Homogeneous and semi-heterogeneous magnetically retrievable bis-n-heterocyclic carbene rhodium(I) based catalysts for selective hydroaminomethylation reactions

Article abstract of DOI:10.1002/ejoc.201403526

The preparation of various bis-N-heterocyclic carbene (bis-NHC) complexes of rhodium and their application in the hydroaminomethylation of vinyl arenes was investigated. Various reaction parameters such as solvent, temperature, and pressure were tested, and the optimized protocol was applied to a wide variety of vinyl arenes and amines to afford the corresponding amines in good yields with high selectivity towards the branched product. The bis-NHC-based catalyst demonstrated superior reactivity and selectivity compared to catalysts containing bidentate phosphine or monodentate NHC ligands. A triethoxysilyl-functionalized bis-NHC ligand was immobilized on magnetic nanoparticles and then utilized to bind a rhodium catalyst. The resulting catalytic system was successfully employed in hydroaminomethylation reactions. The catalyst exhibited excellent reactivity, high selectivity, and was simply recovered from the reaction medium by applying an external magnetic field. Various bis-N-heterocyclic carbene (bis-NHC) ligands are prepared and complexed with rhodium(I). Some of these complexes show excellent reactivity and high selectivity in homogeneous hydroaminomethylation reactions. A bis-NHC ligand derivative is supported on magnetite nanoparticles and after complexation with Rh^I is was applied successfully in semi-heterogeneous hydroaminomethylations.

Full text of DOI:10.1002/ejoc.201403526

FULL PAPER
DOI: 10.1002/ejoc.201403526
Homogeneous and Semi-Heterogeneous Magnetically Retrievable Bis-N-
Heterocyclic Carbene Rhodium(I) Based Catalysts for Selective
Hydroaminomethylation Reactions
Bishnu Dutta,[a] Rony Schwarz,[a] Suheir Omar,[a] Suzana Natour,[a] and Raed Abu-Reziq*[a]
Keywords: Rhodium / Carbene ligands / Nanoparticles / Semi-heterogeneous catalysis / Hydroaminomethylation
The preparation of various bis-N-heterocyclic carbene (bis-
NHC) complexes of rhodium and their application in the
hydroaminomethylation of vinyl arenes was investigated.
Various reaction parameters such as solvent, temperature,
and pressure were tested, and the optimized protocol was
applied to a wide variety of vinyl arenes and amines to afford
the corresponding amines in good yields with high selectivity
towards the branched product. The bis-NHC-based catalyst
demonstrated superior reactivity and selectivity compared to
catalysts containing bidentate phosphine or monodentate
NHC ligands. A triethoxysilyl-functionalized bis-NHC ligand
was immobilized on magnetic nanoparticles and then uti-
lized to bind a rhodium catalyst. The resulting catalytic sys-
tem was successfully employed in hydroaminomethylation
reactions. The catalyst exhibited excellent reactivity, high
selectivity, and was simply recovered from the reaction me-
dium by applying an external magnetic field.
subsequent hydroformylation and hydrogenation reac-
tions.[7] Nonetheless, the commonly used rhodium–
phosphine complexes generally exhibit insufficient hyd-
rogenation activity, and the formation of enamine/imine by-
products is easily promoted.[8]
Introduction
As a class of industrially important compounds, amines
have gained a great deal of interest in multiple applications
including agricultural, pharmaceutical, and chemical manu-
facturing.[1] Even though various methods to synthesize
amines are reliable, including aminations, hydroaminations,
and nucleophilic substitutions of alkyl halides, they are inef-
ficient from a standpoint of atom economy and selectivity.[2]
The hydroaminomethylation of olefins, first reported by
Reppe and Vetter,[3] has experienced remarkable break-
throughs in terms of cost-efficient and environmentally
friendly alternative processes for the generation of amines
from ubiquitous inexpensive feedstock.[4] The one-pot
hydroaminomethylation reaction consists of initial hydro-
formylation of an olefin to an aldehyde followed by conden-
sation with an amine and a subsequent hydrogenation
step.[5] Recently, considerable progress in hydroamino-
methylation reactions has been achieved by several research
groups, and this has led to remarkable advancements and
developments.[6] Despite this, limited catalytic activity and
regioselectivity in this domino reaction remains a funda-
mental drawback to be overcome.
N-Heterocyclic carbenes (NHCs) have attracted much at-
tention in recent years because of their distinctive properties
and because they can serve as alternatives to phosphine-
based ligands in numerous catalytic systems.[9] The coordi-
nation of NHC ligands with metals usually leads to the for-
mation of complexes with strong NHC–metal bonds owing
to the strong σ-donating properties of NHCs.[10] Facile tun-
ing of the electronic and steric properties of NHC-based
organometallic complexes, in addition to their unique sta-
bility, has prompted their application in various homogen-
eous catalytic processes, including cross-couplings, metathe-
sis, oxidations, and carbonylations.[11] Recently, Beller and
co-workers reported the utilization of monodentate NHCs
in hydroaminomethylation reactions for the first time.[12]
Several methods for the heterogenization of homogen-
eous catalysts have been developed in the last three decades
to enable their isolation from the reaction media and their
recycling.[13] Among these methods, the binding of catalysts
to organic polymer solids[14] or inorganic solids[15] has
found broad application in catalysis. However, the reactivity
and selectivity of the heterogenized catalysts are signifi-
cantly lower than the reactivity and selectivity of their
homogeneous analogues. Magnetic nanoparticles have been
used widely in the last years as perfect nanosupports for the
immobilization of homogeneous catalysts, and they can be
isolated easily by application of an external magnetic
field.[16] These nanosupports have shown great potential for
Rhodium-based catalysts have been studied intensively in
this type of reaction, as they are efficient in catalyzing the
[a] Institute of Chemistry, Casali Center for Applied Chemistry
and the Center for Nanoscience and Nanotechnology,
The Hebrew University of Jerusalem,
Jerusalem 91904, Israel
E-mail: Raed.Abu-Reziq@mail.huji.ac.il
http://chemistry.huji.ac.il/casali/Abu-Reziq.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403526.
