Selective hydroformylation of various olefins using diphosphinite ligands

Article abstract of DOI:10.1002/aoc.2983

Novel diphosphinite ligands are synthesized by the reaction of various derivatives of 1,3-diols with chlorodiphenylphosphine. The synthesized ligands exhibited considerable impact on hydroformylation of various olefins with excellent regioselectivity toward branched aldehyde. The effect of solvent, temperature, pressure and catalyst loading on the hydroformylation reaction is also described. The synthesized diphosphinite ligands with rhodium precursor works under milder reaction conditions as compared to traditional phosphine and phosphite-based ligands. Copyright 2013 John Wiley & Sons, Ltd. A novel diphosphinite ligands are synthesized by the reaction of various derivatives of 1,3-diol with chlorodiphenylphosphine. The synthesized ligands exhibited a considerable impact on hydroformylation of various olefins with excellent regioselectivity toward branched aldehyde. The effect of solvent, temperature, pressure and catalyst loading on the hydroformylation reaction is also described. The synthesized diphosphinite ligands with Rhodium precursor works at milder reaction conditions as compared to traditional phosphine and phosphite based ligands. Copyright

Full text of DOI:10.1002/aoc.2983

Full Paper
Received: 28 January 2013
Revised: 25 February 2013
Accepted: 27 February 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2983
Selective hydroformylation of various olefins
using diphosphinite ligands
Shoeb R. Khan and Bhalchandra M. Bhanage*
Novel diphosphinite ligands are synthesized by the reaction of various derivatives of 1,3-diols with chlorodiphenylphosphine.
The synthesized ligands exhibited considerable impact on hydroformylation of various olefins with excellent regioselectivity
toward branched aldehyde. The effect of solvent, temperature, pressure and catalyst loading on the hydroformylation
reaction is also described. The synthesized diphosphinite ligands with rhodium precursor works under milder reaction
conditions as compared to traditional phosphine and phosphite-based ligands. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: hydroformylation; olefins; bis(phosphinite) ligand; regioselective; homogeneous catalysis
Thus the development of new ligands to obtain highly active
and selective catalysts is always a key issue in the case of the
Introduction
Hydroformylation reaction has attracted considerable attention
for the synthesis of aldehydes from olefins and finds large appli-
cations in fine chemical and pharmaceutical industries.[1] The
reaction was first discovered by Otto Roelen in 1938 during
investigations on the formation of oxygenated products in
cobalt-catalyzed Fisher–Tropsch reactions. Based on homoge-
neous catalysis, it is one of the largest industrially applied, clean
and atom-efficient processes.[2,3] Worldwide, several million tons
of aldehyde are produced via hydroformylation reactions, most
of which are reduced to alcohols or oxidized to carboxylic acids
or esters.[4,5] Nonsteroidal anti-inflammatory drugs (NSAIDs) like
ibuprofen (2-arylpropanoic acid) is also synthesized via
hydroformylation followed by oxidation of aldehyde using
styrene derivative as a substrate.[6,7]
Several transition metal-based catalysts involving Rh, Pt, Co
and Ru have been used for this reaction; however, Rh-based
catalysts works at lower temperature and pressure and hence
are generally preferred for hydroformylation reactions.[8,9] Usu-
ally, phosphorus donor ligands employing transition metal
complexes are able to act efficiently under mild conditions and
are extensively used ligands in homogeneous catalysis, whereas
selectivity of desired product can be tuned by varying the ligands
attached to the metal centre.[10]
The literature reveals that in the case of hydroformylation
processes catalysts based on phosphite/phosphinite (weak s-donors
and strong p-acceptors) ligands are more effective than the
conventional Rh-triphenyl phosphine based catalyst.[11–13]
Moreover, phosphite and phosphinite ligands are less prone
to oxidation compared to phosphine-based ligands.[14,15]
However, in spite of their high activity and performance there
are very few reports on diphosphinite ligands. Some of the best
results with diphosphinite ligands have been obtained using the
family of calixarenes, which are known as sophisticated
molecular cages and claw-like ligands,[16] pyranoside[17] and
furanoside[18] ligands. However, difficulties in preparation of
these ligands limit their application.
hydroformylation reaction. As hydroformylation seems to be
attainable only with synthetic catalysts, much attention has been
paid to developing new phosphinite-based ligands. In this regard
we have synthesized and characterized a new class of diphosphinite
ligands based on 1,3-diol as a backbone and applied to the
rhodium-catalyzed hydroformylation of aryl olefins (Scheme 1).
