Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of Rhodium as Catalyst Precursors

Article abstract of DOI:10.1007/s10562-019-02998-y

Abstract: Novel cationic Rh(I) imino-pyridine complexes were evaluated as catalyst precursors in the hydroaminomethylation of 1-octene in conjunction with both primary (aniline and benzylamine) and secondary amines (piperidine). These complexes were found

Full text of DOI:10.1007/s10562-019-02998-y

Catalysis Letters
https://doi.org/10.1007/s10562-019-02998-y
Alkylation of Amines Via Tandem Hydroaminomethylation Using
Imino‑Pyridine Complexes of Rhodium as Catalyst Precursors
Jacquin October1 · Selwyn F. Mapolie1
Received: 10 June 2019 / Accepted: 11 October 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Novel cationic Rh(I) imino-pyridine complexes were evaluated as catalyst precursors in the hydroaminomethylation of
1-octene in conjunction with both primary (aniline and benzylamine) and secondary amines (piperidine). These complexes
were found to be highly efcient catalysts in mediating a one-pot hydroaminomethylation reaction. High chemoselectivities
towards the target secondary and tertiary amines were obtained depending on the reaction components.
Graphic Abstract
Keywords Imino-pyridine complexes · Alkylation of amines · Rhodium catalyzed · Hydroaminomethylation · Tandem
reaction
1 Introduction
substitution reactions, leading to various by-products and
therefore often require multiple purifcation steps. This
usually leads to low yield reactions and corrosive by-
products. A possible solution to this is the employment
of atom-efficient ‘green’ catalytic approaches such as
hydroamination and hydroaminomethylation [3–8]. The
hydroaminomethylation reaction was developed by Reppe
at BASF and consists of the initial hydroformylation of the
alkenes to aldehydes, followed by the condensation of these
aldehydes with primary/secondary amines to form imine/
enamine intermediates. These imine/enamine intermediates
are then subsequently hydrogenated by the same catalyst to
form the saturated amines. There have been several recent
reviews on the process of hydroaminomethylation. This
Amines are important compounds with a wide range of
applications in the pharmaceutical and perfume industries
[1, 2]. They are normally synthesized via nucleophilic
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02998-y) contains
supplementary material, which is available to authorized users.
* Selwyn F. Mapolie
smapolie@sun.ac.za
1
Department of Chemistry and Polymer Science, Stellenbosch
University, Private Bag X1, Matieland, Stellenbosch 7602,
South Africa
Vol.:(0123456789)
1 3

J. October, S. F. Mapolie
includes excellent overviews by, Vanbésien et al. [6]. Kalck
et al. [7] as well as by Hannedouche and Schulz [8]. The
latter publication interestingly looks at the use of early
transition metal catalysts to mediate The tandem process
of hydroaminomethylation.
of 100 bar. In addition to these N-containing catalysts,
Beller and co-workers [23, 24] reported the use of a P,O-
bidentate ligand for the hydroaminomethylation of internal
alkenes and piperidine to prepare the corresponding linear
amine.
This process is regarded as a green and atom-efcient
process since it generates only water as a by-product [9].
Schif base ligands are ubiquitous and have found wide-
spread application owing to their facile preparation via the
condensation of amines with aldehydes or ketones [10].
Imine ligands are quite useful due to the fact that they can
coordinate to a range of metal ions in various oxidation
states [10]. Despite phosphine-based ligands being synony-
mous with hydroformylation reactions producing highly
chemo- and regioselective catalysts, there has been great
interest in investigating alternative ligand systems [11–13].
The facile synthesis of imine ligands together with their low
cost, have led to them being investigated extensively. When
the imine functionality is combined with pyridine, imino-
pyridine ligands are formed, producing a chelate system
which increases the overall stability of the corresponding
complexes. A number of imino-pyridine ligands coordinated
to a range of metal ions have been reported, with a range of
applications in a number of felds [14–16].
From the literature surveyed, it is clear that complexes
based on N-donor ligands can potentially play a role as
efcient catalysts in the hydroaminomethylation reaction.
This paper therefore reports on the investigation into the
use of some imino-pyridine complexes of Rh(I) in the
hydroaminomethylation of an aliphatic, acyclic alkene in
the presence of the secondary amine, piperidine and the
primary amine, aniline.
2 Results and Discussion
2.1 Synthesis and Characterization
of Imino‑pyridine Ligands
L1 is a known ligand, which was synthesized according
to a previously reported literature method [25–27]. L2 is
a novel ligand and was prepared by reacting 2-pyridine-
carboxaldehyde and 4-amino-3,5-dimethylphenol under
the same conditions as reported for L1. This novel ligand
was isolated as a yellow solid in 84% yield and is sta-
ble at room temperature and can be stored for extended
periods without having to take any special precautions.
Full characterization of the ligand was performed using
IR, NMR (1H and 13C) spectroscopy, mass spectrometry,
elemental analysis and melting point determination. The
IR-ATR spectrum shows the expected disappearance of the
aldehyde absorbance around ~ 1710 cm−1 and the appear-
ance of a new ν(C=N) band at 1638 cm−1 due to the newly
We previously reported on the application of a range
of novel rhodium and ruthenium based imino-pyridine
complexes as catalyst precursors in the hydroformylation
of 1-octene [17]. Recently, we shifted our focus to the
hydroaminomethylation reaction using similar rhodium
imino-pyridine complexes.