Eur. J. Org. Chem. 2015, 1961–1969
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1961

B. Dutta, R. Schwarz, S. Omar, S. Natour, R. Abu-Reziq
FULL PAPER
bridging homogeneous and heterogeneous catalysis. Lately, salts were synthesized simply through a quaternization reac-
a few attempts have been directed towards the recycling of tion, in which the appropriate alkylimidazole was mixed
hydroaminomethylation catalysts. The first attempts were with the desired 1,2-bis(chloromethyl)benzene, 1,3-bis-
based on utilizing biphasic systems in which the rhodium (chloromethyl)benzene, or 2,4-bis(chloromethyl)mesitylene
catalysts were dissolved in water, ionic liquids, or polyethyl- and heated in dry toluene at 110 °C for 16 h to give desired
ene glycol phases and the reactants/products existed in a dications 1–6 in good yields in pure form without the need
different phase.[17] Other efforts were related to the hetero- for further purification (Scheme 1). In these bis-imid-
genization of the hydroaminomethylation catalysts, includ- azolium salts, the number of methyl groups substituted on
ing immobilization of the rhodium catalyst on a supported the benzene ring as well as the nature of the group attached
ionic liquid phase,[18] encapsulation of the rhodium com- to the imidazolium ring can affect the steric properties and
plex in amino-functionalized hexagonal mesoporous sil- structural rigidity of the resulting bis-NHC ligands and cer-
ica,[19] cation exchange of rhodium chloride with sodium tainly lead to different catalytic behavior if involved. More-
cations on titanosilicate supports,[20] and covalent entrap- over, in comparison to monodentate NHC ligands, the syn-
ment of the rhodium complex in a silica sol–gel matrix.[21]
thesized bis-NHC ligands can behave like a clamp upon
Herein, we report the preparation of chelating bis-NHC binding to a metal, which would hence affect its reactivity
ligands and their application in hydroaminomethylation re- and selectivity in the catalytic reaction.
actions after coordination with rhodium. Metal complexes
with bis-NHC ligands became popular in catalysis because
they are more stabile than complexes coordinated with
monodentate NHC ligands.[22] The parameters influencing
the hydroaminomethylation reaction by using rhodium co-
ordinated with bis-NHCs were optimized, and a derivative
of the most reactive and selective catalyst was immobilized
on magnetic nanoparticles. The immobilized catalyst could
be easily separated from the reaction medium by applica-
tion of an external magnetic field and recycled up to four
times without losing its reactivity or selectivity.
Homogeneous Catalysis
To evaluate the catalytic behavior, the synthesized bis-
imidazolium salts were mixed with KOtBu in dry methanol
under an inert atmosphere to yield bis-NHCs, which were
then treated with [Rh(cod)Cl]2 (cod = 1,5-cyclooctadiene)
at room temperature. The rhodium-based complexes were
tested in the homogeneous hydroaminomethylation of styr-
ene and morpholine as a model reaction (Scheme 2). The
hydroaminomethylation of olefins basically consists of three
major reaction steps: one, initial alkene hydroformylation
to form an aldehyde; two, condensation of the aldehyde
with an amine to yield an enamine or imine intermediate;
three, hydrogenation of the enamine/imine intermediate to
give the corresponding amine product.
Results and Discussion
Synthesis of Various Bis-imidazolium Salts
In this research, a set of bis-imidazolium salts as starting
materials were used to prepare the bis-NHC ligands. These
Scheme 2. Hydroaminomethylation of styrene with morpholine.
At first, the reaction was performed in dry methanol at
80 °C with constant pressures of H2 (27.6 bar) and CO
(6.9 bar, 1 bar = 0.1 MPa) by utilizing rhodium-based cata-
lysts prepared by complexation of the bis-NHC ligands (ob-
tained after deprotonation of bis-imidazloium cations 1–6
by using potassium tert-butoxide) with [Rh(cod)Cl]2. The
results of these reactions are presented in Table 1. Careful
examination of the obtained yields and selectivities and the
formation of other byproducts such as aldehydes and en-
amines revealed that ligand 1 gave the most promising re-
sults with a good conversion (92%) and high selectivity of
11:1 towards the branched amine (Table 1, entry 1). Rho-
Scheme 1. Preparation of bis-imidazolium salts; Mes = 2,4,6-tri-
methylphenyl.