Results and Discussion
The initial studies were conducted using L1 as a choice of ligand
with Rh(acac)(CO)2 for the hydroformylation of styrene as a
model reaction (Scheme 2).
A series of experiments were performed to optimize various
reaction parameters such as effect of temperature, solvent,
catalyst loading, CO/H2 pressure and time on a model reaction.
The results obtained are summarized in Table 1. Initially, the
reaction was studied at different temperatures in the range
of 50–100ꢀC (Table 1, entries 1–5). High temperature significantly
promotes the side reaction of the hydrogenation process
with containment of a-formylation and thus provides higher
b-aldehyde formation while, lowering the reaction temperature
to 50ꢀC, regioselectivity toward branched aldehyde product
increased at the expense of reaction rate. Thus further studies
were carried out at 60ꢀC, which was found to provide maximum
conversion and selectivity toward the desired product (Table 1,
entry 4). Next we studied the effect of solvent on
hydroformylation reaction. Solvents such as tetrahydrofuran
(THF) and methanol provide lower conversion and selectivity of
*
Correspondence to: Bhalchandra M. Bhanage, Department of Chemistry,
Institute of Chemical Technology, Matunga, Mumbai 400019, India.
E-mail: bm.bhanage@gmail.com
Department of Chemistry, Institute of Chemical Technology, Matunga,
Mumbai 400019, India
Appl. Organometal. Chem. 2013, 27, 313–317
Copyright © 2013 John Wiley & Sons, Ltd.

S. R. Khan and B. M. Bhanage
Scheme 1. Schematic representation of diphosphinite ligand synthesis.
expected product (Table 1, entries 6 and 7). In the case of
methanol as a solvent a considerable amount of acetal formation
was observed. It was observed that the reaction was more
favourable using toluene as a solvent (Table 1, entry 4). Further-
more, the effect of syngas (CO/H2) pressure on reaction rate and
selectivity was studied (Table 1, entries 4 and 8–10). We
observed that lowering the CO/H2 pressure from 35 to 25 bar
did not have any prominent effect on the reaction outcome
but with further decrease in pressure decreased the conversion
as well as selectivity of the desired product. Thus syngas at 25
bar was found to be the optimized pressure for styrene
hydroformylation.
We further studied the substrate:rhodium molar ratio and
observed that with increasing molar ratio from 1000:1 to 2000:1
decreased the conversion and selectivity of desired product
(Table 1, entry 11); this decrease in conversion and selectivity
was due to a decrease in the amount of catalyst from 0.1 mol%
(Sub/Rh molar ratio 1000:1) to 0.05 mol% (Sub/Rh molar ratio
2000:1). Subsequently, we also investigated the effects of
ligand/Rh molar ratio and it was observed that on increasing
the P/Rh molar ratio from 4 to 8 the selectivity for branched
aldehyde increased at the expense of conversion (Table 1, entries
4 and 12). The influence of reaction time ranging from 6 h to 3 h
was also studied, and it was found that within 4 h the reaction
provides maximum yield and selectivity for branched aldehyde
formation (Table 1, entries 4, 13 and 14).
phosphite ligands such as P(OPh)3 and P(OEt)3 showed quite low
conversion in comparison with the developed phosphinite ligands
under optimized reaction conditions (Table 2, entries 3 and 4).
As the phosphine ligands are more basic than their phosphinite
counterparts, they provided less conversion and selectivity toward
the desired product (Table 2, entries 5–9). Compared with
bidentate phosphine ligands, monodentate phosphine (PPh3) gave
better conversion (Table 2, entry 5). In the case of bidentate
phosphine ligands, conversion goes on increasing from dppm to
dppb (Table 2, entries 4–9). Among all the screened phosphine
ligands, dppe offered excellent regioselectivity (97%) for iso
aldehyde, whereas low conversion (36%) confines its applications
to hydroformylation reactions (Table 2, entry 7). A bulky bidentate
phosphine ligand like Xantphos provided a hydroformylation prod-
uct with very low conversion (7%) and aldehyde selectivity (35%),
even after a prolonged reaction period of 8 h (Table 2, entry 10).
