Selective hydroaminomethylation of olefins using simple and efficient Rh-phosphinite complex catalyst

Article abstract of DOI:10.1002/aoc.3046

Hydroaminomethylation of various olefins with primary and secondary amines was carried out using a simple and efficient rhodium-phosphinite complex catalyst. The influence of various reaction parameters including the effects of temperature, pressure, catalyst loading, time and solvents has been investigated. The present protocol is general with wider substrate applicability for the synthesis of an important class of aliphatic amines and arylethylamines. High activity and selectivity for amines was achieved with a very good substrate/catalyst molar ratio (turnover number 2500) under mild reaction conditions. Copyright

Full text of DOI:10.1002/aoc.3046

Full Paper
Received: 11 July 2013
Revised: 25 July 2013
Accepted: 26 July 2013
Published online in Wiley Online Library: 16 September 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3046
Selective hydroaminomethylation of olefins
using simple and efficient Rh–phosphinite
complex catalyst
Shoeb R. Khan and Bhalchandra M. Bhanage*
Hydroaminomethylation of various olefins with primary and secondary amines was carried out using a simple and efficient
rhodium–phosphinite complex catalyst. The influence of various reaction parameters including the effects of temperature,
pressure, catalyst loading, time and solvents has been investigated. The present protocol is general with wider substrate
applicability for the synthesis of an important class of aliphatic amines and arylethylamines. High activity and selectivity
for amines was achieved with a very good substrate/catalyst molar ratio (turnover number 2500) under mild reaction condi-
tions. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: hydroaminomethylation; olefins; amines; Rh–phosphinite complex; homogeneous catalysis
co-workers. It required 120 bar pressure and a temperature of
Introduction
130 °C.[8] Rische and Eilbracht reported the synthesis of biologi-
Amines and their derivatives are highly versatile building blocks
possessing wide applications in the pharmaceutical, agrochemi-
cal, dye and fine chemical industries.[1] Owing to the importance
of amines, several methods are developed for their synthesis.
Alongside various reported protocols,[2] hydroaminomethylation
(HAM) has attracted considerable interest as it minimizes the
waste, the number of reaction steps and the purification pro-
cesses, which fulfill the criteria of sustainability and green chem-
istry. In terms of the atom efficiency, activity, selectivity, and
applicability, HAM of alkenes is one of the most promising
reactions for the production of amines from olefins. The reaction
has an additional advantage of ready availability and inexpensive
starting materials.[3] This domino reaction begins with the
hydroformylation of an olefin to an aldehyde, which then reacts
with the amine to form enamine or imine, followed by hydroge-
nation reaction (Scheme 1).
Although the reaction was discovered in 1949 by W. Reppe,
intensive research with respect to HAM reaction has only been
carried out in the last two decades. In particular, the notable
works of P. Eilbracht and M. Beller for the synthesis of biologically
important amines are remarkable.[4] Until the mid 1990s,
relatively harsh conditions were employed to obtain a good yield
of corresponding amines from olefins with the aid of HAM reac-
tion.[5] Several metal catalysts comprising ruthenium, indium
and bimetallic complexes were developed for HAM reaction.
Either lower activity in the hydroformylation step, high catalyst
loading or harsh reaction conditions were used to limit their
applications.[6] Among all other metals, rhodium found more
success in this reaction; hence it was the most preferred metal
for studying the HAM reaction. Schulte et al. reported HAM of
long-chain alkenes; however, they obtained very poor regioselec-
tivity and required harsh reaction conditions (100–150 bar CO/H2,
150°C),[7] whereas HAM in an aqueous two phase-system is
limited to only the lower alkenes, as reported by Beller and
cally important arylethylamines with a very good yield and selec-
tivity to the corresponding branched amines at high pressure and
temperature (110 bar; 110°C) in the presence of [Rh(cod)Cl]2 cata-
lyst.[9] Arylethylamines synthesis was reported by Kostas using
cationic rhodium complex containing a specially designed P,N-
ligand system, but again it required high pressure (100 bar).[10]
On the other hand, Routaboul et al. reported the HAM of styrene
with aniline catalyzed by [Rh(cod)2BF4] with dppf ligand and
additive like tetrafluoroboric acid (HBF4) providing a good yield
and selectivity toward branched amine.[11] The reaction works
under mild conditions, but the use of additive and its unknown
effects increase the complications in the system. Although these
methods are available for HAM, control of selectivity (chemo-
and regio-), softening of reaction conditions and getting a better
turnover number (TON) are still considered challenging tasks.