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Bis-NHC–RhI-Based Catalysts for Selective Hydroaminomethylation
dium-based complexes with bis-NHC ligands 2, 3, 5, and 6 At elevated temperatures, high yields of the amines were
gave high yields of enamines (Table 1, entries 2, 3, 5, and obtained, while low regioselectivity was observed (Table 2,
6), which were obtained as intermediates. These results indi- entries 5 and 6).
cated that the involved catalysts had low hydrogenation re-
activity. The catalyst resulting from complexation of rhodi-
um(I) with ligand 4 exhibited good reactivity and selectivity
toward the amines, but the ratio between the branched (B)
and linear (L) amines was low. Because there is no signifi-
cant difference between the electron-donating abilities of
NHC ligands substituted with methyl or mesityl (Mes)
groups, variations in the reactivity and selectivity of the cat-
alysts coordinated with ligands 1–6 can be mainly attrib-
uted to steric effects. To compare the reactivity and selectiv-
ity of these bis-NHC complexes with a catalyst based on a
monodentate NHC ligand, [Rh(cod)Cl]2 was complexed
Table 2. Homogeneous hydroaminomethylation of styrene with
morpholine at different temperatures.[a]
Entry
Temp.
[°C]
Conv.[b] Enamines[c] Amines[c] L/B ratio[c]
[%]
[%]
[%]
1
2
3
4
5
6
30
45
60
80
100
150
0
0
4
0
0
6
2
0
–
14
23
92
93
95
10
23
92
87
93
100:0
1:8
1:11
1:6
1:3
[a] Conditions: [Rh(cod)Cl]2 (0.01 mmol), ligand 1 (0.02 mmol),
styrene (5 mmol), morpholine (6 mmol), MeOH (5 mL), H2/CO
(27.6/6.9 bar), 16 h. [b] Determined by GC and 1H NMR spec-
troscopy. [c] Determined by GC.
with
1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene
(IMes) obtained by deprotonation of 1,3-bis(2,4,6-trimeth-
ylphenyl)imidazolium chloride (IMesHCl) with potassium
tert-butoxide. The resulting complex provided the desired
Consequently, the effect of the reaction pressure was
amines with good selectivity and the B/L ratio was low studied in two manners. First, the total pressure of
(Table 1, entry 7). Moreover, the use of the bidentate phos- hydrogen and carbon monoxide was kept at 34.5 bar,
phine ligand rac-2,2Ј-bis(diphenylphosphanyl)-1,1Ј-binaph- whereas the partial pressure of each gas was changed. Sec-
thyl (rac-BINAP) or monodentate triphenylphosphine ond, the total pressure in the reaction vessel was changed,
could certainly contribute to the good to excellent conver- whereas an equal ratio of the partial pressures of the two
sions (Table 1, entries 8 and 9) accompanied by creation of gases was kept constant. From the results summarized in
a considerable amount of the enamine intermediate.
Table 3 it can be concluded that the pressures of carbon
monoxide and hydrogen have a crucial effect on the catalyst
performance. Increasing the CO partial pressure led to a
significant decrease in the reactivity of the catalyst, as re-
flected from the lower conversions (Table 3, entries 2 and
3). These results can be explained by catalyst deactivation
as a result of the high concentration of CO, which reduces
the affinity of the catalyst to coordinate with the olefinic
substrates. At the same time, the low partial pressure of
H2 could also decrease the activity of the catalyst in the
hydrogenation of the enamine intermediates to yield the de-
sired amines. If the partial pressure of H2 was much higher
than the partial pressure of CO, the catalyst exhibited high
activity and regioselectivity (Table 3, entry 1). In addition,
increasing the total pressure of the reaction improved both
the yield and regioselectivity. These results correlate with
Le Chatelier’s principle, in which formation of the product
is encouraged by an increase in the concentration of the
reactants. It was also noticed that raising the total pressure
from 13.8 (Table 3, entry 4) to 34.5 bar (Table 3, entry 5)
had a moderate effect on the reactivity and regioselectivity
Table 1. Homogeneous hydroaminomethylation of styrene with
morpholine by using various NHC and phosphine ligands.[a]
Entry
Ligand
Conv.[b] Enamines[c] Amines[c]
L/B
[%]
[%]
[%]
ratio[c]
1
2
3
4
5
6
1
2
3
4
5
6
92
86
87
89
60
85
83
91
100
0
92
76
64
89
35
60
83
49
60
1:11
1:8
1:9
1:7
1:8
1:6
1:3
1:2.7
1:5
10
23
0
25
24
0
7[d]
8[e]
9[f]
IMes
rac-BINAP
Ph3P
36
40
[a] Conditions: [Rh(cod)Cl]2 (0.01 mmol), ligand (0.02 mmol), styr-
ene (5 mmol), morpholine (6 mmol), MeOH (5 mL), 80 °C, H2/CO
(27.6/6.9 bar), 16 h. [b] Determined by GC and 1H NMR spec-
troscopy. [c] Determined by GC. [d] The reaction was performed
with IMes (0.04 mmol). [e] The reaction was performed with rac-
BINAP (0.02 mmol); 6% of aldehydes were also formed. [f] The
reaction was performed with Ph3P (0.04 mmol).
Optimizing different parameters such as temperature, of the catalyst. On the other hand, increasing the total pres-
pressure, and reaction solvent was performed by utilizing sure to 69 bar dramatically enhanced the conversion of the
ligand 1 under various conditions. In this manner, the reaction to 95% and increased the L/B selectivity to 1:20
hydroaminomethylation of styrene with morpholine was (Table 3, entry 6).
performed at various temperatures ranging from 30 to
Another fundamental parameter to be optimized in the
150 °C. The results are summarized in Table 2. Clearly, hydroaminomethylation reaction was the type of solvent.