As alluded to earlier, the literature is dominated by
reports on phosphine based systems to catalyze the
hydroaminomethylation reaction [18–20]. However, there
are relatively few reports where nitrogen-based ligands
have been used. One example is a report by Alper and
co-workers [21] where they studied the intramolecular
hydroaminomethylation of 2-isopropenylamine using
[Rh(NN)(CO)2]+[RhCl2(CO)2]− (NN = N,N,N′,N′-
tetramethylethane-1,2-diamine). Kostas [22] made use of
a P,N-bidentate ligand, (phosphino amino alcohol ligand)
complexed to Rh(I) to study the hydroaminomethyla-
tion of styrene and morpholine. Good regioselectivities
towards the branched amine was observed (86%), however,
the reaction was performed under high CO:H2 pressure
1
formed imine bond. Similarly, in the H NMR spectrum
the imine proton resonates at 8.35 ppm while the 13C NMR
spectrum (Supp. Info, Fig. S4) further confrms the suc-
cessful preparation of the ligand. In the ESI–MS (positive
mode) spectrum of this ligand, the [M + H]+ ion, which is
also the base peak, is observed at m/z 227.1181 with the
sodium adduct, [M + Na]+ also observed at m/z 249.1000
(Fig. 1).
1
1
Fig. 1 Synthesis of imino-pyri-
dine Ligands, L1–L2
R
R
MeOH
H2N
OH
N
N
OH
+
O
Reflux, 3 h
N
1
1
R
R
1
L1
R = H (
)
1
L2
R = CH3(
)
1 3

Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of…
upon complexation from 8.35 ppm in the ligand to 8.45 ppm
2.2 Synthesis and Characterization of Rh(I)
Iminopyridine Complexes
while the proton on the carbon next to the pyridine nitrogen
shifted upfeld from 8.72 ppm in the ligand to 8.07 ppm in
the complex. 13C NMR spectroscopy further confrmed the
successful preparation of all complexes (Supp. Info, Figs.
S9 and S13).
The complexes (C1–C2) were synthesized by reacting the
ligands (L1–L2) with [RhCl(COD)]2 in DCM at room tem-
perature in a 2:1 ratio. The complexes were isolated in good
to high yields (77–85%) as brown solids. These complexes
are stabilized by isolating them as their tetraphenylborate
salts. In terms of solubility, both complexes are soluble in
acetone, acetonitrile, tetrahydrofuran and dimethyl sulfoxide
while C2 shows additional solubility in DCM and CHCl3
(Fig. 2).
2.3 Hydroaminomethylation of 1‑Octene
and Piperidine
The complexes were subsequently evaluated as catalyst
precursors in the hydroaminomethylation of 1-octene in the
presence of piperidine (Fig. 3).
Complexation of the ligands to the Rh(I) centre was
monitored using IR spectroscopy by following changes in
the absorption bands of the imine and pyridyl moieties. The
relatively strong band observed for the imine moiety of L1
around 1622 cm−1 shifted to 1614 cm−1 along with a simul-
taneous decrease in the intensity of the band. The pyridyl
imine moiety, however, showed a shift to higher wavenum-
bers from around 1578 cm−1 in L1 to 1593 cm−1 in C1.
Similarly, for C2 the imine shifted from 1638 cm−1 in the
ligand to 1614 cm−1 in the complex, while the pyridyl C=N
stretch shifted slightly from 1586 cm−1 to 1590 cm−1.
1H NMR spectroscopy was also used in order to confrm
successful complexation of the ligands to Rh(I). Upon com-
plexation, the imine proton shows a slight shift from around
8.61 ppm in L1 to 8.68 ppm in C1. However, the proton
on the carbon next to the pyridine nitrogen showed a more
signifcant shift from 8.71 ppm in the ligand to 8.14 ppm in
the complex. For C2, the imine proton also shifted downfeld
Diferent reaction parameters were evaluated with the aim
to establish the optimum reaction conditions for the process.
The results from the optimization experiments are discussed
below.
2.3.1 Efect of Time and Diferent Catalyst Precursors
The hydroaminomethylation of 1-octene and piperidine was
performed at three diferent reaction times (1 h, 1.5 h and
2 h; entries 1–3) using C1 as pre-catalyst. As expected, an
increase in the conversion of 1-octene as well as an increase
in the yield of the amine is observed at extended reac-
tion times. After 1.5 h, the high chemoselectivity towards
amines is maintained even up to 2 h. The efcacy of the
complexes with N-donor ligands were evaluated against a
standard phosphine complex, viz. HRhCO(PPh3)3. For this
purpose, C1–C2 was compared against the HRhCO(PPh3)3
R1
R1
-
BPh4
DCM, RT
[RhCl(COD)]2
N
N
OH
+
N
N
OH
NaBPh4, MeOH
Rh
R1
R1
1
C1
R = H (
)
1
C2
R = CH3 (
)
Fig. 2 Synthesis of Rh(I) iminopyridine complexes, C1–C2
Fig. 3 Hydroaminomethylation
N
of 1-octene and piperidine
Rh(I),
CO:H2 (30 bar)
+
Piperidine
75 °C, 1-2 h
N
1 3

J. October, S. F. Mapolie
(Entries 4–5). The phosphine complex showed superior
hydroformylation activity when compared against C1 and
C2 as exemplifed by the higher octene conversion. However
the reaction mixture consists essentially of the aldehydes
and enamines with only just over 10% of the target amines
being formed. The novel N-donor complexes show superior
hydrogenation activity with over 50% of the reaction mixture
being the target amines. It has previously been shown that
generally, phosphine-based hydroaminomethylation catalysts
require temperatures in excess of 100 °C in order to mediate
the reduction of imine/enamine intermediates [20, 28, 29].