The reaction was also studied in the absence of ligand using only
Rh(acac)(CO)2; however, low conversion and poor selectivity of
the desired product address the importance of the ligand in
hydroformylation reaction (Table 2, entry 11). Hence, among
several screened ligands, diphosphinite ligand L1 with Rh(acac)
(CO)2 precursor was found to be the best catalytic combination
for hydroformylation reaction of styrene, providing admirable
conversion and selectivity for the desired product.
In order to examine the general applicability of the developed
protocol, we studied the range of substrates for this process.
Table 3 shows the results from the hydroformylation of a variety
of olefins using optimized reaction conditions.
Hence the optimized reaction conditions for the hydroformylation
of styrene were: styrene (5 mmol), Rh(acac)(CO)2 (0.1 mol%), ligand
(L1) (0.2 mol%), 25 bar CO/H2 (1:1) at a temperature of 60ꢀC for 4 h
in toluene (15 ml) as solvent.
The model reaction of styrene under optimized reaction
conditions provided excellent conversion and very good selec-
tivity (91%) toward branched aldehyde (Table 3, entry 1).
Substituted styrenes like p-tert-butylstyrene and 3-methylstyrene
were found to react smoothly, furnishing good yields and selectiv-
ity of the corresponding product (Table 3, entries 2 and 3). Styrene
with an electron-donating group, such as p-chlorostyrene, also
provides almost complete conversion and immense selectivity
for branched aldehyde formation (Table 3, entry 4). It was
observed that the regioselectivity for branched product faintly
increases with electron-withdrawing substituent on a phenyl
ring in the order p-(CH3)3CPh < m-CH3Ph < HPh < ClPh. This
might be due to increase in p-electron density on the a-carbon
of styrene, which favours the attack of electropositive Rh
metal, providing higher selectivity for branched aldehyde.[21]
Allylbenzene and 4-methoxy allylbenzene offered good selec-
tivity for aldehyde formation with almost 100% conversion. As
these olefins having an isolated olefinic CC bond form a phenyl
In order to compare the activity and selectivity of the developed
diphosphinite ligands, various phosphite and phosphine ligands
were screened for hydroformylation of styrene under optimized
reaction conditions. It is well known that, especially, ligands that
are good electron acceptors have been found to be effective
ligands for hydroformylation reactions. Since these ligands
decrease the electron density of rhodium and weaken the p-back
donation from rhodium to CO, resulting in rapid CO dissociation,
this swift mechanistic step in hydroformylation accelerates the rate
of reaction.[19,20] The synthesized diphosphinite ligands used in the
present study are good electron acceptors and show little differ-
ence in their electronic and steric properties. However, it was
observed that the conversion continues to decrease from L1 to L2
because of different substituents at the 1,3-diol backbone, though
L2 shows very good selectivity towards branched aldehyde but
gave low conversion (Table 2, entries 1 and 2). Screened
Scheme 2. Hydroformylation of styrene.
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 313–317

Regioselective hydroformylation using bis(phosphinite) ligand
Table 1. Effect of reaction parameters on hydroformylation of styrene using L1 as a liganda
Entry
Temp. (ꢀC)
Solvent
Sub/Rh (molar ratio) Pressure (bar) Time (h) Conv.c (%) Aldehyde (%) Iso:linearc (%) Reduction(%)
1
100
80
70
60
50
60
60
60
60
60
60
60
60
60
Toluene
Toluene
Toluene
Toluene
Toluene
THF
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
2000
1000
1000
1000
35
35
35
35
35
35
35
30
25
20
25
25
25
25
6
6
6
6
6
6
6
6
6
6
6
6
4
3
100
100
100
100
47
92
94
64:36
73:27
81:19
90:10
96:4
8
6
5
1
1
5
9
2
1
3
7
1
1
2
2
3
95
4
99
5
99
6
79
95
88:12
83:17
88:12
89:11
87:13
85:15
92:8
7
MeOH
85
64:27*
98
8
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
99
9
100
92
99
10
11
12b
13
14
97
81
93
87
99
100
93
99
91:9
98
93:7
aReaction conditions: styrene (5 mmol), Rhacac(CO)2 (0.1 mol%), L1 (0.2 mol%), 800 rpm; *acetal formation.
bP/Rh (8/1).
cConversion and selectivity (iso/linear) were determined by GC analysis.
ring they gave three structural aldehydes due to isomerization
reaction (Table 3, entries 5 and 6). Moreover, linear aliphatic
alkenes such as 1-hexene gave a remarkable yield of the desired
Experimental
Materials and Instruments
product as well (Table 3, entry 7). Cyclic olefins such as
cyclopentene and cyclohexene also provided a satisfactory yield
of expected products (Table 3, entries 8 and 9).
