PEG-anchored rhodium polyether diphosphinite complex as an efficient homogeneous and recyclable catalyst for hydroaminomethylation of olefins

Article abstract of DOI:10.1016/j.catcom.2011.08.033

A recyclable rhodium polyether diphosphinite complex anchored in polyethylene glycol [RhPEGD] was studied for hydroaminomethylation of various olefins with primary and secondary amines. The protocol was optimized with respect to various reaction parameters and the general applicability of catalyst for hydroaminomethylation of different functionalized olefins with corresponding amines was investigated. During the course of reaction, catalyst was soluble with reactants/products while could be quantitatively separated from reaction media in biphasic form by addition of anti-solvent on completion of reaction. The catalyst exhibited remarkable activity and was subsequently recycled up to five consecutive cycles.

Full text of DOI:10.1016/j.catcom.2011.08.033

Catalysis Communications 15 (2011) 141–145
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
PEG-anchored rhodium polyether diphosphinite complex as an efficient
homogeneous and recyclable catalyst for hydroaminomethylation of olefins
⁎
Shoeb R. Khan, Mayur V. Khedkar, Ziyauddin S. Qureshi, Dattatraya B. Bagal, Bhalchandra M. Bhanage
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2011
Received in revised form 19 August 2011
Accepted 29 August 2011
Available online 9 September 2011
A recyclable rhodium polyether diphosphinite complex anchored in polyethylene glycol [RhPEGD] was stud-
ied for hydroaminomethylation of various olefins with primary and secondary amines. The protocol was op-
timized with respect to various reaction parameters and the general applicability of catalyst for
hydroaminomethylation of different functionalized olefins with corresponding amines was investigated.
During the course of reaction, catalyst was soluble with reactants/products while could be quantitatively sep-
arated from reaction media in biphasic form by addition of anti-solvent on completion of reaction. The cata-
lyst exhibited remarkable activity and was subsequently recycled up to five consecutive cycles.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Hydroaminomethylation
Olefins
Amines
Recyclable Rh complex
Homogeneous catalysis
1. Introduction
or by supported liquid phase catalysis [11,12]. On other hand, Alper and
co-workers employed ionic diamine rhodium complex for hydroamino-
The hydroaminomethylation reaction has attracted a great atten-
tion for synthesis of amines finding large applications in pharmaceu-
tical and chemical industries [1,2]. It is a tandem reaction involving
hydroformylation of an olefin to an aldehyde followed by the reaction
of resulting aldehyde with a primary or secondary amine to provide
the corresponding enamine or imine and finally the intermediate is
hydrogenated to amine (Scheme 1).
From economical and environmental point of view, one pot syn-
thesis of amines from inexpensive feedstock like alkenes sounds to
be a superior method over conventional ones which comprises of
usually multi-step and less atom-efficient processes [3].
In 1949, W. Reppe discovered the hydroaminomethylation using stoi-
chiometric amount of Fe(CO)5 as a catalyst. Till mid of 1990s, relatively
harsh conditions were employed to obtain good yield of corresponding
amines from simple olefins with the aid of hydroaminomethylation reac-
tion [4–7]. However in the past two decades, several other catalysts com-
prising of rhodium, ruthenium and bimetallic complexes were developed
[8–10]. It is also noteworthy to mention that, the several other methodol-
ogies developed for hydroaminomethylation using homogeneous cata-
lytic system suffer with the limitation of catalyst-product separation
and catalyst recyclability. However, some investigations have been
made in this regard and the possible outcome includes anchoring of ho-
mogeneous catalyst, either by using a liquid–liquid two phase system
methylation reaction but the system fails to recover an expensive rhodi-
um metal catalyst on completion of the reaction [13,14]. Hence, still there
lies a great scope in development of an efficient and facile metal complex
catalyst with an industrially feasible separation strategy for the hydroa-
minomethylation reaction.
Considering the above issues, the aim of present work highlights
use of liquid phase catalyst soluble in organic solvent thus retaining
the merits of homogeneous systems with an additional advantage of
biphasic separation strategy. On completion of reaction, the catalyst
can be quantitatively separated in liquid form from the reaction mix-
ture by addition of a volatile anti-solvent such as n-hexane which
forms two phases and thus the liquid phase catalyst can be subse-
quently reused (Fig. 1). In continuation to our interest in application
of biphasic separation methodology [15], we herein report a facile
and highly efficient protocol for homogeneous liquid phase catalytic
system for hydroaminomethylation reaction.