The dimeric complex, [Rh(COD)Cl]2, and the cationic
species, [Rh(COD)2]+ were also evaluated as catalyst pre-
cursors in the hydroaminomethylation of 1-octene and
piperidine (Table 1, Entry 5–6). These catalyst precursors
produced much less of the amine products when compared
to our complexes, C1–C2. It is thought that in the case of
the Rh-COD dimer and the cationic species [Rh(COD)2]+,
inactive Rh-carbonyl species are generated in situ which
are unable to mediate the hydrogenation of the enamine
intermediates formed during the Schif- base condensation
reaction of the aldehyde intermediate and piperidine. The
high concentration of enamines at the present at the end
of the reaction lends credence to the lower hydrogenation
ability exhibited by the COD complexes in the absence of
an auxiliary diimine ligand. In addition, under the reaction
conditions employed it appears that the cationic complex
Rh-COD is far less efcient in mediating the hydrogena-
tion of the enamines when compared to the neutral dimeric
precursor, [Rh(COD)Cl]2.
In addition, the hydrogenation of 1-octene to octane was
not detected. At extended reaction times, a small amount of
aldehyde is detected in the reaction mixture. The residual
aldehyde fraction was found to be exclusively branched in
nature (Table 1, Entries 1–4). This is attributed to the higher
reactivity of the linear aldehyde in hydroaminomethylation
compared to the less reactive branched aldehydes [7]. The
result of this is a slightly higher regioselectivity towards
the linear amine (1-nonylpiperidine) at short reaction times.
This linear regioselectivity decreases slightly over prolonged
reaction times as more of the branched aldehydes gradually
react with piperidine to form enamine intermediates which
ultimately lead to branched amines.
2.3.2 Infuence of Olefn:Amine Ratio
Using catalyst precursor C1, we investigated the efect of
the olefn:amine ratio on the activity and selectivity of the
hydroaminomethylation reaction of 1-octene in the presence
of piperidine (Table 2). Performing the reaction in the pres-
ence of an excess of amine (Table 2, Entry 3) positively
infuenced the catalytic activity producing higher conver-
sions of 1-octene and higher yields of amines. Higher con-
centrations of the amine thus suppress the extent of side
reactions such as isomerization (19% at an octene:amine
ratio of 1:0.5 compared to 14% at 1:1.5) as well as alkene
hydrogenation. Piperidine is most likely able to compete
with octenes (terminal and internal) for vacant coordina-
tion sites on the metal. Since the termination step in alkene
isomerization, β-hydride transfer requires a vacant site on
the metal to accommodate the hydrogen, piperidine can to
some extent inhibit this process [18]. On the other hand,
it is expected that the rate of the condensation of the alde-
hydes with piperidine would also be faster due to the higher
Using C1 as precursor at slightly extended reaction time
of 1.5 h leads to the complete hydrogenation of the enam-
ines to amines. In all cases, the amount of internal octenes
obtained as a result of alkene isomerization, was below 20%.
Table 1 Hydroaminomethylation of 1-octene and piperidine
Entry
Catalyst
t (h)
Conv. (%)
Int.
Aldehyde (%)
Enamine (%)
Amines (%)
Regioselectivity
octenes
(%)
l:b (ald)a
l:b (am)b
1
2
3
4
5
6
7
C1
1
1.5
2
1
1
1
1
56
77
84
53
52
49
98
15
16
16
15
17
18
3
11
6
7
11
14
14
33
22
0
0
21
36
61
53
52
78
77
53
33
7
0:100
0:100
0:100
0:100
0:100
0:100
17:83
83:17
76:24
75:25
81:19
85:15
86:14
94:6
C1
C1
C2
[Rh(COD)Cl]2
[Rh(COD)2]BF4
HRhCOL3
11
(L=PPh3)
The reactor was charged with 1-octene (1.6 mmol), piperidine (1.6 mmol), 30 bar CO:H2 (1:1), toluene (5 mL), catalyst loading of
0.1 mol%. The reactor was heated to 75 °C
al:b (ald) (normal aldehyde:iso aldehyde)
bl:b (amine) (normal amine:iso amine)
1 3

Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of…
Table 2 Olefn:amine ratio
Entry
Olefn:amine
Conver-
sion (%)
Int.
