Highly regioselective homogeneous isomerization-hydroformylation of 2-butene with water- and air-stable phosphoramidite bidentate ligand

Article abstract of DOI:10.1016/j.mcat.2021.111598

Highly selective isomerization-hydroformylation of 2-butene was achieved with the presence of Rh(acac)(CO)₂ and a phosphoramidite bidentate ligand which bearing 2,2′-dihydroxy-1,1′-binaphthyl backbone and N-indolyl substitute. The molar ratio of n- to isovaleraldehyde (217) is distinctly higher than the reported systems. NMR and IR revealed that the five-coordinate HRh(ligand)(CO)₂ was an equatorial-equatorial configuration which contributed to the n-selectivity of valeraldehyde. The strong π-acceptor ability of ligand was suggested to play a key role in fast isomerization of 2-butene. Hydrolysis and oxidation experiments demonstrated that the ligand was water- and air-stable. Cyclic voltammetry measurement confirmed that this phosphoramidite ligand is more difficult to be oxidized, compared with the phosphine, phosphinite and phosphite ligands. Inspiringly, recycling experiments showed the catalytic system could work for at least 7 runs with unchanged selectivity.

Full text of DOI:10.1016/j.mcat.2021.111598

Molecular Catalysis 508 (2021) 111598
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Highly regioselective homogeneous isomerization-hydroformylation of
2-butene with water- and air-stable phosphoramidite bidentate ligand
Songbai Tang, Conceptualization; Methodology; Formal analysis; Investigation; Writing -
original draft; Visualizationa, Yanxin Jiang, Validation; Visualizationa, Jiwei Yia,
Xiaoxia Duana, Haiyan Fua, Ruixiang Li, Methodology; Funding acquisitiona, Maolin Yuana,
Hua Chen, Project administration; Supervision; Resources; Methodologya, Chunji Yangb,
Xueli Zheng, Conceptualization; Writing - review & editing; Project administration;
,
*
Supervisiona
a Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, PR China
b Petrochemical Research Institute QaQing Petrochemical Research Center, Daqing, 163000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly selective isomerization-hydroformylation of 2-butene was achieved with the presence of Rh(acac)(CO)2
and a phosphoramidite bidentate ligand which bearing 2,2′-dihydroxy-1,1′-binaphthyl backbone and N-indolyl
substitute. The molar ratio of n- to isovaleraldehyde (217) is distinctly higher than the reported systems. NMR
and IR revealed that the five-coordinate HRh(ligand)(CO)2 was an equatorial-equatorial configuration which
Isomerization-hydroformylation•2-butene
Diphosphoramidite•linear selectivity
Cyclic voltammetry
contributed to the n-selectivity of valeraldehyde. The strong π-acceptor ability of ligand was suggested to play a
key role in fast isomerization of 2-butene. Hydrolysis and oxidation experiments demonstrated that the ligand
was water- and air-stable. Cyclic voltammetry measurement confirmed that this phosphoramidite ligand is more
difficult to be oxidized, compared with the phosphine, phosphinite and phosphite ligands. Inspiringly, recycling
experiments showed the catalytic system could work for at least 7 runs with unchanged selectivity.
1. Introduction
been an increasing demand in the production of alternative plasticizer
alcohols due to environmental concern about current products. As an
Hydroformylation (oxo) is one of the most important reactions in
industrial. Various aldehydes and alcohols, which are important in-
termediates for the synthesis of bulk chemicals, such as detergents,
cosmetics, air fresheners, plasticizers, and food additives, are produced
by this highly atom-economical process [1,2]. The most representative
rhodium catalyzed plants are the homogeneous low-pressure oxo pro-
important fungible plasticizer alcohol, 2-propylheptanol could be syn-
thesized by the aldol condensation of n-valeraldehyde and the subse-
quent hydrogenation of the condensation product [6,7]. During the
hydroformylation of Raffinate-2, n- and isovaleraldehydes are produced
and the production of n-valeraldehyde is more desirable due to better
physical properties of the resulting plasticizers. As demonstrated in
Scheme 1, direct hydroformylation of 2-butene may yield to iso-
valeraldehyde product (reaction a). Hoechst Company developed a new
method for the hydroformylation of mixed butene, in which two
different catalytic systems are combined to produce n-valeraldehyde
[8]. The first catalytic system allows 2-butene isomerization to 1-butene
and the second one catalyzes the hydroformylation of 1-butene to yield
n-valeraldehyde. To obtain n-valeraldehyde from 2-butene in one cat-
alytic system, an ideal catalyst has to catalyze the competitive
ˆ
cess (LPO) and the biphasic Ruhrchemie/Rhone-Poulenc process
(RCH/RP) based on a water soluble catalyst [3,4]. Compared with
biphasic process, rhodium-catalyzed homogeneous hydroformylation
has the advantages of high activity and selectivity for aldehydes under
mild conditions. Raffinate-2, which consists of 1-butene, most prefer-
entially 2-butene and small amount of butane, is widely used in
hydroformylation, not only for the cheap and extensive C4 feedstock
[5], but also for the production of n-valeraldehyde. Recently there has
* Corresponding author.