80 °C was found to be the optimal temperature at which the Thus, the hydroaminomethylation of styrene with morphine
reactivity and selectivity of the catalyst were high (Table 2, was performed in four different solvents: MeOH, THF, tol-
entry 4). At a temperature of 30 °C, the reaction did not uene, and CH2Cl2. Under the reaction conditions of 80 °C
occur (Table 2, entry 1), whereas at mild temperatures lower and a H2/CO pressure of 27.6/6.9 bar, CH2Cl2 was found
than 60 °C, the catalyst produced the product in poor yield to be the best solvent of those tested, and a conversion of
with high to moderate selectivity (Table 2, entries 2 and 3). 97% and a L/B selectivity of 12:1 were achieved (Table 4,
Eur. J. Org. Chem. 2015, 1961–1969
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B. Dutta, R. Schwarz, S. Omar, S. Natour, R. Abu-Reziq
FULL PAPER
Table 3. Homogeneous hydroaminomethylation of styrene with the reaction, whereas the type of the amine involved in the
morpholine at different pressures.[a]
hydroaminomethylation significantly affected the selectivity
of the reaction.
Entry P(H2)/P(CO) Conv.[b] Enamines[c] Amines[c] L/B ratio[c]
[bar]
[%]
[%]
[%]
Table 5. Homogeneous hydroaminomethylation of various alkenes
and amines.[a]
1
2
3
4
5
6
27.6/6.9
20.7/13.8
6.9/27.6
6.9/6.9
17.2/17.2
34.5/34.5
92
16
11
25
64
95
0
11
1
10
4
92
5
10
15
60
95
1:11
1:17
1:18
1:12
1:17
1:20
0
[a] Conditions: [Rh(cod)Cl]2 (0.01 mmol), ligand 1 (0.02 mmol),
styrene (5 mmol), morpholine (6 mmol), MeOH (5 mL), 80 °C,
1
16 h. [b] Determined by GC and H NMR spectroscopy. [c] Deter-
mined by GC.
entry 4). Noteworthy, the conversion of styrene proceeded
smoothly in the other solvents (Table 4, entries 1–3), but the
degree of selectivity towards the corresponding amine was
lower than that obtained in CH2Cl2 as was the L/B ratio.
Consequently, on the basis of previous findings, the reac-
tion was performed in CH2Cl2 under a total pressure of
69 bar (1:1 H2/CO) at 80 °C for further evaluation. In that
case, the performance of the catalyst was superior: a conver-
sion of 98% and a L/B ratio of 1:24 were attained (Table 4,
entry 5).
Table 4. Homogeneous hydroaminomethylation of styrene with
morpholine in different solvents.[a]
Entry Solvent
Conv.[b] Enamines[c] Amines[c] L/B ratio[c]
[%]
[%]
[%]
1
2
3
MeOH
THF
toluene
CH2Cl2
CH2Cl2
92
96
91
97
98
0
10
7
0
0
92
86
84
97
98
1:11
1:9
1:10
1:12
1:24
4
5[d]
[a] Conditions: [Rh(cod)Cl]2 (0.01 mmol), ligand 1 (0.02 mmol),
styrene (5 mmol), morpholine (6 mmol), solvent (5 mL), H2/CO
(27.6/6.9 bar), 80 °C, 16 h. [b] Determined by GC and 1H NMR
spectroscopy. [c] Determined by GC. [d] Reaction was performed
at a H2/CO pressure of 34.5/34.5 bar.
[a] Conditions: [Rh(cod)Cl]2 (0.01 mmol), ligand 1 (0.02 mmol),
alkene (5 mmol), amine (6 mmol), CH2Cl2 (5 mL), H2/CO (34.5/
34.5 bar), 80 °C, 16 h; yields were determined by GC and 1H NMR
spectroscopy.
The hydroaminomethylation reaction was further tested
under the established optimal conditions by employing a
set of various alkenes and a wide variety of amines. The
reaction of styrene with different amines such as morph-
oline, piperidine, N-phenylpiperidine, and N-methylpiper-
idine proceeded smoothly in high yields with high selectivity
Immobilization of Bis-imidazolium Salts on Magnetite
Nanoparticles
The immobilization of bis-imidazolium salts on magne-
ratios in favor of the branched amines (Table 5, entries 1– tite nanoparticles can be performed after its functionaliza-
4). In addition, high yields and selectivities were obtained tion with trialkoxysilane groups, which react with the
upon treating styrene derivatives such as 4-methylstyrene, 4- hydroxy groups on the surface of Fe3O4 nanoparticles.
chlorostyrene, and 4-methoxystyrene with different amines Thus, a bis-imidazolium salt functionalized with triethoxy-
(Table 5, entries 5–12). Moreover, the catalyst efficacy was silane groups was synthesized by treating 1,2-bis(chloro-
tested with aliphatic substrates such as 1-decene and 5-de- methyl)benzene with 1-[3-(triethoxysilyl)propyl]-4,5-di-
cene (Table 5, entries 13 and 14). High yields were obtained hydro-1H-imidazole at 120 °C in dry toluene under an inert
with both substrates. 1-Decene exhibited moderate selectiv- atmosphere (Scheme 3). Supporting resulting salt 7 on mag-
ity towards the linear amine, whereas 5-decene gave the netic nanoparticles was accomplished by mixing it with a
branched amine as the exclusive product. From the results freshly prepared solution of nanomagnetite in ethanol
shown in Table 5 it can be noted that the electronic proper- (95%) under basic conditions for 36 h at room temperature.