Moreover, new-generation hydroformylation catalysts (rhodium
metal with an excess of ligand) often do not show enough activity
for enamine or imine hydrogenation.[12] Hence, in continuation of
our interest in the development of a homogeneous catalytic
system,[13] we herein report a facile and highly efficient protocol
for HAM reaction.
Results and Discussion
In order to optimize the reaction conditions, the reaction of
hexene with piperidine in the presence of Rh–phosphinite
*
Correspondence to: Bhalchandra M. Bhanage, Department of Chemistry, Insti-
tute 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, 711–715
Copyright © 2013 John Wiley & Sons, Ltd.

S. R. Khan and B. M. Bhanage
It is well known that the regioselectivity in HAM is determined
at the hydroformylation step; it might be possible to obtain a
high regioselectivity for the desired amine by varying the ligands
and the reaction conditions, especially temperature. To examine
the effect of temperature, the reaction was carried out at differ-
ent temperatures ranging from 120 to 70°C (Table 1, entries 4
and 6–8). No profound decrease in the yield of the desired
product was observed when the reaction temperature decreased
from 120 to 80°C but further decrease in the temperature
decreases the conversion of hexene. High temperature signifi-
cantly promotes the formation of thermodynamically stable
branched amine, which is not part of the present protocol;
therefore, 80°C was considered as an optimum reaction temper-
ature for further studies.
Next, we studied the substrate/Rh molar ratio and observed
that increasing the molar ratio from 1000:1 to 2500:1 had no
significant effect on conversion and selectivity of desired product
(Table 1, entries 9–11), while a further increase in substrate to a
rhodium molar ratio of 3333:1 results in a slight decrease in the
conversion (Table 1, entry 11). This decrease in conversion was
due to a decrease in the amount of catalyst from 0.002 mmol
(substrate/Rh molar ratio 2500:1) to 0.0015 mmol (substrate/Rh
molar ratio 3333:1). The present catalytic system shows a good
substrate/catalyst molar ratio for the HAM reaction, which is
useful for potential applications. The influence of solvents on
the HAM reaction was then investigated. Solvents such as
toluene and methanol provided lower conversion and selectivity
for the desired products and thus were not suitable solvents for
the present catalytic system (Table 1, entries 12 and13). THF
provided an excellent yield and selectivity of the desired product
and hence was used for further study (Table 1, entry 4). We
further studied the effect of reaction time ranging from 8 h
to 4 h and it was found that within an appreciable period of
6 h the reaction provides maximum yield and selectivity for
linear amine formation (Table 1, entries 4, 14 and 15). In
Scheme 1. HAM of alkenes.
complex catalyst was chosen as a model reaction (Scheme 2) and
the influence of various reaction parameters such as CO/H2
pressure, temperature, catalyst loading, solvent and time was
studied. The results obtained are summarized in Table 1.
Initially, the effect of CO/H2 pressure on reaction rate and selec-
tivity was studied in the range of 60–20 bar at different molar ratios
(Table 1, entries 1–5). Lowering the CO/H2 (1:2) pressure from 60 to
30 bar showed no prominent effect on reaction outcome, whereas
further decrease in pressure to 20 bar decreases the conversion and
selectivity of the desired product. The title reaction requires a 1:2
proportion of CO/H2 consumption and increase of H2 concentration
to 7/23 bar leads to a gradual increase in the hydrogenation activity
of the catalyst, providing high selectivity toward amine formation
(Table 1, entry 4).
Scheme 2. HAM of hexene with piperidine using Rh–phosphinite
complex catalyst.