E-mail addresses: 1451298321@qq.com (S. Tang), 997373774@qq.com (Y. Jiang), 2720673131@qq.com (J. Yi), 1638372111@qq.com (X. Duan), scufhy@scu.
edu.cn (H. Fu), liruixiang@scu.edu.cn (R. Li), cdscyml@sina.com (M. Yuan), ycj459@petrochina.com.cn (C. Yang), zhengxueli@scu.edu.cn (X. Zheng).
https://doi.org/10.1016/j.mcat.2021.111598
Received 21 February 2021; Received in revised form 30 March 2021; Accepted 20 April 2021
Available online 7 May 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
2. Experimental
2.1. General reagents and instruments
2-butene (95%, mixture of cis and trans) and ligand BISBI (L5, Fig. 1)
was supplied by Chengdu Xinhuayuan Technology Co., Ltd. Rh(acac)
(CO)2 (acac = acetylacetone) [23], ligand L1-L4 and L6-L8 were pre-
pared according to literatures [12,24–29]. H2O2 (30%), toluene and
xylene were supplied by Chengdu Chron Chemicals Co., Ltd and tetra-
hydrofuran was purchased from Tianjin Guangfu Fine Chemical
Research Institute. The solvents were purified by standard methods.
NMR was performed on Bruker AVANCE III HD-400 MHz spectrometer.
1H NMR was reported with TMS as an internal standard. 31P{1H} NMR
was reported with H3PO4 as an external reference. High-resolution mass
spectra were recorded on a SHIMADZU LCMS-IT-TOF mass spectrom-
eter. FTIR spectra were recorded on an IR Tracer-100 spectrometer. Gas
chromatography was analyzed on PANNA A91 (KB-1, 30 m × 0.25 mm
× 0.50 µm, FID). Cyclic voltammetry measurements were performed on
electrochemical workstation (CHI760E), the working electrode is a Pt
plate, counter electrode is a carbon rod and the reference electrode is
Ag/AgCl.
Scheme 1. Isomerization-hydroformylation of 2-butene to yield n-
valeraldehyde.
isomerization of 2-butene to 1-butene (reaction b) and then predomi-
nantly convert 1-butene to n-valeraldehyde (reaction c) rather than
isovaleraldehyde, which is quite essential to the industrial utilization of
Raffinate-2.
Phosphorus ligands modified rhodium catalyst usually allows good
activity and selectivity in hydroformylation, and the steric as well as the
electronic properties of ligand are always the key issues. Casey et al.
examined the steric effects of different ligands and concluded that a
wide bite angle between bidentate ligand (i.e., BISBI) and rhodium
would improve the n-selectivity through the formation of equatorial-
equatorial (ee) coordination in which the ligand occupied the two
equatorial positions of the rhodium complex [9,10]. Similar to the
backbone of BISBI, 2,2′-dimethyl-1,1′-binaphthalene [11], 2,2′-dihy-
droxy-1,1′-biphenyl [12] and 2,2′-dihydroxy-1,1′-binaphthyl [13] were
proved to be suitable backbone of phosphorus ligands to achieve ee
configuration of rhodium-ligand complex. Furthermore, xanthene and it
derivatives [14] as well as some spiroketal-based chemicals [15] were
also good backbones. Hydroformylation of 2-butene with the presence of
spiroketal-based diphosphite ligands could obtain the n/i ratio (n- to iso-
valeraldehyde) of 34.5. It is also reported that the rhodium catalysts
2.2. Hydroformylation and recycle procedures
The homogeneous hydroformylation was carried out in a 25 mL
stainless steel autoclave with a pressure gage, a magnetic stirrer and a
thermocouple. Typically, the precursor Rh(acac)(CO)2 (0.008 g, 0.031
mmol), the ligand (in the P/Rh molar ratio of 5), the internal standard
dodecane (100 μL, 0.0753 g) and the solvent toluene or xylene (8 mL)
were loaded into the autoclave, and then 2-butene (1.8 g, 32 mmol) was
transferred into the autoclave at low temperature using Schlenk tech-
nique. After being filled with definite constant pressure of syngas and
heated to the desired temperature, the autoclave was vigorously stirred.