ties of the substituents negligibly influenced the progress of This process afforded a bis-imidazolium salt that was cova-
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Bis-NHC–RhI-Based Catalysts for Selective Hydroaminomethylation
lently linked to the surface of the magnetic nanoparticles MNPs exhibited a slight weight loss of 3.56% (Figure 2,
(see compound 8, Scheme 4).
curve a), which is mostly attributed to solvent residues or
adsorbed moisture on the surface of the magnetite nano-
particles. On the other hand, the immobilized bis-imid-
azolium salt showed a total mass loss of 51% (Figure 2,
curve b). The TGA curve shows three stages of weight loss.
The first, a small weight loss of 1.1%, is centered at 97 °C
and probably results from desorption of solvent residues.
The weight losses centered at 354 and 700 °C are likely
ascribed to decomposition of the organic elements of the
bis-imidazolium salt. Consequently, the loading of the bis-
imidazolium ligand is 1.2 mmolg–1. After synthesis of the
carbene ligand by deprotonation by using potassium-tert-
butoxide and complexation with [Rh(cod)Cl]2, inductively
coupled plasma mass spectrometry (ICP-MS) measure-
ments were undertaken to determine the loading of rho-
dium. This analysis indicated a rhodium loading of
0.29 mmolg–1.
Scheme 3. Synthesis of bis-imidazolium salt functionalized with tri-
ethoxysilane groups.
Scheme 4. Grafting bis-imidazolium salt functionalized with tri-
ethoxysilane groups on magnetite.
Supported ligand 8 was then treated with an excess
amount of tBuOK for 30 min under an atmosphere of
nitrogen to generate the grafted bis-carbene ligand MNP-
NHC; this was followed by the addition of [Rh(cod)Cl]2
to yield a rhodium-based catalyst immobilized on magnetic
nanoparticles (MNPs). The resulting MNP-NHC did not
exhibit a significant change in the particle size relative to
that of bare MNPs, as it was determined by TEM analysis
that exhibited a particle size in the range of 5 to 15 nm
(Figure 1). Nonetheless, the modified magnetic nanopar-
ticles differ in character from the bare magnetic nanopar-
ticles. Whereas the bare MNPs can easily aggregate, the
modified MNPs can be handled as a solid and are easily
redispersed in solvents such as methanol and ethanol.
Figure 2. TGA analysis of (a) bare magnetic nanoparticles and
(b) magnetic nanoparticles grafted with bis-imidazolium salt (8).
Semi-heterogeneous Catalysis
The catalytic performance of the rhodium catalyst was
tested in the hydroaminomethylation of various styrenes
and amines. Similar to the homogeneous route, the reaction
was performed under equivalent conditions of time, pres-
sure, temperature, and molar ratios. According to the re-
sults summarized in Table 6, the catalyst exhibited high
catalytic activity, and the reaction progress was barely affec-
ted by the electronic nature of the substrates involved. In
comparison to the homogeneous hydroaminomethylation
reactions performed with the use of the rhodium catalyst
with ligand 1, semi-heterogeneous rhodium-based catalyst
8 revealed a slight decrease in its selectivity.
Figure 1. TEM images of (a) bare magnetic nanoparticles and
(b) magnetic nanoparticles grafted with bis-imidazolium salt (8).
The distinctive importance of developing the semi-
heterogeneous rhodium catalyst is the ability to separate it
Thermogravimetric analysis (TGA) of the pure MNPs easily from the reaction media by application of an external
and supported bis-imidazolium salt 8 is shown in Figure 2. magnetic field and to recycle it. Recyclability of the catalyst
Thermal decomposition was performed in the temperature was evaluated over four consecutive catalytic cycles, for
range of 25 to 950 °C under an inert atmosphere. The bare which the catalyst was washed several times with the reac-
Eur. J. Org. Chem. 2015, 1961–1969
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B. Dutta, R. Schwarz, S. Omar, S. Natour, R. Abu-Reziq
FULL PAPER
Table 6. Semi-heterogeneous hydroaminomethylation of various homogeneous hydroaminomethylation reactions. Some of
alkenes and amines.[a]
these complexes showed excellent reactivity and high selec-
tivity. In comparison with rhodium complexes coordinated
with the monodentate 1,3-bis(2,4,6-trimethylphenyl)imid-
azole-2-ylidene NHC, triphenylphosphine, or the bidentate
rac-2,2Ј-bis(diphenylphosphanyl)-1,1Ј-binaphthyl
ligand,
the catalyst with bis-NHC ligands was much more selective.
In addition, a simple and efficient catalytic system for the
semi-heterogeneous hydroaminomethylation reaction was
developed by immobilization of a bis-NHC ligand function-
alized with silane groups on magnetic nanoparticles and co-
ordination with rhodium. In this case, product separation
and catalyst recovery were accomplished simply by applying
an external magnetic field. The separated catalyst was
recycled four times without an appreciable loss in catalytic
activity. We believe this catalytic system can be used in
many other essential organic transformations requiring im-
proved selectivities.