Table 1. The effect of various reaction parameters on hydroaminomethylation of hexene and piperidinea
Entry
CO/H2 (bar)
Temp. (°C)
Sub./Cat
(molar ratio)
Solvent
Time (h)
Conv. (%)b
Selectivity (%)b
Amine Enamine
n/iso
1
20/40
15/30
10/20
7/23
7/13
7/23
7/23
7/23
7/23
7/23
7/23
7/23
7/23
7/23
7/23
7/23
80
80
80
80
80
100
120
70
80
80
80
80
80
80
80
80
1000
1000
1000
1000
1000
1000
1000
1000
2000
2500
3333
2500
2500
2500
2500
2500
THF
8
8
8
8
8
8
8
8
8
8
8
8
8
6
4
6
100
100
100
100
62
99
97
96
99
92
99
99
71
99
99
98
99
89
99
97
87
—
61:39
62:38
65:35
66:34
68:32
57:43
51:49
69:31
65:35
66:34
68:32
47:53
61:39
66:34
67:33
53:47
2
THF
3
4
1
5
3
THF
4
THF
5
THF
6
THF
100
100
45
—
—
7
THF
8
THF
29
9
THF
100
100
94
—
10
11
12
13
14
15
16[c]
THF
—
—
—
THF
2
7
3
Toluene
MeOH
THF
95
77
100
93
THF
THF
65
13
aReaction conditions: hexene (5 mmol), piperidine (5 mmol), solvent (15 ml), 600 rpm.
bConversion and selectivity were determined by GC analysis.
cCatalyst precursor [Rh(cod)Cl]2.
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Appl. Organometal. Chem. 2013, 27, 711–715

Hydroaminomethylation of olefins using Rh–phosphinite complex
addition, the activity of Rh–phosphinite complex with a ligand-
free system was compared. The reaction of 1-hexene with pi-
peridine was also performed with [Rh(cod)Cl]2 as the catalyst
precursor; however, low conversion and poor selectivity of
the desired product address the importance of the ligand in
HAM reaction (Table 1, entry 16). Hence the optimized reaction
conditions for the HAM of hexene are: hexene (5 mmol),
piperidine (5 mmol), Rh–phosphinite complex (0.002 mmol),
CO/H2 (7:23) bar at a temperature of 80°C for 6 h in THF (15 ml)
as solvent.
Table 2. Hydroaminomethylation of various olefins and aminesa
The catalytic system with high TON and wider substrate
applicability has significant importance in the chemical industry
and to synthetic organic chemists. Hence, with these optimized
reaction conditions, we investigated the scope of the developed
protocol for the synthesis of diverse secondary as well as tertiary
amines from various aliphatic, cyclic and aromatic olefins. Table 2
shows the results obtained from the HAM of a variety of olefins
and amines using optimized reaction conditions.
Entry
R
R1, R2
Conversion Yield of n/isoc
(%)
amine
(%)b
1
n-Bu
(CH2)5
100
100
99
99
98
97
95
93
84
99
99
99
99
99
99
66:34
63:37
65:35
61:39
64:36
57:43
69:31
61:39
59:41
—
2
n-Bu
(CH2)4O
(CH2)4
3
n-Bu
Beginning with the cyclic amines, piperidine, morpholine and
pyrrolidine smoothly reacted with hexene, providing a very good
yield of the desired product with remarkable selectivity toward
the amine formation (Table 2, entries 1–3). Similarly, secondary
aliphatic amines such diethylamine also gave N,N-diethylheptan-
1-amine in a good yield (Table 2, entry 4). In addition to secondary
amines, primary amines also reacted well with 1-hexene, providing
moderate selectivity for amine formation (Table 2, entry 5). The
reaction of hexene with aromatic amines such as aniline provided
an acceptable yield of N-heptylaniline under optimized reaction
conditions (Table 2, entry 6). We are pleased to mention that the
higher aliphatic olefins such as 1-dodecene, which often react
slowly, showed very good conversion and selectivity for amine for-
mation under the present reaction conditions (Table 2, entry 7).
Moreover, allylbenzene and p-methoxy allylbenzene also provided
a good yield of expected products with appreciable selectivity
for amine formation. It is important to note that in all the above
cases kinetically stable linear amine formation is predominant
and the reaction occurs with high chemoselectivity, giving the
amine in remarkable yield. It is known that morpholine often
reacts more sluggishly, probably due to its chelating coordi-
nation mode,[14] and hence not preferred for providing good
results in HAM reaction. In the present catalytic system
morpholine reacts very well with different olefins. The cyclic
olefins such as cyclopentene and cyclohexene with morpholine
furnished very good yield and selectivity for their corre-
sponding product (Table 2, entries 10 and 11). Bicyclo
[2.2.1]hept-2-ene also provided a remarkable yield of the
desired product (Table 2, entry 12).