2. Experimental methods
2.1. Materials
All chemicals like olefins, amines, chlorodiphenylphosphine,
[Rh(acac)(CO)2] and Xantphos were purchased from Sigma-Aldrich
and Alfa Aesar. PEG-600 was supplied by S. D. Fine Chemicals India.
All other reagents were of analytical grade and were used without
further purification. Syn gas (CO and H2, 1:1), with a purity of
99.9% was obtained from Rakhangi Gases Ltd., Mumbai, India.
⁎
Corresponding author. Tel.: +91 22 33611111; fax: +91 22 33611020.
E-mail addresses: bm.bhanage@gmail.com, bm.bhanage@ictmumbai.edu.in
(B.M. Bhanage).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.033

142
S.R. Khan et al. / Catalysis Communications 15 (2011) 141–145
NR
+
2
Catalyst
CO/H
+
HNR
R
2
R
NR
2
2
R
CO/H
2
H
Catalyst
2
CHO
HNR
2
CHO
R
+
Imine / Enamine
R
-H O
2
Condensation / Isomeriation
Hydroformylation
Scheme 1. Hydroaminomethylation of olefins.
in THF (20 mL) at room temperature. The reaction mixture was stirred
for 3 h at room temperature (30 °C). Precipitated Py.HCl was filtered
off, and the THF solution was passed through a silica gel column to re-
move the dissolved Py.HCl. The ligand α,ω-bis(diphenylphosphino)
poly(ethylene glycol-600) was obtained in 80% yield (4.0 g) after evapo-
ration of THF under rotary evaporator.
2.2. General procedure for hydroaminomethylation
In a typical hydroaminomethylation procedure, to a high pressure re-
actor of 100 mL, olefin (2 mmol), amine (2 mmol) and toluene (10 mL)
were added with polymeric RhPEGD-600 (2.5 μmol) (3.9 mg). The reac-
tor was then flushed with nitrogen followed by syn gas (1:1 mixture of
H2 and CO gas) at room temperature; the reaction was then pressurized
to 400 psi and was heated to 100 °C with stirring speed of 800 rpm. After
completion of the reaction, the reactor was cooled to room temperature
and remaining syn gas was carefully vented. The reaction mixture was
analyzed by gas chromatography (Perkin Elmer, Clarus 400 GC) equipped
with capillary column (30 m×0.25 mm×0.25 μm) and a flame ioniza-
tion detector (FID). All the products obtained are well known in literature
[8] and were confirmed by GC–MS analysis (Shimadzu, GCMSQP2010)
equipped with Rtx wax capillary column (30 m length and 0.25 mm
diameter).
31P NMR (400 MHz, DMSO): δ 19.98 (s).
2.3.2. Synthesis of polyether phosphinite Rh (I) complex (RhPEGD-600)
A solution of RhCl3·H2O (1 g, 3.8 mmol) in ethanol (30 mL) was
added to a refluxing solution of polyether diphosphinite (11.4 mmol)
in ethanol (150 mL). After 30 min, on dropwise addition of 40% aqueous
formaldehyde (5 mL) the solution turned pale yellow. Further, addition
of sodium borohydride (1 g) in ethanol to this hot mixture yielded the
greenish yellow product, as described above with 70% yield.
31P NMR (400 MHz, DMSO): δ 73.88 (d, J(Rh,P) 144 Hz).
2.4. Catalyst recyclability study
2.3. Catalyst preparation and characterization
The catalyst was investigated for five consecutive recycles for
hydroaminomethylation reaction using the above experimental pro-
cedure (described in Section 2.2). During recyclability study, addi-
tional PEG-600 (1 mL) was added for recovery of the catalyst. On
completion of reaction, 20 mL n-hexane was added to the reaction
mixture, which separated the lower catalyst-philic PEG phase from
upper product phase. After phase separation PEG phase containing
polyether Rh(I) catalyst was subjected to reuse by charging the
same amount of substrate (2 mmol) in toluene as a solvent.