Experimental Section
Materials and Methods: α,αЈ-Dichloro-o-xylene, α,αЈ-dichloro-m-
xylene, N-methylimidazole, triethoxy-3-(2-imidazolin-1-yl)propyl-
silane, 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride (IMes-
HCl), and rac-2,2Ј-bis(diphenylphosphanyl)-1,1Ј-binaphthyl (rac-
BINAP) were purchased from Sigma–Aldrich. FeCl3·6H2O,
FeCl2·4H2O, and ammonium hydroxide (25%) were purchased
from Acros Organics. All substrates for the hydroaminomethylation
reactions were purchased from either Sigma–Aldrich or Acros and
were used without further purifications. Transmission electron
microscopy (TEM) was performed with a (S) TEM Tecnai F20
G2 (FEI Company, USA) operated at 200 kV. Thermogravimetric
analysis (TGA) was performed with a Mettler Toledo TG 50 ana-
lyzer. Measurements were performed in the temperature range that
extended from 25 to 950 °C at a heating rate of 10 °Cmin–1 under
an atmosphere of nitrogen. Gas chromatography (GC, Agilent
Technologies, 7890A) with a capillary column (HP-5, 30 m) was
used to determine the conversions and the yields of the hydro-
aminomethylation reactions. 1H NMR and 13C NMR spectra were
recorded with a Bruker DRX-400 instrument in CDCl3 or [D6]-
DMSO. Inductively coupled plasma (ICP) mass spectrometry mea-
surements were performed with a 7500cx (Agilent company) by
using external standard calibration.
[a] Conditions: Suspension of magnetite in CH2Cl2 (5 mL) contain-
ing the rhodium catalyst (0.01 mmol), alkene (5 mmol), amine
(6 mmol), and H2/CO (34.5/34.5 bar) at 80 °C for 16 h; yields were
1
determined by GC and H NMR spectroscopy.
tion solvent directly after completion of the reaction and
used in the next catalytic cycle without further purification.
The reusability of the magnetically retrievable catalyst was
tested in the hydroaminomethylation of styrene with
morpholine. As depicted in Table 7, the efficiency and the
selectivity of the catalyst were preserved over four catalytic
cycles. After each catalytic cycle and after removal of the
catalyst, the solutions were analyzed by ICP-MS to detect
if any leaching of the rhodium catalyst took place during
the reactions. There was no detectable rhodium in these
solutions.
Table 7. Recycling of rhodium catalyst supported on magnetic
nanoparticles in the hydroaminomethylation of styrene and morph-
oline.[a]
3,3Ј[1,2-Phenylenebis(methylene)]bis(1-methyl-1H-imidazolium)
Chloride (1):[23] A mixture of N-methylimidazole (16 mL, 0.2 mol)
and α,αЈ-dichloro-o-xylene (17.6 g, 0.1 mol) was heated at reflux in
toluene (40 mL) for 16 h. The mixture was cooled to room tem-
perature, and a white solid precipitated. The product was filtered
and washed with dry diethyl ether and dried under vacuum, yield
Run
Yield [%][b]
Selectivity (L/B ratio)[b]
1
2
3
4
98
98
97
97
1:16
1:16
1:15
1:14
1
27.2 g (80%). H NMR (400 MHz, [D6]DMSO): δ = 4.05 (s, 6 H),
5.92 (s, 4 H), 7.49–7.53 (m, 2 H), 7.61–7.65 (m, 2 H), 7.96 (s, 2 H),
8.02 (s,2 H), 9.67 (s, 2 H) ppm. 13C NMR (100 MHz, [D6]DMSO):
δ = 36.39, 49.45, 122.98, 124.33, 130.01, 130.20, 133.56,
137.61 ppm.
[a] Conditions: Suspension of magnetite in CH2Cl2 (5 mL) contain-
ing the rhodium catalyst (0.01 mmol), styrene (5 mmol), morph-
oline (6 mmol), and H2/CO (34.5/34.5 bar) at 80 °C for 16 h.
1
[b] Determined by GC and H NMR spectroscopy.
1-(2,4,6-Trimethlphenyl)-1H-imidazole (1-mesityl-H-imidazole):[24]
A mixture of formaldehyde (3.0 mL, 40 mmol) and glyoxal
(4.6 mL, 40 mmol) in glacial acetic acid (20 mL) was heated at
70 °C. A solution of 2,4,6-trimethylaniline (5.6 mL, 40 mmol) and
ammonium acetate (3 g) in water (5 mL) was added. The mixture
Conclusions
In conclusion, different bis-N-heterocyclic carbene
(NHC) rhodium complexes were prepared and utilized in was heated for 18 h at 70 °C and then cooled and poured into a
1966 Eur. J. Org. Chem. 2015, 1961–1969
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Bis-NHC–RhI-Based Catalysts for Selective Hydroaminomethylation
solution of NaHCO3 (60 g) in water (600 mL). The crude product
under vacuum, yield 7.5 g (72 %). 1H NMR (400 MHz, [D6]
was collected by filtration and crystallized from ethyl acetate, yield
2.97 g (40%). H NMR (400 MHz, CDCl3): δ = 2.04 (s, 6 H), 2.34 H), 7.12 (s, 4 H), 7.18 (s, 1 H), 7.90 (s, 2 H), 7.99 (s, 2 H), 9.45 (s,
(s, 3 H), 6.77 (s, 1 H), 6.97 (s, 2 H), 7.23 (s, 1 H), 7.43 (s, 1 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 17.29, 20.99, 120.01,
128.96, 129.53, 133.37, 135.37, 137.44, 138.79 ppm.