4
n-Bu
C2H5, C2H5
n-Bu, H
Ph, H
99
5
n-Bu
100
73
6
n-Bu
7
n-Dec
Ph-CH2
(CH2)5
100
99
8
(CH2)5
9
p-OCH3-Ph-CH2 (CH2)5
99
10
11
12
Cyclopentene
Cyclohexene
Bicyclo[2.2.1]
hept-2-ene
Ph
(CH2)4O
(CH2)4O
(CH2)4O
99
95
—
97
—
13
14d Ph
15d p-tBu-Ph
16d m-CH3-Ph
17d p-Cl-Ph
(CH2)5
(CH2)5
(CH2)5
(CH2)5
(CH2)5
99
100
100
100
100
97
99
99
99
99
28:72
9:91
14:86
13:87
5:95
aReaction conditions: olefin (5 mmol), amine (5 mmol), THF (15 ml),
Rh–phosphinite complex (0.002 mmol), CO/H2 (7:23 bar),
reaction time (6 h), temperature (80°C), 600 rpm.
bYields were determined by GC analysis using 2-methoxyethyl
ether as an internal standard.
cSelectivity was determined by GC analysis.
dToluene as a reaction solvent.
excellent selectivity for branched amine formation (Table 2,
entry 17). In the case of aryl olefin HAM, the branched amine forms
preferentially due to the generation of thermodynamically stable
benzylrhodium intermediate.
The scope of the developed catalytic protocol was further extended
for aromatic olefins, which are known either to produce potentially im-
portant aldehydes via hydroformylation or biologically important
arylethylamines via HAM reactions. Styrene with piperidine
reacts efficiently in both THF and toluene as a solvent but
excellent regioselectivity for branched amine in toluene was
observed (Table 2, entry 14), indicating toluene as the best
suitable solvent for aromatic olefins. This change in selectivity
might be due to the interaction of THF (polar solvent) with
benzylrhodium intermediate inhibiting the arylethylamine
formation. Substituted styrenes like 4-tert-butylstyrene and 3-
methylstyrene were found to react smoothly with piperidine,
furnishing a good yield and selectivity of corresponding
branched product (Table 2, entries 15 and 16). Furthermore,
p-chlorostyrene also provided almost complete conversion and
Conclusion
The present study reports a simple and efficient protocol for
hydroaminomethylation reactions by using
a well-defined
Rh–phosphinite complex as a versatile catalyst. The reaction
system was optimized with respect to various reaction parame-
ters and applied to HAM of a range of olefins with different types
of amines, furnishing good to excellent yields of the correspond-
ing products. The developed protocol works under milder
reaction conditions with the additional advantage of high TON,
in comparison to most of the previously reported protocols.
Furthermore, the catalytic system was also useful for the
Appl. Organometal. Chem. 2013, 27, 711–715
Copyright © 2013 John Wiley & Sons, Ltd.
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S. R. Khan and B. M. Bhanage
synthesis of arylethylamines, with excellent regioselectivity for
branched amine formation. Thus we believe that the developed
catalytic system constitutes an economically attractive and
environmentally favorable method for the synthesis of valuable
chemicals from inexpensive feedstock.
with 4 ml petroleum ether and kept at À25°C to afford
Rh–phosphinite complex as a yellow crystalline compound. Yield
69% (0.067 g, 0.069 mmol). Characterization results of ligand and
metal complex are in accordance with literature data.[16] TGA of
the Rh–phosphinite catalyst showed that the catalyst was
thermally stable at the reaction temperature.
Experimental
General procedure for catalytic HAM
Materials and Instruments
In a typical experiment, to a 100 ml autoclave, olefin (5 mmol),
amine (5 mmol) and solvent (15 ml) were added with
Rh–phosphinite complex (0.002 mmol, 2 mg). The reactor was
then flushed with nitrogen followed by CO/H2 gas (7:23 mixture
of CO and H2 gas) at room temperature. Next, the reactor was
pressurized to 30 bar CO/H2 (7:23) and heated to 80°C for 6 h with
a stirring speed of 600 rpm. After completion of the reaction, the
reactor was cooled to room temperature, and the remaining
CO/H2 gas was carefully vented. The reaction mixture was then
analyzed by gas chromatography using 2-methoxyethyl ether as
internal standard. Characterization data of the reaction products
are in accordance with the literature data.