Octane (%)
Aldehydes (%)
Enamines (%)
Amines (%)
Regioselectivity
octenes
(%)
l:b (ald)a
l:b (am)b
1c
2
1:0.5
1:1
1:1.5
46
56
65
19
15
14
0
0
0
16
11
8
14
22
17
50
52
62
10:90
0:100
0:100
83:17
83:17
79:21
3d
The reactor was charged with 1-octene (1.6 mmol), piperidine (1.6 mmol), 30 bar CO:H2 (1:1), Toluene (5 mL), catalyst loading of 0.1 mol%
(C1). The reactor was heated to 75 °C for 1 h
al:b (ald) (normal aldehyde:iso aldehyde)
bl:b (amine) (normal amine:iso amine)
c0.8 mmol piperidine
d2.4 mmol piperidine
concentration of the amine. Similar results were previously
reported by Wang et al. in their study of the hydroamino-
methylation of dimethylamine and 1-dodecene [30]. In
the example reported they found that an increase in the
dimethylamine:1-dodecene ratio from 1:1 to 1:4, isomeriza-
tion and hydrogenation was signifcantly suppressed result-
ing in higher amine yields. Furthermore, they also observed
a decrease in the amount of aldehydes remaining with a
concomitant increase in the concentration of the amines
produced [30]. Since side reactions, such as isomerization
of 1-octene and alkene hydrogenation, are inhibited, alde-
hydes are produced in much higher concentrations resulting
in more of the Schif base product forming with piperidine.
The Schif base product is subsequently hydrogenated. Of
the aldehyde products detected in the reaction mixture,
only branched aldehydes are observed when olefn:amine
ratios of 1:1 and 1:1.5 are used. This is as a result of their
lower reactivity which means that subsequent transforma-
tions of the branched aldehydes are slow. In the case of a
1:0.5 olefn:amine ratio, some linear aldehydes still remains.
Slightly less branched aldehydes remained when using a
ratio 1:1.5 compared to the other ratios. As a result of this,
more branched amines is obtained at 1:1.5 ratio, thus leading
to a lower regioselectivity towards the linear amine com-
pared to when other amine:olefn ratios are employed [30].
2.3.3 Infuence of Pressure and Temperature
Here we investigated the infuence of pressure and tempera-
ture (Table 3) on the reaction. With an increase in pressure
from 20 bar to 30 bar (Table 3, Entries 1–3), an increase in
the conversion of 1-octene was observed, however, only a
slight increase occurred at 40 bar. At 30 bar, lower amounts
of enamines were observed while it resulted in the highest
percentage of amines. Lower enamine hydrogenation activi-
ties are thus observed at the higher pressures (40 bar). Both
the H2 and CO partial pressures increase when the total pres-
sure is increased. At higher CO partial pressures, Rh species
Table 3 Infuence of pressure and temperature
Entry
P (bar)
T (°C)
Conv. (%)
Int.
Octane (%)
Aldehyde (%)
Enamines (%)
Amines (%)
Regio-selectivity
Octenes
(%)
l:b (ald)a
l:b (am)b
1
2
3
30
20
40
30
30
30
75
75
75
65
85
85
65
56
69
42
93
99
14
15
12
11
17
10
0
0
0
0
0
1
8
7
12
10
3
17
21
26
24
0
62
57
49
55
80
89
0:100
5:95
7:93
0:100
0:100
~
79:21
80:20
83:17
79:21
75:25
63:37
4
5
6c
0
0
The reactor was charged with 1-octene (1.6 mmol), piperidine (2.4 mmol), Toluene (5 mL), catalyst loading of 0.1
mol % (C1) for 1 h
al:b (ald) (normal aldehyde:iso aldehyde)
bl:b (amine) (normal amine:iso amine)
c2 h
1 3

J. October, S. F. Mapolie
rich in CO ligands can potentially form, which is inactive in
the hydrogenation of enamines. The pressure had no marked
efect on the l:b ratio of the amines.
the branched aldehydes was overcome and thus leading to
branched amines. This modest regioselectivity is comparable
to that of other non-phosphine based systems in the literature
[23, 24].
Performing the reaction at 85 °C (Table 3, Entry 5)
allowed for the complete consumption of the enamine
intermediates while only 3% branched aldehydes remained.
Extending the reaction time to 2 h allowed for the chem-
oselective synthesis of amines with a GC yield up to 89%
(10% internal octenes). A modest regioselectivity of 63%
towards the linear amine is obtained. Therefore, as a result
of the higher temperature, the thermodynamic stability of
In order to broaden the scope of the hydroaminomethyla-
tion reaction, styrene was also evaluated in the presence of
an excess of piperidine (1.5 eq) as shown in Fig. 4. Full con-
version of the substrate was obtained, highly chemoselective
towards amines. In addition, the process is regioselective
towards the branched amine (1-(2-Phenylpropyl)piperidine),
corresponding to other reports in literature using non-phos-
phine based catalysts [23, 31].
2.4 Hydroaminomethylation of 1‑Octene
and Aniline
2.4.1 Efect of Temperature and Catalyst Concentration
Following the evaluation of the complexes in the hydroami-
nomethylation of 1-octene and piperidine, we shifted our
attention towards the use of a primary amine, viz. aniline
(Fig. 5).
The initial reaction conditions for the hydroamino-
methylation of 1-octene and aniline were chosen as 30 bar
(CO:H2, 1:1), 85 °C, 0.1 mol% catalyst loading and 1 h.