DMSO): δ = 2.04 (s, 12 H), 2.32 (s, 9 H), 2.38 (s, 6 H), 5.66 (s, 4
1
2 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 16.73, 17.77,
20.30, 21.05, 48.84, 123.06, 124.09, 125.44, 129.97, 130.69, 132.15,
135.37,138.00, 139.12, 140.43, 141.24, 141.40 ppm.
3,3Ј-[1,2-Phenylenebis(methylene)]bis(1-mesityl-1H-imidazolim)
3,3Ј-[1,2-Phenylenebis(methylene)]bis{1-[3-(triethoxysilyl)propyl]-
Chloride (2): A mixture of 1-mesityl-H-imidazole (3.72 g, 20 mmol) 4,5-dihydro-1H-imidazolium} Chloride (7): A mixture of triethoxy-
and α,αЈ-dichloro-o-xylene (1.75 g, 10 mmol) was heated at 110 °C
in toluene (40 mL) for 10 h. The mixture was cooled to room tem-
perature, and a white solid precipitated. The product was filtered
and washed with dry diethyl ether and dried under vacuum, yield
3-(2-imidazolin-1-yl)propylsilane (10 g, 36.6 mmol) and α,αЈ-
dichloro-o-xylene (3.2 g, 18.3 mmol) in dry toluene (45 mL) was
heated at reflux for 18 h. Progress of the reaction was checked by
1H NMR spectroscopy. The mixture was cooled to room tempera-
4.36 g (80%). 1H NMR (400 MHz, [D6]DMSO): δ = 2.08 (s, 12 H), ture. Toluene was removed under reduced pressure. The product
2.36 (s, 6 H), 5.92 (s, 4 H), 7.15 (s, 4 H), 7.22–7.43 (m, 2 H), 7.51–
7.54 (m, 2 H), 8.03 (s, 2 H), 8.19 (s, 2 H), 9.98 (s, 2 H) ppm.
was washed with diethyl ether and dried under vacuum, yield 9.24 g
1
(75%), H NMR (400 MHz, CDCl3): δ = 0.59–0.60 (m, 2 H), 1.21
13C NMR (100 MHz, [D6]DMSO): δ = 17.63, 21.04, 50.07, 123.28, (t, J = 2.8 Hz, 18 H), 1.72–1.75 (m, 4 H), 3.56–3.57 (m, 2 H), 3.77
124.49, 129.29, 129.92, 130.83, 132.81, 134.24, 138.17, 141.19 ppm.
C32H36Cl2N4 (547.57): calcd. C 70.19, H 6.63, N 10.23; found C
69.23, H 6.66, N 10.12.
(q, J = 7.2 Hz, 12 H), 3.83–93 (m, 4 H), 3.99–4.02 (m, 4 H), 5.34
(s, 4 H), 7.35 (s, 4 H), 9.93 (s, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 7.18, 18.20, 21.07, 47.93, 48.20, 49.39, 50.75, 58.45,
129.28, 129.69, 132.24,158.64 ppm. C32H60Cl2N4O6Si2 (723.93):
calcd. C 53.09, H 8.35, N 7.74; found C 53.24, H 8.61, N 7.60.
3,3Ј-[1,3-Phenylenebis(methylene)]bis(1-methyl-1H-imidazolium)
Chloride (3): Prepared by following the procedure outlined for the
preparation of 1 by using N-methylimidazole (0.2 mol) and α,αЈ-
General Procedure To Support Bis-imidazolium Salt 7 on Magnetic
Nanoparticles: The magnetic nanoparticles were prepared accord-
ing to Massart’s method.[27] By this synthesis, a mixture of
FeCl3·6H2O (11.6 g) and FeCl2·4H2O (4.3 g) in degassed water
(400 mL) was heated at 85 °C under an atmosphere of nitrogen.
Liquid ammonia (25%, 15 mL) was added quickly, and the heating
was continued for an extra 30 min. The mixture was cooled to room
temperature, and the black particles were separated magnetically
and washed with water (5ϫ 200 mL). The particles were then sus-
pended in ethanol (500 mL) and sonicated for 1 h. The resulting
suspension was stirred mechanically, and a solution of bis-imid-
azolium salt 7 (30 mmol) in ethanol (100 mL) and liquid ammonia
(2 mL) were added. Stirring was continued for an additional 36 h
under an atmosphere of nitrogen. The modified magnetic nano-
particles were separated by applying an external magnetic field and
washed with ethanol (200 mL). Ether (100 mL) was added, and the
modified magnetic nanoparticles were magnetically separated,
washed with ether (100 mL), and dried under vacuum. Typically,
6.5–7.0 g of black powder was obtained.
1
dichloro-m-xylene (0.1 mol), yield 70%. H NMR (400 MHz, [D6]-
DMSO): δ = 3.85 (s, 6 H), 5.49 (s, 4 H), 7.37–7.42 (m, 3 H), 7.66
(s, 1 H), 7.73 (s, 2 H), 7.88 (s, 2 H), 9.61 (s, 2 H) ppm. 13C NMR
(100 MHz, [D6]DMSO): δ = 36.35, 51.79, 122.83, 124.33, 129.14,
129.95, 136.24, 137.49 ppm. C16H20Cl2N4 (339.27): calcd. C 56.64,
H 5.94, N 16.51; found C 55.71, H 6.21, N 16.14.