When the reaction completed, the autoclave was cooled down and
carefully vented. The reaction mixture was analyzed by GC immediately
using internal standard method.
modified with strong
π
-acceptor ligands could achieve higher activity
-donor ligands, since the
and n-selectivity compared to those with
σ
lower basicity of the ligand could facilitate olefin coordination to
rhodium [16–18]. In 2001, Beller group successfully applied
electron-deficient NAPHOS-type diphosphines to rhodium-catalyzed
homogeneous hydroformylation of 2-butene, the n/i was 95:5 and the
TOF was up to 900 hꢀ 1 [19]. Later, similarly strongly electron-deficient
sulfonated ligand BINAS was utilized in a water-organic biphasic system
in the isomerization-hydroformylation of internal olefins and the n/i
ratio could reach 49 in the hydroformylation of 2-butene [20]. More
recently, some new techniques were developed in continuous C4
hydroformylation industrially such as supporting ligands on ionic liquid
phase (SILP) [21] or applying porous organic copolymer phosphine
ligand [22]. High TOF could be obtained with moderate n-selectivity by
these methods.
The recycling runs were similar to typical hydroformylation pro-
cedure. After each run completed, the reaction mixture was collected
and analyzed by GC. Then the liquid mixture was removed by a rotary
evaporator, and the solid residue, including the catalyst and ligand, was
dissolved by another portion of solvent and transferred into the same
autoclave for the next run.
2.3. Stability test of ligands
The hydrolysis and oxidation experiments were performed in a 15
mL flask. For hydrolysis, ligand L2 (0.1 mmol, 81 mg) was dissolved in
toluene (2 mL) and then H2O (1 mL) was added and stirred at 80 ◦C for
24 h. For oxidation experiments, L2 (0.1 mmol, 81 mg) was stirred with
toluene (2 mL) at 80 ◦C for 24 h under O2 balloon, or L2 was stirred with
As mentioned above, these catalytic systems have achieved some
outstanding developments, there still remain some problems prior to
more efficient industrialization, such as the cost and the stability of the
ligand in terms of hydrolysis and oxidation, which are quite crucial for
their industrial application to the hydroformylation of Raffinate-2.
Phosphine ligands show good resistance to hydrolysis but they are
sensitive to oxygen. Phosphites with P-O bonds exhibit high resistance to
oxidation owing to the absence of P-C bond, however, they are sensitive
to moisture. P-N bond might be more stable to moisture than P-O bond.
Herein, we reported an efficient and low-cost 2-butene isomerization-
hydroformylation system with a readily accessible, water- and air-
stable diphosphoramidite ligand bearing 2,2′-dihydroxy-1,1′-
binaphthyl skeleton and indolyl substituent. The linear selectivity to-
wards valeraldehyde was up to 99.6% with the aldehyde yield above
93.4%. The stability and the reusability of the catalyst were further
explored.
◦
toluene and 30% H2O2 (1 mL) at 80 C for 2 h. After hydrolysis and
oxidation reactions, the solvents were removed and the residue was
analyzed by 31P NMR and HRMS. In addition, cyclic voltammetry
measurements were also used to analyze the ligand stability.
3. Result and discussion
3.1. Influence of different ligands
Due to the goal of achieving more linear valeraldehyde, different
types of diphosphorus ligands (Fig. 1) based on 2,2′-dihydroxy-1,1′-
biphenyl or 2,2′-dihydroxy-1,1′-(±)binaphthyl backbone, which the
phosphorus atoms linked to can bear moderate bite angle with rhodium,
were explored in the isomerization-hydroformylation of 2-butene with
the presence of catalyst precursor Rh(acac)(CO)2, and the results were
shown in Table 1. It can be seen that except phosphine ligand BISBI (L5)
2

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
Fig 1. Structure of ligands L1-L8.