These conditions is the optimized reaction conditions for
the hydroaminomethylation of 1-octene and piperidine, as
Fig. 4 Hydroaminomethylation of styrene and piperidine. The reac-
tor was charged with freshly distilled styrene (1.6 mmol), piperidine
(2.4 mmol), Toluene (5 mL), catalyst loading of 0.1 mol% (C1) at
30 bar CO:H2 (1:1), 85 °C for 2 h
H
N
Fig. 5 Hydroaminomethylation
of 1-octene and aniline
Rh(I),
CO:H2 (30-50 bar)
+
Aniline
75-85 °C, 3-18 h
H
N
Rh(I),
1-Octene
CO:H2 (30-50 bar) 75-85 °C, 3-18 h
N
+
N
1 3

Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of…
seen in Table 3, Entry 5. Furthermore, the hydroamino-
methylation of 1-octene and aniline was evaluated using an
olefn:amine ratio of 2:1. In the hydroaminomethylation of
primary amines, tertiary amines are in some cases obtained
as a result of selectivity issues [32]. Therefore, we studied
the reaction at an initial olefn:amine ratio of 2:1 in order to
circumvent these issues.
case of isomerization, it can block coordination sites on
the metal which to some degree inhibits β-hydride transfer
since there are no vacant sites to accommodate the hydride
being transferred. Therefore, due to the lower basicity of
aniline, it is less likely that it would occupy any coordina-
tion sites on the metal with the result that its competition
with any incoming hydride is less. Therefore, isomerization
of 1-octene would be likely, which explains the higher level
of internal octenes. In a similar manner, the hydrogenation
of 1-octene to octane was not a signifcant side reaction in
reactions when aniline is used as the amine.
The hydroaminomethylation of 1-octene and aniline
proved to be a challenging reaction under these conditions,
judging by the amount of amines formed (Table 4). Increas-
ing the temperature (Table 4, Entry 2) and catalyst concen-
tration (Table 4, Entries 3–4) had no signifcant efect on the
yield of amines while only secondary amines were obtained.
The hydroformylation reaction proceeds relatively smoothly
under these conditions, however, the hydrogenation process
is less favoured under the same conditions. It was further
found that almost half of the product stream was comprised
of internal octenes. Isomerization of 1-octene in the pres-
ence of aniline is therefore a quite signifcant side reaction,
compared to in the case of piperidine (Sect. 2.3). As alluded
to earlier, this indicates that it is likely that piperidine is able
to have an infuence on the chemoselectivity of the reaction,
where it can compete with octenes (terminal and internal)
for vacant coordination sites on the metal. Especially in the
2.4.2 Efect of Pressure
Next, we studied the efect of pressure on the hydroamino-
methylation of 1-octene and aniline (Table 5). From Table 5
we can conclude that the hydroformylation reaction proceeds
relatively smoothly under the reaction conditions employed,
however, the hydrogenation reaction required much harsher
conditions.
Therefore, the ratio of CO:H2 was varied, moving into
a more hydrogen rich environment in an attempt to boost
the hydrogenation activity of the catalyst (Table 5, Entries
1–5) [5–8, 23, 24, 31]. However, increasing the CO:H2
Table 4 Efect of temperature and catalyst concentration on the hydroaminomethylation of 1-octene and aniline
Entry
mol% (C1)
T (°C)
Conv. (%)
Int.
Octane (%)
Ald. (%)
Im. (%)
2° Am (%)
Regioselectivity
octenes
(%)
l:b (ald.)a
l:b (am)b
1
2
3
4
0.1
0.1
0.2
0.5
85
100
85
98
99
98
98
45
42
42
41
4
7
6
6
15
21
17
19
35
28
33
29
1
2
2
4
35:65
36:64
37:63
35:65
100:0
100:0
100:0
100:0
85
The reactor was charged with 1-octene (1.6 mmol), aniline (0.8 mmol), 30 bar CO:H2 (1:1), toluene (5 mL), 1 h
al:b (ald) (normal aldehyde:iso aldehyde)
bl:b (amine) (normal amine:iso amine)
Table 5 The efect of pressure on the hydroaminomethylation of 1-octene and aniline
Entry
P (Bar)
CO:H2 ratio
Conv. (%)
Int.
Octane (%)
Ald. (%)
Im. (%)
2° Am (%)
Regioselectivity
octenes
(%)
l:b (ald)a
l:b (am)b
1
2
3
4
5
30
30
30
40
50
1:1
1:3
1:5
1:3
1:3
98
98
98
99
98
41
45
52
31
19
6
6
7
5
3
19
14
9
31
38
29
28
23
26
33
4
6
10
7
7
35:65
32:68
28:72
40:60
42:58
100:0
93:7
93:7
93:7
93:7
The reactor was charged with 1-octene (1.6 mmol), aniline (0.8 mmol), Toluene (5 mL), catalyst loading of 0.5 mol % (C1), 85 °C, 1 h
al:b (ald) (linear aldehyde:branched aldehyde)
bl:b (amine) (linear amine:branched amine)
1 3

J. October, S. F. Mapolie
ratio from 1:1 to 1:5 only produced a 6% increase in the
yield of secondary amines. Unfortunately, it was found
that in the presence of a H2-rich environment, isomeriza-
tion of 1-octene was even more prevalent, although hav-
ing little if any effect on the hydrogenation of 1-octene
to octane. This is consistent with the mechanism of the
hydroformylation reaction. Upon formation of a branched
Rh-alkyl species, another CO ligand can coordinate fol-
lowed by carbonylation to form the Rh-acyl species,
alternatively β-hydride transfer can occur, leading to the
isomerization of the olefin.
2.4.3 Efect of Time
The reaction was then conducted at extended reaction times
with the aim of increasing the yield of amines (Table 6).
Extending the reaction time from 1 to 18 h allowed for
the complete consumption of internal octenes. More signif-
cantly, the yield of secondary amines could be increased to
49% after 18 h and only at this point were tertiary amines
observed. Since only internal octenes are converted from 1 h
onwards, only branched aldehydes are expected to form. This
explains the increase in the amount of branched aldehydes
relative to the linear aldehydes over extended reaction times.