All chemicals, e.g. olefins, chlorodiphenylphosphine, [Rh(acac)
(CO)2] and phosphorus ligands, were purchased from Sigma-
Aldrich and Alfa Aesar. All other reagents were of analytical grade
and were used without further purification. Syngas (CO and H2,
1:1) with a purity of 99.9%, was obtained from Rakhangi Gases
Ltd, Mumbai, India.
Conclusion
Novel diphosphinite ligands have been synthesized and applied
in a rhodium-catalyzed hydroformylation reaction. The catalyst
systems were optimized with respect to various parameters and
enabled hydroformylation of different olefins with electron-rich
and electron-deficient substituent. In all cases the developed
system afforded excellent yields and high regioselectivity toward
branched aldehyde under mild reaction conditions. Thus the
wider substrate applicability and high regioselectivity make this
catalyst system attractive for further investigations.
The 1,3-diol backbones were synthesized from their corre-
sponding 1,3-dione according to the literature procedure.[22]
Diphosphinite ligands were also prepared by the reported
methods[23–25] under an atmosphere of dry nitrogen or argon
using standard Schlenk techniques. Solvents were dried and
distilled by conventional methods prior to use.
The 1H and 13C NMR (d in ppm) spectra were recorded on a Varian
VXR 300 spectrometer at operating frequencies of 300 MHz and 75
MHz, respectively, in CDCl3 solvent using tetramethylsilane as
Table 2. Effect of ligands on hydroformylation of styrenea
Entry
Ligand (P/Rh = 4)
Conversionc (%)
Aldehyde (%)
Iso:linearc (%)
Reduction (%)
1
L1
100
84
83
88
85
29
36
66
84
07
65
99
100
98
97
100
97
96
98
97
35
98
91:9
1
2
L2
96:4
—
2
3
P(OPh)3
P(OEt)3
PPh3
95:5
4
74:26
86:14
91:9
3
5
—
3
6
dppm
dppe
dppp
dppb
Xantphos
—
7
97:3
4
8
81:19
73:27
26:74
56:44
2
9
10b
3
65
2
11
aReaction conditions: styrene (5 mmol), Rhacac(CO)2 (0.1 mol%), ligand (0.2 mol%), CO/H2 (1:1) 25 bar, temperature 60ꢀC, time 6 h, 800 rpm;
btime (8 h).
cConversion and selectivity (iso/linear) were determined by GC analysis.
Appl. Organometal. Chem. 2013, 27, 313–317
Copyright © 2013 John Wiley & Sons, Ltd.
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S. R. Khan and B. M. Bhanage
Table 3. Hydroformylation of different olefins using Rhacac(CO)2 with L1 liganda
Entry
Substrate
Conversionc (%)
Aldehyde selectivityd (%)
Iso:linearc (%)
Reduction (%)
1
Ph-CH=CH2
100
100
99
99
91:9
87:13
89:11
92:8
1
2
p-tBu-Ph-CH=CH2
m-CH3-Ph-CH=CH2
p-Cl-Ph-CH=CH2
Ph-CH2-CH=CH2
p-OMe-Ph-CH2-CH=CH2
CH3-(CH2)3-CH=CH2
Cyclopentene
99
1
3
98
2
4
100
97
99
1
5
99
1:40:59
2:37:61
4:33:63
—
1
6
99
100
99
—
1
7
100
87
8b
9b
100
100
—
—
Cyclohexene
72
—
aReaction conditions: olefin (5 mmol), Rhacac(CO)2 (0.1 mol%), ligand L1 (0.2 mol%), toluene (15 ml), CO/H2 (1:1) 25 bar, temperature 60ꢀC, time 4
h, 800 rpm;
btemperature 80ꢀC.
cConversion and selectivity (iso/linear) were determined by GC analysis.
dChemoselectivity for aldehyde product to total reaction product.
3
internal standard. 31P NMR spectra were obtained at an operating
frequency of 162 MHz on a Varian VXR 400 spectrometer.