exploration.
and phosphinite L6, the other ligands (including phosphoramidite and
phosphite) with strong electron-withdrawing groups achieved good
selectivity to n-valeraldehyde (Table 1, entry 1–4 and entry 7–8), and
the highest molar ratio of n- to isovaleraldehyde (n/i) was obtained as
40.9 with a good valeraldehyde yield of 61.6% (Table 1, entry 2) when
phosphoramidite ligand L2 was used. The phosphite ligands L7 and L8
achieved moderate activity of 23.3% and 31.0%. The regioselectivity
3.2. Optimization of hydroformylation conditions with L2
Since the hydroformylation results highly rely on the reaction con-
ditions, the influence of P/Rh ratio, temperature, reaction time and
pressure were evaluated. As demonstrated in Table 2, in the absence of
ligand, the non-modified rhodium catalyst yielded only 4.7% of alde-
hydes and the n/i ratio was only 0.61 (Table 2, entry 1). This situation
was greatly improved when L2 was added. However, increasing the P/
Rh ratio from 5 to 15, the yield slightly decreased (entry 2–4) and
sharply dropped to 17.6% when the P/Rh ratio came to 20. The
follows the trend L2 > L7 > L6 > L5, which correlates to the
π
π
-acceptor
-acceptor
properties of these ligands [30]. Because ligand with strong
property would facilitate the isomerization of 2-butene to 1-butene.
Ligand L2 showed excellent results in terms of activity and regiose-
lectivity, therefore, it was selected for the following reaction conditions
3

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
to 7.5 bar (Table 2, entry 13–15), and the best one was found at 7.5 bar,
where the n/i ratio was 57.0. However, the n/i ratio and the yield of
aldehyde decreased as the pressure increased from 10 to 25 bar (entry
16–19).
Table 1
Rh-catalyzed hydroformylation of 2-butene with different ligandsa.
Entry
Ligand
n/ib
Aldehyde (%)c
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
14.7
40.9
16.5
21.3
0.5
51.5
61.6
54.1
38.2
22.9
14.8
23.3
31.0
Some researches demonstrated that the partial pressure of hydrogen
(PH2) and carbon monoxide (PCO) had a prominent impact, not only on
the β-H elimination but also on the carbonyl insertion [33–35]. Fig. 2
showed the n/i ratio and the yields of aldehydes when the ratio of
1.1
P
H2/PCO varied from 1/4 to 4/1 with the presence of Rh(acac)(CO)2 and
17.0
20.1
ligand L2 (Rh/L2 for short). It is not difficult to figure out that the
isomerization-hydroformylation of 2-butene had poor regioselectivity
and activity when PH2/PCO was 1/4 (Fig. 2a, 2b). On the contrary, when
the PH2/PCO ratio ascended to 4/1, the regioselectivity increased
dramatically (Fig. 2c, 2d). Representatively, n/i ratio was up to 217 as
the partial pressure of hydrogen and CO was 10 bar and 2.5 bar,
respectively, which was higher than the reported catalytic systems.
Landis demonstrated that regioselectivity of styrene hydroformylation
might depend on CO pressure at very low pressure [33,35,36], while for
the isomerization-hydroformylation of 2-butene, the tendency was that
the partial pressure of hydrogen made more contribution to the regio-
selectivity. Hydroformylation of internal olefin is slower than isomeri-
zation and hydroformylation of terminal olefin [37], it is reasonable to
infer that the partial pressure of hydrogen would influence the regio-
selectivity via the isomerization of 2-butene. Under high syngas pressure
or high partial pressure of CO, low regioselectivity was probably caused
by that carbonyl insertion was more competitive than β-H elimination
[19], and low yield of aldehydes was likely due to that the excess CO
would somehow inhibit the coordination of olefin to Rh center.
a
Reaction conditions: [Rh] = 410 ppm, 2-butene (1.8 g), P/Rh = 5, S/C
(Molar ratio of substrate to catalyst) = 1040, 10 bar syngas constant pressure,
P
H2/PCO = 1/1, 70 ◦C, 2 h, xylene (8.0 mL) as the solvent, dodecane (100 uL,
0.0753 g) as the internal standard.
b
Molar ratio of n- to iso-valeraldehyde, determined by GC.
c
The yield of all aldehydes, determined by GC.