At 18 h, of the aldehydic products only branched aldehydes
remained in the reaction mixture since the more reactive lin-
ear aldehyde is consumed more rapidly. Furthermore, more
branched secondary amines are also obtained over longer
reaction times, resulting in a lower regioselectivity towards
the linear secondary amine.
Increasing the CO:H2 ratio from 1:1 to 1:5, lowers the
CO partial pressure and therefore the concentration of
CO in solution. At this lower CO concentration, the CO
coordination step is most likely much slower, allowing
the β-hydride elimination reaction to be more likely to
occur. As a result, less aldehydes are produced in the
more H2-rich environment (Table 5, Entry 1 vs. Entry 3).
Since linear aldehydes are more reactive in reductive ami-
nation than branched aldehydes, they will be consumed
faster and therefore more unreacted branched aldehydes
remains as can be seen in the l:b ratio of the aldehydes in
Table 5, Entry 1 and Entry 3.
2.4.4 Efect of Octene:Aniline Ratio
The formation of tertiary amines was found to be not an
issue in these reactions. Therefore, the hydroaminomethyla-
tion of 1-octene and aniline was evaluated using a 1:1 ratio
of octene:aniline as well (Table 7).
The pressure was then increased from 30 to 50 bar
while keeping the CO:H2 ratio constant at 1:3 (Table 5,
Entries 2, 4–5). This also had no significant effect on the
yield of secondary amines, however, it had a significant
effect on the hydroformylation reaction. Less isomeriza-
tion occurred at higher pressures since only 19% inter-
nal octenes were obtained at 50 bar compared to 45% at
30 bar. The higher pressure thus favoured the formation of
the aldehydes as a result of the increase in the CO partial
pressure and thus the concentration of CO in solution.
This accelerates the carbonylation step in the reaction
mechanism and thus inhibiting the competing β-hydride
transfer reaction. Therefore, more linear aldehydes are
also obtained relative to the branched aldehydes at this
higher pressure. Similar observations were previously
made by Li et al. [33].
Comparing Entries 1 and 3, Table 7, the higher aniline
concentration appears to have no efect on the regioselec-
tivity of the obtained secondary amines. Furthermore, sig-
nifcantly less aldehydes are obtained in Entry 3, Table 7
compared to Entry 1, Table 7. This is as a result of the higher
concentration of aniline which translates into faster Schif
base condensation with the aldehydes and therefore greater
proportion of imine intermediates being obtained. Moreo-
ver, since in the Schif-base condensation reaction, the linear
aldehydes are more reactive than their branched counter-
parts, more branched aldehydes relative to linear aldehydes
are obtained when employing an olefn to aniline ratio of
1:1. Extending the reaction time to 6 h (Table 7, Entry 4),
the highest amine yield (69%) was obtained. Again, due to
Table 6 Extension of reaction time
Entry
t (h)
Conv. (%)
Int. octenes Octane (%)
(%)
Ald. (%)
Im. (%)
2° Am (%)
Other
Regioselectivity
l:b (ald)a
l:b (am)b
1
2
3
4
1
3
6
18
98
19
7
2
3
2
1
1
38
54
51
41
33
14
3
7
0
0
0
10
42:58
39:61
25:75
0:100
93:7
100
100
100
23
43
49
88:12
81:19
70:30
0
0
The reactor was charged with 1-octene (1.6 mmol), aniline (0.8 mmol), 50 bar CO:H2 (1:3), Toluene (5 mL), catalyst loading of 0.5 mol% (C1),
85 °C
al:b (ald) (linear aldehyde:branched aldehyde)
bl:b (amine) (linear amine:branched amine)
1 3

Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of…
Table 7 The efect of octene:aniline ratio
Entry
t (h)
Octene:aniline
ratio
Conv. (%)
Int.
Octane (%)
Ald. (%)
Im. (%)
2° Am (%)
Regioselectivity
octenes
(%)
l:b (ald)a
l:b (am)b
1
2
3
4
5
3
6
3
6
6
2:1
2:1
1:1
1:1
100
100
100
100
100
7
2
8
2
1
2
1
2
1
1
54
51
23
19
2
14
3
46
8
23
43
22
69
96
39:61
25:75
20:80
5:95
88:12
81:19
87:13
75:25
56:44
1:1.5
0
0:100
The reactor was charged with 1-octene (1.6 mmol), corresponding amount of aniline, 50 bar CO:H2 (1:3), Toluene (5 mL), catalyst loading of
0.5 mol% (C1), 85 °C
al:b (ald) (linear aldehyde:branched aldehyde)
bl:b (amine) (linear amine:branched amine)
the higher reactivity of the linear aldehydes, they are con-
sumed at a much faster rate than their branched counterparts.
However, analysis of the regioselectivity of the secondary
amines indicates that branched amines are also produced in
signifcant amounts. We further evaluated the hydroamino-
methylation reaction of 1-octene in the presence of an excess
of aniline (Table 7, entry 5). Under these conditions, the
chemoselectivity is dramatically impacted in that we observe
a signifcant increase in the amount of amines produced.