The polyether ligand α,ω-bis(diphenylphosphino)-poly
(ethyleneglycol-200) [DPPPEG-200], α,ω-bis(diphenylphosphino)-
poly(ethyleneglycol-400) [DPPPEG-400], α,ω-bis (diphenyl phosphino)-
poly(ethyleneglycol-600) [DPPPEG-600], Bu(OPPh2)2 and Rh(I) carbonyl
phosphinite complex [RhPEGD-600] were prepared by procedures
reported in literature [15–19]. The complex HRhCO(PPh3)3 was also pre-
pared by reported method [20,21]. 31P NMR analysis of the
diphosphinite ligand [DPPPEG-600] and polyether Rh metal complexes
were carried out using a Varian 300 MHz instrument. Thermogravi-
metric analyses (TGA) of metal complex were performed using an
SDT-Q600 instrument under N2 atmosphere (refer supporting
information).
3. Results and discussion
The initial studies were conducted using RhPEGD-600 as a choice
of catalyst for the hydroaminomethylation of cyclopentene with mor-
pholine as a model reaction (Scheme 2).
2.3.1. Synthesis of α,ω-bis(diphenylphosphino)-poly(ethylene glycol)
[DPPPEG-600] ligand
In an inert atmosphere, chlorodiphenylphosphine (2.3 g, 10.0 mmol)
in THF (2.0 mL) was added slowly into a stirred solution of polyethylene
glycol-600 (PEG-600) (5.0 mmol) and pyridine (Py) (0.8 g, 10.0 mmol)
Various reaction conditions for this transformation were exten-
sively studied and the results obtained were summarized in Table 1.
Initially, the reaction was studied at different temperatures in the
Product Separation
Product /
Organic
Addition of
Antisolvent
and Purification
Homogeneous Catalyzed
Hydroaminomethylation
Feed
Catalyst /
PEG-600
PEG-600 / Catalyst Recycle
Fig. 1. Diagrammatic representation for catalyst separation and reusability.

S.R. Khan et al. / Catalysis Communications 15 (2011) 141–145
143
O
O
RhPEGD-600
N
+
CO / H2
N
H
Ph
Ph
P
O
DPPPEG =
PEG =
Ph
O
P
n
Ph
H
OH
O
n
(PEG av. MW = 200, 400, 600)
Scheme 2. Hydroaminomethylation of cyclopentene with morpholine.
range of 80–120 °C (Table 1, entries 1–3) where 100 °C was found to
provide the maximum conversion with high selectivity. However,
with further increase in reaction temperature up to 120 °C, reduction
of aldehyde to alcohol in small amount was observed. The influence of
reaction time ranging from 2 to 6 h was studied and the best results
were obtained within an appreciable period of 4 h (Table 1, entries
2, 4 and 5). Furthermore, the effect of syn gas pressure on reaction
rate and selectivity of hydroaminomethylation reaction was studied
in the range of 200–600 psi (Table 1, entries 2, 6 and 7). We observed
that lowering the reaction pressure results in decrease of conversion
as well as selectivity of desired product, whereas with further in-
crease in pressure up to 600 psi did not have any prominent effect
on the reaction outcome. While studying the hydroaminomethylation
reaction at 400 psi of CO/H2 (in 1:2 stoichiometric amounts), reduc-
tion of starting olefins (6%) was observed providing lower yield of de-
sired product (Table 1, entry 8). Hence, further studies were carried
out at 400 psi of CO/H2 with 1:1 molar concentrations.
that increasing the molar ratio from 400:1 to 800:1 had no significant
effect on conversion and selectivity of desired product (Table 1, en-
tries 2, 11) while further increase in substrate to rhodium mole
ratio of 1600:1 decreased the conversion and selectivity of the reac-
tion (Table 1, entry 12), this decrease in conversion and amine selec-
tivity was due to decreases in the amount of catalyst from 2.5 μmol
(Sub/Rh mole ratio 800:1) to 1.25 μmol (Sub/Rh mole ratio 1600:1).
Hence, the optimized reaction conditions for the hydroaminomethy-
lation using RhPEGD-600 catalyst were cyclopentene (2 mmol), mor-
pholine (2 mmol), RhPEGD-600 (2.5 μmol), 400 psi CO/H2 (1:1),
temperature 100 °C for 4 h in toluene (10 mL) as a solvent.