All chemicals and reagents were purchased from reputable firms
at the highest purity available and were used without further
purification. CO and H2 gases with a purity of 99.9% were
obtained from Rakhangi Gases Ltd, Mumbai, India.
The products are well known in the literature[15] and were com-
pared with authentic samples. Progress of the reaction was monitored
by gas chromatography (PerkinElmer Clarus 400 GC) equipped
with a capillary column (Elite-1, 30 m × 0.32 mm × 0.25 μm) and
a flame ionization detector. Mass of the reaction products was
confirmed by using a Shimadzu GC/MS-QP 2010 instrument
equipped with a capillary column (Rtx-17, 30 m × 25 mm i.d., film
thickness (df) 0.25 μm) having a column flow (2 ml minÀ1
,
80–240°C at 10°C minÀ1 rise). The product was purified by
1-(4-(4-Methoxyphenyl)butyl)piperidine
1
column chromatography on silica gel (100–200 mesh). The H
Yield 58% (GC).1H NMR (300 MHz, CDCl3): δ = 7.07 (d, J = 6.6 Hz,
2H, arom. CH of CH2-C(CH)2), 6.80 (d, J = 6.6 Hz, 2H, arom. CH of
CH3O-;C(CH)2), 3.76 (s, 3H, OCH3), 2.55 (t, J = 7.2 Hz, 2H, Ar-CH2),
2.28–2.36 (m, 6H, N-(CH2)2 and N-1CH2), 1.42–1.62 ppm (m, 10H,
remaining H); 13C NMR (75 MHz, CDCl3): δ = 157.71 (Carom),
134.66 (Carom), 129.30 (Carom), 113.73 (Carom), 59.43 (N-1C), 55.28
(OCH3), 54.65 (NC), 34.98 (ArCH2), 29.88 (ArCH2CH2), 26.48 (NCC),
25.91 (NCCC), 24.47 ppm (N-2C); IR (neat): ν = 2933, 2854, 2763,
1612, 1512, 1246, 1176, 1117, 1038, 820 cmÀ1; GC-MS (EI, 70
eV): 247 (8.2, M+), 161 (0.4), 134 (0.8), 121 (7.6, [M À (CH2)3pipy]+),
111 (1.1), 98 (100, [M À MeOC6H4(CH2)3]+), 91 (2.3), 85 (4.1), 70
(3.4), 55 (4.3), 41 (5.7); HRMS (EI) (M+). Calcd for C16H25NO:
247.1936. Found 247.1928.
and 13C NMR (δ in ppm) spectra were recorded on a Varian VXR
300 spectrometer at operating frequencies of 300 and 75 MHz,
respectively, in CDCl3 as a solvent. Chemical shifts are reported in
parts per million (δ) relative to tetramethylsilane (TMS) as an
internal standard. J (coupling constant) values were reported in
hertz. Proton splitting patterns are described as s (singlet), d
(doublet), t (triplet), and m (multiplet).
The Rh–phosphinite complex used was prepared according to
the reported procedure in the literature.[16] 31P and 1H NMR
analysis of the diphosphinite ligand and Rh complex were carried
out using a Varian 400 MHz instrument. TGA of metal complex
was performed using a PerkinElmer (STA 6000) instrument under
N2 atmosphere (see supporting information).
Supporting Information
Supporting information may be found in the online version of
this article.
Catalyst Preparation and Characterization
Synthesis of diphosphinite ligand [Ph2POC6H4OPPh2]
A solution of ClPPh2 (1.32 g, 6 mmol) in diethyl ether (5 ml)
was added slowly to a stirred solution of 1,4-dihydroxybenzene
(0.33 g, 3 mmol) and triethylamine (0.61 g, 6 mmol) in 25 ml
diethyl ether over a period of 15 min at 0°C. Following the
addition, the reaction mixture was left to warm at room temper-
ature and stirring was continued for 16 h. The triethylamine
hydrochloride salt was filtered off using frit containing celite
under nitrogen atmosphere and the filtrate was evaporated to
dryness. The residue, dissolved in dichloromethane and petroleum
ether, was added to afford diphosphinite ligand as a white crys-
talline compound and was stored under nitrogen atmosphere.
Yield 78% (1.11 g, 2.32 mmol).
Acknowledgments
The author S. R. Khan is highly indebted to the Council of
Scientific and Industrial Research (CSIR, India) for providing a
Senior Research fellowship (SRF).
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