Selectivity of 96% towards the amines was obtained. How-
ever the regioselectivity remains relatively moderate with
a linera:branched ratio of 56:44 (l:b). As alluded to earlier,
Schif-base condensation of the intermediate aldehyde is
facilitated in the presence of higher aniline concentrations.
facile that it overcomes the thermodynamic stability of the
branched aldehydes. Furthermore, in the Schif base prod-
ucts formed, the imine moiety is not conjugated and thus
stabilized by an aromatic ring as is the case for the Schif
base product derived from aniline. It is thus expected that the
hydrogenation of the imine should also be more facile. This
was indeed the case, with 95% secondary amines obtained
after only 3 h. Moreover, no aldehydes were observed, which
is in agreement for what is expected for the more nucleo-
philic benzylamine.
To the best of our knowledge, the hydroaminomethylation
reaction has not been previously investigated using imino-
pyridine complexes of rhodium, while reports on the use of
non-phosphine ligands in these reactions are also extremely
limited. The literature is dominated by phosphine-based
catalysts, therefore, the performance of our catalysts in the
hydroaminomethylation of aliphatic alkenes with piperidine
and aniline was compared to previously reported studies on
similar substrates using phosphine-based rhodium cata-
lysts [29, 34–37]. In terms of the hydroaminomethylation
of 1-octene and piperidine, our catalyst facilitates this pro-
cess at much lower catalyst concentration in comparison to
the Rh/TPPTS and Rh/Xantphos systems reported in the
2.4.5 Aniline Versus Benzylamine
The hydroaminomethylation of 1-octene and benzylamine
was also investigated and compared to that of aniline
(Table 8). Benzylamine is more basic than aniline and
therefore more nucleophilic than aniline. The former is thus
expected to react faster with the aldehydes in the Schif base
condensation step. The reaction with benzylamine is so
Table 8 Hydroaminomethylation of aniline versus benzylamine
Entry
Amine
t (h)
Conv. (%)
Int.
Octane (%)
Ald. (%)
Im. (%)
2° Am (%)
Regioselectivity
Octenes
(%)
l:b (ald)a
l:b (am)b
1
2
3
Aniline
Aniline
Benzylamine
3
6
3
100
100
100
8
2
4
2
1
1
23
19
0
46
8
0
22
69
95
20:80
5:95
~
87:13
75:25
59:41
The reactor was charged with 1-octene (1.6 mmol), amine (1.6 mmol), 50 bar CO:H2 (1:3), Toluene (5 mL), catalyst loading of 0.5 mol % (C1),
85 °C
al:b (ald) (linear aldehyde:branched aldehyde)
bl:b (amine) (linear amine:branched amine)
1 3

J. October, S. F. Mapolie
literature [34, 37]. In addition, our reactions are performed
at much shorter reaction times (2 h). While our catalyst and
the Rh/Xantphos systems employ reaction temperatures
around~80 °C, the Rh/TPPTS systems requires much higher
temperatures (130 °C). Even though not directly compara-
ble since we studied the hydroaminomethylation of 1-octene
and piperidine, the Rh-Phosphinite performs better than
our catalyst in mediating the hydroaminomethylation of
1-hexene and piperidine at a catalyst loading of 0.04 mol %
[36]. However, longer reaction times were required. Similar
regioselectivities were obtained for the Rh-phosphinite and
our system. Although the Rh-phosphinite system operate
at much lower pressures in the hydroaminomethylation of
1-hexene and aniline, our system produces full conversion
of the substrate with much higher regioselectivities towards
the linear amine. The comparison of our catalyst system with
examples from the literature is summarized in Table 9.
4 Experimental
4.1 General Considerations
All of the reagents were purchased from either Merck or
Sigma Aldrich and were used without further purifcation.
Solvents were purchased from Merck or Kimix, and were
dried by conventional distillation, Pure Solv™ micro solvent
purifers ftted with activated alumina columns or simple
storing over molecular sieves. Preparation of the ligands was
performed in round-bottom fasks under a stream of N2 gas.
The [RhCl(COD)]2 dimer and HRhCO(PPh3)3 was prepared
according to literature procedures [38, 39]. The complexes
were prepared using standard Schlenk techniques under
a stream of N2. FTIR spectra were recorded on a Thermo
Nicolet Avatar 330 FTIR spectrometer equipped with a
Smart performer (Zn/Se) ATR attachment. NMR spectra
were recorded on a Varian Unity Inova NMR Spectrometer
(300, 400 and 600 MHz), with the peaks referenced relative
to residual signals of the solvent and relative to the stand-
ard. ESI mass spectra were obtained using a Waters Synapt
G2 spectrometer. Elemental analyses were performed at the
University of Cape-Town on a Thermo Flash 1112 Series
CHNS-O analyser. Melting point determinations were per-
formed on a Stuart Scientifc melting point apparatus SMP3.
Syngas (CO:H2, 1:1) was purchased from Afrox SA.
3 Conclusion
In conclusion, two novel Rh(I) imino-pyridine complexes
were synthesized and fully characterized using a range of
analytical techniques. Both complexes were found to be
active for the hydroaminomethylation of 1-octene with
amines such as piperidine, aniline and benzylamine. It was
found that the basicity of the amine impacts on the rate of
the reaction. The more basic the amine, the faster is the
reaction and the higher the yield of the targeted N-alkylated
amines. The hydroaminomethylation of 1-octene with pri-
mary amines was successfully performed without the forma-
tion of tertiary amines except when the reaction was per-
formed over extended reaction times (18 h).
