 2
Effect of reaction conditions on Rh-catalyzed hydroformylation of
a
-methylstyrene.a
e
Entry
L1/Rh
T (8C)
P (MPa)
t (h)
Con. (%)b
Sel. of aldehydes (%)c
Regiosel. of linear aldehyde(%)d
TOF (hÀ1
)
1
2
3
4
5
6
7
8
9
10
5
10
20
10
10
10
10
10
10
10
100
100
100
90
5
5
5
5
5
5
5
5
4
6
2
2
2
1
1
1
1
1
1
1
85.3
86.2
57.6
38.2
65.3
73.1
60.8
47.3
64.0
80.8
96.9
97.6
97.1
98.5
96.9
95.4
92.1
91.2
94.1
95.8
99.2
99.2
99.0
99.0
99.1
99.1
99.1
99.1
99.1
99.2
426
431
288
382
653
731
608
473
640
808
100
110
120
130
110
110
b–e
See Table 1.
a
[Rh(acac)(CO)2] =1.5 mmol/L,
a-methylstyrene (1.0 mL), S/C = 1000, CO/H2 =1 (pressure ratio), toluene 4.0 mL.
Please cite this article in press as: X.-L. Zheng, et al., Highly active rhodium/phosphorus catalytic system for the hydroformylation of
methylstyrene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.01.028
a-

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3
Table 3
Effect of substrate/catalyst molar ratio on Rh-catalyzed
a
-methylstyrene hydroformylation.a
e
Entry
S/C
1000
[Rh] (mmol/L)
[S] (mol/L)
Con. (%)b
Sel. of aldehydes (%)c
Sel. of linear aldehyde(%)d
TOF (hÀ1
)
1
2
3
4
5
6
1.5
1.5
1.5
1.5
1.0
0.6
1.5
3.1
4.6
6.2
5.2
6.2
80.8
79.4
81.4
70.3
60.2
56.0
95.8
96.2
94.6
95.4
96.7
95.5
99.2
99.2
99.3
99.1
99.1
99.2
808
1640
2496
2821
3130
5786
2066
3067
4013
5200
10333
b–e
See Table 1.
a
L/[Rh(acac)(CO)2] = 10, 110 8C, 6 MPa (CO/H2 = 1, pressure ratio), toluene as solvent, 1 h.
was also a key element in the hydroformylation. Increasing
the pressure from 4 to 6 MPa resulted in higher activity and
selectivity for aldehydes. The catalytic system has good activity:
when the S/C was increased from 1000 to 3067, TOF of up to
Acknowledgments
The authors thank the financial support from the National
Natural Science Foundation of China (No. 201202108), from the
Opening Project of Key Laboratory of Green Chemistry of Sichuan
Institutes of Higher Education (No. LZJ1402), and from the Sichuan
university outstanding scholar research fund (No. 2015SCU04A05).
2496 hÀ1 was attained and the conversion of
a-methylstyrene was
maintained at 79.4%–81.4% (Table 3, entries 1–3). The highest TOF
(5786 hÀ1) was achieved at S/C of 10333 (Table 3, entry 6), although
the conversion decreased to 56.0%. To our knowledge, this is the
best result for the hydroformylation of
[1,2].
a-methylstyrene so far
Appendix A. Supplementary data
The process of styrene hydroformylation may form stable
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2016.01.028.
h3-allyl-Rh complexes, which could contribute to the formation of
branched aldehyde [8]. In the case of
a-methylstyrene, the steric
hindrance caused by the phenyl and methyl groups on C-1
predisposes it to the formation of h1-allyl-Rh complexes. This
ensured the linear aldehyde 3-phenylbutanal as the dominant
product (> 99.0%).
References
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In summary, the influence of the steric and electronic properties
of the ligand on the activity and the regioselectivity was
investigated in the Rh-catalyzed hydroformylation of
styrene. The results demonstrated that tris(N-pyrrolyl)phosphine
L1 with good -acceptability could efficiently improve the
catalytic activity, and the steric hindrance of -methylstyrene at
-C effectively enhanced the regioselectivity toward the linear
a-methyl-
p
a
a
aldehyde. A high TOF (5786 hÀ1) with 99% yield of linear aldehyde
was obtained at relatively mild conditions (syngas pressures
6 MPa, 110 8C), when Rh(acac)(CO)2/L1 was used as the catalyst,
which is the highest activity reported for the hydroformylation of
a
-methylstyrene so far.
[8] E. Boymans, M. Janssen, C. Muller, M. Lutz, D. Vogt, Rh-catalyzed linear hydro-
formylation of styrene, Dalton Trans. 42 (2012) 137–142.
Please cite this article in press as: X.-L. Zheng, et al., Highly active rhodium/phosphorus catalytic system for the hydroformylation of
methylstyrene, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.01.028
a-