Table 2
Rh/L2 catalyzed isomerization-hydroformylation of 2-butene under different
conditions.a
Entry
P/Rh
T ( ◦C)
P (bar)
t(h)
n/ib
Yield (%)c
1
0
70
70
70
70
70
50
60
80
90
100
70
70
70
70
70
70
70
70
70
10
10
10
10
10
10
10
10
10
10
10
10
2.5
5
2
2
2
2
2
2
2
2
2
2
4
6
2
2
2
2
2
2
2
0.61
40.9
42.8
35.5
35.5
33.2
38.4
38.2
21.9
5.8
4.7
2
5
61.6
54.9
46.9
17.6
10.8
13.0
68.1
89.5
80.1
64.7
92.7
45.4
49.8
46.0
61.6
42.9
29.5
26.0
3
10
15
20
5
4
5
6
7
5
8
5
9
5
3.3. Catalyst characterization studies
10
11
12
13
14
15
16
17
18
19
5
5
43.2
44.8
34.5
50.6
57.0
40.9
35.5
31.4
18.8
The coordination mode of phosphorus ligands to the rhodium center
is quite important to the regioselectivity of hydroformylation. A
bidentate ligand can coordinate with rhodium in equatorial-equatorial
(ee) or equatorial-axial (ea) configuration at the resting state of five-
coordinate HRh(L)(CO)2. In this study, to clarify the configuration of
HRh(L2)(CO)2, Rh(acac)(CO)2 and L2 were mixed in the molar ratio of
1/1 and stirred in tol-d8 under 10 bar of syngas (PH2/PCO = 1/1) for 1 h
at 80 ◦C, and after the syngas was released, the mixture was analyzed by
NMR and IR characterization immediately. 31P{1H} NMR showed a
broad doublet at 137.0 ppm and the corresponding hydride resonance
was observed as a broad peak at ꢀ 10.6 ppm in 1H NMR (Fig. 3b), the
JRhP (226.8 Hz) coupling constant and the chemical shifts of P and H are
close to those for the ee configuration of its analogue HRh(L4)(CO)2
[32]. While it is unable to determine JHP and JHRh coupling constants
since the resonance is a broad signal, and the JPP is undetected which
probably hints that the two phosphorus atoms are equivalent. IR spec-
trum could give complementary evidence for the coordination of HRh
(L2)(CO)2, where the CO vibration were observed at 2068 and 2018
cmꢀ 1 (Fig. 4), which were quite close to the signals in ee configuration
(2075–1970 cmꢀ 1) [32,38]. Combining with the NMR results above, it is
believed that the two phosphorus atom of L2 is dominantly coordinated
with Rh at ee configuration, which is the preferred isomer to achieve
linear product. When increasing PH2/PCO to 4/1, the major configuration
was also in an ee fashion.
5
5
5
5
7.5
10
15
20
25
5
5
5
5
a
[Rh] = 410 ppm, 2-butene (1.8 g), S/C = 1040, PH2/PCO = 1/1, xylene (8.0
mL), dodecane (100 uL, 0.0753 g).
b,c
see Table 1.
selectivity did not change too much. It can be explained by that the high
concentration of ligand around the metal would inhibit the coordination
of olefin to metal and led to low activity. The temperature had a sig-
nificant impact on the selectivity and the activity. The yield of aldehyde
increased from 10.8% to 89.5% when the temperature varied from 50 to
90 ◦C (entry 2, 6–9), but decreased to 80.1% at 100 ◦C (entry 10). The
selectivity slightly changed from 50 to 80 ◦C but had a great decline over
◦
90 C, which indicated that the isomerization-hydroformylation of 2-
butene could get better results under 70–80 ◦C with the presence of
ligand L2 and Rh(acac)(CO)2. Compared with ligand L2, NAPHOS [19],
Ding’s spiroketal-based diphosphite ligands [15] and our previous work
[31] could also achieve good n-selectivity but require higher tempera-
ture at 100–120 ◦C. Probably because a certain high temperature would
favor the β-H elimination of branched Rh-alkyl species [32] which leads
to the isomerization of 2-butene to 1-butene. Upon heating at 70 ◦C for 6
h, an excellent valeraldehyde yield of 92.7% with the n/i ratio of 44.8
could be obtained (entry 12).