Moreover, various Rh-phosphine and Rh-phosphinite catalysts
were screened for hydroaminomethylation of cyclopentene with
morpholine under optimized reaction conditions (Table 2, entries
1–6). Other phosphinite ligands such as DPPPEG-200, DPPPEG-400
and Bu(OPPh2)2 having similar steric and electronic properties to
that of DPPPEG-600 revealed good selectivity toward amine forma-
tion however decrease in conversion was observed (Table 2, entries
2–4). Whereas, the bulky bidentate phosphine ligand like Xantphos
provided the hydroaminomethylated product with comparatively
low conversion (87%) and selectivity (72%) even after a prolonged re-
action period of 6 h (Table 2, entry 5). On the other hand, extensively
used hydroformylation catalyst HRh(CO)(PPh3)3 furnished good
The influence of solvents on the hydroaminomethylation reaction
was then investigated. We demonstrated the hydroaminomethyla-
tion reaction using PEG-600 as environmentally benign solvent but
observed only 13% of conversion promising toluene as a best suitable
solvent for the present reaction system (Table 1, entry 2, 9–10). In ad-
dition, we studied the substrate to rhodium mole ratio and observed
Table 1
Hydroaminomethylation of cyclopentene using RhPEGD-600 catalyst.a
Entry
Solvent
Sub/Cat
(mole ratio)
CO/H2
(1:1)
psi
Temp.
(°C)
Time
(h)
Conversion
(%)b
Selectivity (%)b
Amine
Enamine
Hydrogenation
1
2
3
4
5
6
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
n-Hexane
PEG-600
Toluene
Toluene
800
800
800
800
800
800
800
800
800
800
400
1600
400
400
400
400
400
200
600
400
400
400
400
400
80
100
120
100
100
100
100
100
100
100
100
100
4
4
4
6
2
4
4
4
4
4
4
4
68
100
100
99
79
75
100
100
32
13
100
87
92
99
95
99
98
83
96
94
87
17
100
97
7
1
1
–
1
–
4
1
–
2
3
6
4
81
–
2
2
15
1
7
8c
9
–
9
02
–
10
11
12
1
a
Reaction conditions: cyclopentene (2 mmol), morpholine (2 mmol), toluene (10 mL), 800 rpm.
Conversion and selectivity were determined by GC analysis.
CO/H2 (1:2).
b
c

144
S.R. Khan et al. / Catalysis Communications 15 (2011) 141–145
Table 2
appreciable selectivity (Table 3, entries 4–7). Similarly, secondary al-
iphatic amines like dibutyl amine and diethyl amine also offered good
yield of desired product (Table 3, entries 8–9). The reaction of cyclo-
pentene with aromatic amine such as aniline provided considerable
yield of hydroaminomethylated product (Table 3, entry 10).
The scope of developed catalytic protocol was further extended for
hydroaminomethylation of various olefins as substrates with mor-
pholine as amine source (Table 4, entries 1–5). The reaction of cyclo-
hexene with morpholine resulted in good conversion and selectivity
for the expected product (Table 4, entry 1). The reaction of morpho-
line with aromatic olefins such as styrene and 4-tert-butyl styrene of-
fered good yield of hydroaminomethylated products (Table 4, entries
2–3), whereas Bicyclo[2.2.1]hept-2-ene also provided remarkable
yield of desired product (Table 4, entry 4). Moreover, the linear ali-
phatic alkene such as 1-hexene also furnished a good conversion
and selectivity toward amine formation (Table 4, entry 5).
In consideration of economical view of the developed methodology,
reusability of catalytic system was examined for hydroaminomethylation
reaction of cyclopentene with morpholine. The organometallic complex
prepared using PEG supported diphosphinite ligand is highly soluble in
the PEG. PEG-600 (1 mL) was added along with starting material and tol-
uene solvent during the study. The addition of PEG-600 makes the reac-
tion medium viscous which decreases the yield of desired product at
100 °C, however at 120 °C excellent yield of desired product was
obtained. Hence, 120 °C was used during the study. The RhPEGD-600
was found to be recycled for five consecutive runs with excellent catalytic
activity and selectivity toward the formation of desired product (Table 5).
In context, we performed the ICP-AES analysis of 1st and 5th recycle runs
and observed only 0.01 ppm of rhodium in solution which revealed no
significant leaching of rhodium.