CCHOP), 31.6 (d, Jp,c = 8.3 Hz, CH2CHOP), 25.6 (s, (CH3)3C); 31P
NMR (162 MHz, CDCl3, 25ꢀC): d = 110.42 ppm.
Preparation of 1,3-Bis((diphenylphosphino)oxy)-1,3-
Procedure for Hydroformylation Reaction of Styrene
diphenylpropane (L1)
In a typical experiment, to a high pressure reactor of 100 ml
capacity, Rhacac(CO)2 (0.1 mol%), ligand L1 (0.2 mol%), styrene
(5 mmol) and toluene (15 ml) were added. The reactor was then
flushed with nitrogen, followed by syngas (1:1 mixture of CO and
H2 gas) at room temperature; next, the reaction was pressurized
to 25 bar syngas and heated to 60ꢀC at a stirring speed of 800
rpm for 4 h. After completion of the reaction, the reactor was
cooled to room temperature and remaining syngas was carefully
released. The reaction mixture was analysed by gas chromatog-
raphy (GC; Clarus 400, PerkinElmer) equipped with a capillary
column (30 m  0.25 mm  0.25 mm) and a flame ionization
detector (FID). All products obtained are well known in the
A solution of PPh2Cl (1.32 g, 6 mmol) in dry THF (5 ml) was added
slowly, with stirring, to a mixture of 1,3-diphenylpropane-1,3-diol
(0.68 g, 3 mmol) and pyridine (0.47 g, 6 mmol) in dry THF (15 ml)
at 0ꢀC. Following the addition, the reaction mixture was left to
warm at room temperature and stirred overnight. The pyridine
hydrochloride was filtered off under nitrogen atmosphere, the fil-
trate evaporated to dryness and the residue dissolved in 7 ml
diethyl ether. When this solution was cooled to À5ꢀC, the prod-
uct separated in the form of white crystals and was stored under
nitrogen atmosphere: yield 1.5 g (2.52 mmol, 84%).
NMR spectra of L1: 1H NMR (300 MHz, CDCl3, 25ꢀC): d = 7.68–7.75
(m, 8H, PhP), 7.24–7.47 (m, 12H, PhP), 7.31–7.37 (m, 10H, PhCHOP),
literature and were confirmed by GC-MS analysis on
a
4.98 (t, 2H, J = 5.8 Hz, CHOP), 2.17 (t, 2H, J= 5.8 Hz, CH2CHOP); 13
C
Shimadzu GCMS-QP 2010 instrument (Rtx-17, 30 m  25 mm
ID, film thickness 0.25 mm df) (column flow 2 ml minÀ1, 80–240ꢀC
at 10ꢀ/minÀ1 rise) see supporting information.
1
NMR (75 MHz, CDCl3, 25ꢀC): d = 144.3 (d, Jp,c = 18.6 Hz, Cipso PhP),
2
133.7 (s, Cipso Ph), 131.3 (d, Jp,c = 7.8 Hz, Co PhP), 129.6 (s, Co
3
Ph), 128.9 (d, Jp,c = 5.7 Hz, Cm PhP), 128.1 (s, Cp PhP), 127.3
2
(s, Cm Ph), 125.7 (s, Cp Ph), 71.5 (d, Jp,c = 22.8 Hz, CHOP), 46.9
Acknowledgements
3
(d, Jp,c = 8.7 Hz, CH2CHOP); 31P NMR (162 MHz, CDCl3, 25 ꢀC):
The author (S. R. Khan) is greatly thankful to the Council of
Scientific and Industrial Research (CSIR, India) for providing a
Senior Research Fellowship (SRF) and to M. M. Siddiqui from
IIT-B for fruitful discussions.
d = 111.66 ppm.
Preparation of ((2,2,6,6-Tetramethylheptane-3,5-diyl)bis
(oxy))bis(diphenylphosphine)(L2)
A solution of PPh2Cl (1.32 g, 6 mmol) in dry THF (5 ml) was added
slowly, with stirring, to a mixture of 2,2,6,6-tetramethylheptane-
3,5-diol (0.56 g, 3 mmol) and pyridine (0.47 g, 6 mmol) in dry
THF (15 ml) at 0ꢀC. Further procedures were the same as
discussed earlier. The product was obtained as a white solid: yield
1.3 g (2.3 mmol, 79%).
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