3.4. Stability testing of ligand L2
Excellent catalysis results were gained with the presence of Rh/L2, in
order to probe if the Rh/L2 catalyst could be long-time used to catalyze
the isomerization-hydroformylation of 2-butene in industrial, the recy-
cling of catalyst was performed. Mild reaction conditions such as 80 ◦C
The total syngas pressure was also a key parameter in the hydro-
formylation of 2-butene. Under low pressure (below 7.5 bar), the
regioselectivity was slightly improved as the pressure increased from 2.5
4

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
Fig. 2. Rh/L2 catalyzed isomerization-hydroformylation of 2-butene at different PH2/PCO. a) PH2 = 2.5 bar, b) PH2 = 3.75 bar, c) PCO = 2.5 bar, d) PCO = 3.75 bar.
Other reaction conditions: see Table 1.
Fig. 3. 31P{1H} and 1H NMR of HRh(L2)(CO)2.
5

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
decomposition or hydrolysis during the recycling operation, which
might be responsible for the decline of the selectivity and aldehyde
yield. In order to probe the exact reason, hydrolysis and oxidation ex-
periments were performed on ligand L2, and the reaction mixture along
with the hydroformylation residue after 1st, 7th, 8th and 10th runs were
collected for 31P{1H} NMR and HRMS characterization. In CDCl3, ligand
L2 showed a singlet at 104.6 ppm (Fig. 6a). After stirring with H2O at
80 ◦C for 24 h (Fig. 6b), the chemical shift was not changed compared
with L2 which revealed that ligand L2 may be water-stable. For oxida-
tion experiments, oxygen and hydrogen peroxide were selected as the
oxidants. After heating L2 at 80 ◦C under O2 balloon for 24 h, the
chemical shift was maintained at 104.6 ppm (Fig. 6c), which hinted that
phosphoramidite (III) L2, (L2 (III) for short) was hardly oxidized in O2 at
80 ◦C. However, heating L2 in H2O2 at the same temperature within 2 h,
the singlet signal at 104.6 ppm (Fig. 6d) disappeared while another
resonance at ꢀ 11.9 ppm was observed, which could be assigned to
phosphoramide (Ⅴ) species (L2 (Ⅴ) for short), and this assumption was
proved via mass spectra (SI). In the hydroformylation recycling runs,
ligand L2 was not oxidized after 1st run since only the chemical shift of
104.6 ppm was observed (Fig. 6e). As mentioned in Fig. 5, the selectivity
was extremely high and was kept for 7 runs, but the activity was grad-
ually falling during the former 7 runs. The resonances were found at
104.6 ppm and ꢀ 11.9 ppm after the 7 runs, which revealed that L2 (III)
partially transformed into L2 (Ⅴ) species during the recycling process,
probably due to the products separation via distillation at high tem-
perature for more than 110 ◦C. Due to the oxidation of L2 (III), there was
a large decrease from 6th run to 7th run and when L2 (III) was completely
oxidized, the yield increased from 7th run to 8th run.
Fig. 4. Carbonyl stretching absorption of HRh(L2)(CO)2.
and 10 bar constant pressure (PH2/PCO = 1/1) were chosen. To our
delight, extremely high selectivity of n-valeraldyhyde was obtained
(more than 99%) and it could be almost maintained for 7 runs, in other
words, the Rh/L2 catalyst could work well for beyond 40 h (Fig. 5).
Yields of the aldehydes slightly decreased during the 1–6 runs and
maintained about 80% while dropped to 62.7% at the 7th run. However,
dramatic changes appeared at the 8th run, in which the yield of alde-
hydes increased from 62.7% to 98.8% and kept steady in the latter 9th
and 10th run, while the selectivity definitely dropped to 59.0% and kept
going down at the following runs.
As a matter of fact, the coordination ability of L2 (III) is stronger than
that of L2 (Ⅴ). During the former 7 runs, L2 (III) chelating to Rh was
dominant and was favorable for the formation of n-valeraldehyde. The
selectivity to n-valeraldyhyde fell sharply during 8th -10th runs, since L2
(III) completely transformed to L2 (Ⅴ) after 8th run (Fig. 6g-6h). But
Considering that the ligand L2 has the risk of oxidation,
Fig. 5. Recycling tests: [Rh] = 328 ppm, P/Rh = 5, toluene (30 mL) as the solvent, 80 ◦C, 6 h, 10 bar constant pressure (PH2/PCO = 1/1), dodecane as internal
standard. Mass of 2-butene for each run: 1) 6.0 g, 2) 5.4 g, 3) 4.8 g, 4) 4.5 g, 5) 6.0 g, 6) 4.8 g, 7) 4.8 g, 8) 7.2 g, 9) 6.3 g, 4 h,10) 6.0 g, 4 h.