3,3Ј-[1,3-Phenylenebis(methylene)]bis(1-mesityl-1H-imidazolium)
Chloride (4): Prepared by following the procedure outlined for the
preparation of 2 by using 1-mesityl-H-imidazole (20 mmol) and
α,αЈ-dichloro-m-xylene (10 mmol), yield 80%. 1H NMR (400 MHz,
[D6]DMSO): δ = 2.05 (s, 12 H), 2.37 (s, 6 H), 5.65 (s, 4 H), 7.13 (s,
4 H), 7.50–7.58 (m, 3 H), 7.95 (s, 1 H), 7.98 (s, 2 H), 8.28 (s, 2 H),
10.09 (s, 2 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 17.48,
21.18, 52.50, 123.28, 124.40, 129.74, 129.96, 130.82, 131.31, 134.09,
135.07, 137.63, 141.02 ppm. C32H36Cl2N4 (547.56): calcd. C 70.21,
H 6.58, N 10.24; found C 69.0, H 6.58, N 10.27.
2,4-Bis(bromomethyl)-1,3,5-trimethylbenzene:[25] A solution of mes-
itylene (13.9 mL, 0.134 mol) and paraformaldehyde (6.2 g,
0.206 mol) in glacial acetic acid (50 mL) was heated at 80 °C. Then,
hydrobromic acid (33 %, 40 mL) was added. The mixture was
heated for an extra 8 h. A bright solid was obtained, which was
washed with water and dried, yield 84%. 1H NMR (400 MHz,
CDCl3): δ = 2.38 (s, 6 H), 2.44 (s, 3 H), 4.57 (s, 4 H), 6.90 (s, 1 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 14.7, 19.51, 29.82, 130.74,
132.62, 137.21, 138.09 ppm.
Procedure To Support the Rhodium Catalyst on Magnetite Nanopar-
ticles: The magnetite nanoparticles modified with the bis-imid-
azolium salt (0.5 g) and potassium-tert-butoxide (90 mg, 0.8 mmol)
were added to dry CH2Cl2 (100 mL), and the resulting mixture was
stirred for 20 min at room temperature under an inert atmosphere
of nitrogen. [Rh(cod)Cl]2 (112 mg, 0.227 mmol) was added, and the
mixture was stirred for 1 h. Then, the magnetic nanoparticles were
concentrated by application of an external magnetic field, and the
solution was decanted. The magnetic nanoparticles were washed
with dry CH2Cl2 (5ϫ 20 mL) and finally dispersed in dry CH2Cl2
(120 mL).
3,3Ј-(2,4,6-Trimethyl-1,3-phenylene)bis(methylene)bis(1-methyl-1H-
imidazolium) Bromide (5):[26] Prepared by following the procedure
outlined for the preparation of 1 by using N-methylimidazole
(0.2 mol) and pre-prepared 2,4-bis(bromomethyl)-1,3,5-trimeth-
ylbenzene (0.1 mol), yield 75%. 1H NMR (400 MHz, [D6]DMSO):
δ = 2.31 (s, 9 H), 3.83 (s, 6 H), 5.47 (s, 4 H), 7.16 (s, 1 H), 7.78 (d,
J = 2.8 Hz, 2 H), 9.04 (s, 2 H) ppm. 13C NMR (100 MHz, [D6]
DMSO): δ = 16.25, 20.04, 36.40, 47.84, 122.74, 124.22, 128.96,
131.72, 136.48, 139.56, 140.22 ppm.
General Procedure for the Hydroaminomethylation Reaction under
Homogeneous Conditions: The proper bis-imidazolium salt
(0.02 mmol) and potassium-tert-butoxide (5.6 mg, 0.05 mmol) were
mixed in the desired solvent (5 mL). After stirring at room tem-
perature under an atmosphere of nitrogen for 30 min, [Rh(cod)Cl]2
(0.01 mmol) was added and stirring was continued for 1 h. A mix-
ture of the appropriate olefin (5 mmol) and the corresponding
amine (6 mmol) was added to the catalyst solution, and then the
resulting mixture was transferred to a 45 mL glass-lined autoclave.
The autoclave was sealed, purged with hydrogen (3ϫ), and pressur-
ized to the required pressure with different ratios of a mixture of
3,3Ј-(2,4,6-Trimethyl-1,3-phenylene)bis(methylene)bis(1-mesityl-
1H-imidazolium) Bromide (6):[26] A mixture of 1-mesityl-H-imid-
azole (5.88 g, 30 mmol) and 2,4-bis(bromomethyl)-1,3,5-trimeth-
ylbenzene (4.6 g, 15 mmol) in toluene (90 mL) was heated at reflux
for 10 h. The precipitate was washed with diethyl ether and dried
Eur. J. Org. Chem. 2015, 1961–1969
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1967

B. Dutta, R. Schwarz, S. Omar, S. Natour, R. Abu-Reziq
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General Procedure for the Hydroaminomethylation Reaction under
Semi-heterogeneous Conditions: A mixture of the appropriate olefin
(5 mmol) and the corresponding amine (6 mmol) was added to the
magnetite-supported rhodium suspension (5 mL). The mixture was
transferred to a glass-lined autoclave. The autoclave was sealed,
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Received: December 5, 2014
Published Online: February 11, 2015
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