Hydroaminomethylation of cyclopentene using various Rh catalysts: effect of ligands.a
Entry
Catalysts
Time
(h)
Conversion
(%)
Amine selectivity
(%)b
1
2
3
4
5
6
RhPEGD-600
RhPEGD-400
RhPEGD-200
Rh(acac)(CO)2/2Bu(OPPh2)2
Rh(acac)(CO)2/2Xantphos
HRh(CO)(PPh3)3
4
4
4
4
6
4
100
93
90
89
87
82
99
98
99
98
72
92
a
Reaction conditions: cyclopentene (2 mmol), morpholine (2 mmol), toluene
(10 mL), catalyst (2.5 μmol), temperature (100 °C), CO/H2 (1:1) (400 psi), 800 rpm.
b
Conversion and selectivity were determined by GC analysis.
conversion and selectivity toward the desired product but was quite dif-
ficult to recycle in a homogeneous system (Table 2, entry 6). Hence,
among the several screened rhodium based catalysts, RhPEGD-600
was found to be the best catalyst for hydroaminomethylation reaction
providing admirable conversion and selectivity for the desired product.
In order to investigate the general applicability of developed pro-
tocol, we studied the hydroaminomethylation of cyclopentene with
various primary or secondary amines (Table 3). Morpholine and pi-
peridine smoothly reacted with cyclopentene providing excellent
yield of desired product with remarkable selectivity toward the
amine formation (Table 3, entries 1–2). However, piperazine un-
dergoes the reductive amination resulting in mono-hydroamino-
methylated as well as di-hydroaminomethylated product in ratio of
62:38 respectively (Table 3, entry 3). Various primary amines such
as cyclohexyl amine, tertiary butyl amine, n-butyl amine and benzyl
amine provided good to excellent yield of expected products with
Table 3
Hydroaminomethylation of cyclopentene with different amines.a
Entry
1
Amines
Major product
Conv. (%)
100
Amine selectivity (%)
99
Yield (%)b
99
99
99
99
95
99
99
99
92
98
2
3c
4
62 (62:38)
98
94
5
6
7
97
99
95
97
92
98
8
9
94
96
98
98
92
96
10
89
99
87
a
Reaction conditions: cyclopentene (2 mmol), amine (2 mmol), toluene (10 mL), RhPEGD-600 (2.5 μmol), CO/H2 (1:1) (400 psi), time (4 h), temperature (100 °C), 800 rpm.
GC yield.
Formation of monohydroaminomethylated and dihydroaminomethylated product.
b
c

S.R. Khan et al. / Catalysis Communications 15 (2011) 141–145
145
Table 4
Hydroaminomethylation of different olefins with morpholine.a
Entry
1
Olefins
Major product
Conversion (%)
99
Amine selectivity (%)
95
Yield (%)b
94
n:ib
–
2
89
99
89
39:61
3
4
92
98
98
99
91
96
33:67
n.d.
96
98
92
44:42:14
5c
a
Reaction conditions: olefin (2 mmol), morpholine (2 mmol), toluene (10 mL), RhPEGD-600 (2.5 μmol), CO/H2 (1:1) (400 psi), reaction time (4 h), temperature (100 °C),
800 rpm, n.d. = not determined (Exo/Endo).
b
GC yield, selectivity (n/i) was determined by GC analysis.
Iso-product formed namely 4-(2-methyl-hexyl)-morpholine (42% yield) and 4-(2-ethyl-pentyl)-morpholine (14% yield).
c
Table 5
Appendix A. Supplementary data
Catalyst recyclability study.a
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.08.033.
Entry
RhPEGD-600 catalyst
Yield (%)b
1
2
3
4
5
6
Fresh
99
99
98
98
98
98
Recycle 1
Recycle 2
Recycle 3
Recycle 4
Recycle 5
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a
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In conclusion, we developed an efficient, homogeneous and reusable
catalytic system for the hydroaminomethylation reaction. The present
protocol is widely useful for one pot synthesis of secondary and tertiary
amines from olefins via hydroformylation and subsequent reductive
amination with CO and H2 in the presence of polymeric rhodium dipho-
sphinite as a catalyst. In addition, on completion of reaction the catalyst
could be easily separated from the product and efficiently reused for
five consecutive recycles without significant decrease in its catalytic ac-
tivity. Thus, the developed catalytic system sounds to be more general
with wider applicability for range of substrates appealing for its large
scale application.
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Acknowledgment
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The author (S.R. Khan) is greatly thankful to Council of Scientific
and Industrial Research (CSIR, India) for providing Junior Research
fellowship (JRF).