6

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
Fig. 6. Stacked 31P{1H}NMR spectra of L2, and the reaction residue after hydrolysis, oxidation experiments and the 1st, 7th, 8th and 10th hydroformylation runs.
much to our surprise, high activity could also be obtained after the 8th
run which was probably due to the coordination of L2 (Ⅴ) to rhodium via
oxygen [39,40]. To understand the different catalytic performance of
Rh/L2 (III) and Rh/L2 (Ⅴ), their hydroformylation samples were
continuously taken out from the autoclave at different time and
analyzed by GC. The results revealed that in the presence of Rh/L2 (III),
the n/i ratio was high all the time and 2-butene conversion to n-valer-
aldehyde was the major process (Fig. 7a). It is probably that the strong
represent phosphine, phosphinite, and phosphite respectively. Accord-
ing to the electricity peaks in Fig. 8, the oxidation potential of L5, L6 and
L7 located at around 1.07 V, 1.28 V and 1.65 V, respectively. The
oxidation potential of L2 was not observed from ꢀ 1.4 V to +2.62 V
versus Ag/AgCl. It may be explained that ligand L2 is hard to be oxidized
under this condition. The oxidation possibility might follow the ten-
dency L5 > L6 > L7 > L2, which is consistent with the σ-donor ability
[30]. Combined with the above NMR analyses (Fig. 6), it is shown that
the phosphoramidite ligand L2 exhibits good resistance to water and
oxygen and has better stability than other ligands.
π
-accepting bidentate ligand L2 (III) coordinates with Rh in an ee HRh
(L2)(CO)2 configuration, which allows fast isomerization of 2-butene to
1-butene and facilitates the hydroformylation of 1-butene to proceed
immediately once 1-butene is formed [26]. Interestingly, in Rh/L2 (Ⅴ)
system, the n/i ratio was rather low in all time and remained constant
and the n- and i-aldehyde formation rate was close (Fig. 7b). The yield of
all aldehydes was beyond 90% in 4 h, which was in consistent with the
good aldehyde yields at 8th -10th runs when L2 (III) has changed into L2
(Ⅴ).
4. Conclusion
In conclusion, different types of ligands, including phosphine,
phosphinite, phosphite and phosphoramidite were applied to Rh-
catalyzed isomerization-hydroformylation of 2-butene. An easily-
accessible phosphoramidite ligand L2 demonstrated the best selec-
tivity (beyond 99%) towards n-valeraldehyde as well as superior activity
under mild conditions (80 ◦C, 10 bar). Further investigation of [HRhL2
(CO)2] showed that ligand L2 coordinate in an equatorial-equatorial (ee)
fashion to the trigonal bipyramidal rhodium, which seems to be the
To further confirm the stability of phosphoramidite ligand L2, cyclic
voltammetry measurement was performed in tetrahydrofuran at room
temperature, with 0.1 M t-Bu4NBF4 as the supporting electrolyte. For
comparison, the profiles of L5, L6 and L7 were also recorded which
7

S. Tang et al.
Molecular Catalysis 508 (2021) 111598
Fig. 8. Cyclic voltammograms of different ligands in tetrahydrofuran, [n-
Bu4NBF4] = 0.01 M (supporting electrolyte). Working electrode: Pt plate,
counter electrode: carbon rod, reference electrode: Ag/AgCl. Scan rate = 100
mV/s.
the work reported in this paper.
Acknowledgement
This work was supported by Sichuan Science and Technology pro-
gram (2019YFG0146) and the Sichuan University Innovation Spark
research Project (2018SCUH0079). The authors are grateful to the
Centre of Testing & Analysis and the comprehensive training platform of
specialized laboratory, College of Chemistry, Sichuan University for the
analysis work. And the authors declare that they have no conflict of
interest.
Supplementary materials
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111598.
Fig. 7. The distribution of substrate and products with time: a) L2 (III), 2-
butene (4.8 g), 5 h; b) L2 (Ⅴ), 2-butene (6.0 g), 4 h. Reaction conditions:
80 ◦C, 10 bar, [Rh] = 328 ppm, P/Rh = 5, tol (30 mL) as solvent, dodecane as
internal standard.
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