4.2 Instrumentation
Catalytic reactions were performed in a 50 ml Amar pressure
reactor equipped with a Tefon liner. Catalytic reactions were
monitored using a Varian 3800 Gas Chromatograph con-
taining a Petrocol DH 50.2 (50 m ×0.2 mm×0.5 μm) col-
umn equipped with a FID detector. For piperidine as amine
Table 9 Comparison of the performance of our catalyst with literature
Alkene
Amine
Catalyst
Conditions
% Conv.
% Am.
L:b
References
1-Octene
1-Hexene
Piperidine
Piperidine
0.42 mol% Rh/TPPTS (1/64)
60 bar CO:H2 (1:3),130 °C, 4 h
67.5 bar CO:H2 (1:4), 120 °C, 18 h
87
100
72
96
n.r
50:50
[34]
[35]
50 mg HRh(CO)(PPh3)3
immobilized on mesoporous
silica
1-Hexene
1-Hexene
Piperidine
Piperidine
0.04 mol% Rh-phosphinite
30 bar CO:H2 (1:3.3), 80 °C, 6 h
40 bar CO:H2 (1:7), 125 °C, 8 h
100
99
99
92
66:34
97:3
[36]
[29]
0.1 mol% Rh and pyrrole-
based tetraphosphorous
ligand
1-Octene
Piperidine
0.2 mol% Rh/Xantphos (1/5)
20 bar CO:H2 (1:1), 80 °C, 18 h then n. r
20 bar (H2), 80 °C, 10 h
96
n.r
[37]
1-Octene
1-Hexene
1-Octene
Piperidine
Aniline
Aniline
0.1 mol% Rh-Imino-pyridine
0.04 mol% Rh-phosphinite
0.5 mol% Rh-Imino-pyridine
30 bar CO:H2 (1:1), 85 °C, 2 h
30 bar CO:H2 (1:3.3), 80 °C, 6 h
50 bar CO:H2 (1:3), 85 °C, 6 h
99
73
100
89
84
69
63:37
57:43
75:25
This work
[36]
This work
n.r not reported
1 3

Alkylation of Amines Via Tandem Hydroaminomethylation Using Imino-Pyridine Complexes of…
the following temperature program was used: 80 °C (6 min
hold), 100 °C (10 °C/min), 100 °C (10 min hold), 220 °C
(20 °C/min), 220 °C (16 min hold), with total run time of
40 min. For aniline and benzylamine as amine the following
temperature program was used: 80 °C (6 min hold), 100 °C
(10 °C/min), 100 °C (10 min hold), 220 °C (20 °C/min),
220 °C (36 min hold), with total run time of 60 min. Helium
was used as carrier gas at constant fow with a 1.1 mL/min
fowrate. P-Xylene was used as the internal standard.
and washed extensively with MeOH after which the product
was isolated as a brown solid (0.125 g, 85%). IR (ATR) ʋ
(C=N) 1614, 1593 cm−1. 1H NMR (300 MHz, DMSO-d6) δH:
1.93 (m, 4 H, COD), 2.42 (m, 4 H, COD), 4.25 (broad s, 4 H,
COD), 6.84 (comp, 6 H, Ar–H, BPh4), 6.94 (m, 8 H, BPh4),
7.18 (comp, 10 H, Ar–H, BPh4), 7.80 (m, 1 H, pyr–H), 8.12
(comp, 2 H, pyr, CH=N, pyr–H), 8.29 (m, 1 H, pyr–H), 8.63
(d, J=1.5 Hz, 1 H, CH=N). 13C NMR (151 MHz, DMSO-d6)
δC: 29.9, 84.5, 84.6, 115.5, 121.5, 122.9, 125.3, 129.1, 129.5,
135.5, 139.3, 141.2, 149.9, 154.7, 157.4, 163.8, 171.4 ppm.
MS (ESI+mode, m/z): Calcd for [Rh(COD)]+ 211.0, found
211.0, Calcd for [Rh(COD)(MeCN)]+ 252.0, found 252.0,
Calcd for [Rh(COD)(MeCN)2]+ 293.1, found 293.1. Anal.
Calcd (%) for C44H42BN2ORh: C, 72.54; H, 5.81; N, 3.85;
found: C, 72.39; H, 4.65; N, 3.44. Melting point: 146–148 °C.
4.3 Synthesis and Characterization
of Imino‑pyridine Ligand L2
4.3.1 Synthesis
of 3,5‑Dimethyl‑4‑[(pyridin‑2‑ylmethylidene)amino]
phenol (L2)
4.4.2 Synthesis and Characterization of C2
Pyridine-2-carbaldehyde (0.234 g, 2.19 mmol) and 4-amino-
3,5-dimethylphenol (0.300 g, 2.19 mmol) were added into
20 mL of MeOH forming a brown solution. The resulting
brown solution was stirred under refux for 4 h. After the
allotted 4 h, the solvent was removed and the resulting solid
was recrystallized from EtOH. Product was then recovered
via fltration as a brown crystalline solid (0.410 g, 84%).
The [RhCODCl]2 dimer (50 mg, 0.10 mmol) was dis-
solved in DCM (5 mL) forming a bright yellow solution.
3,5-Dimethyl-4-[(pyridin-2-ylmethylidene)amino]phenol
(46 mg, 0.20 mmol) was then added as a solid to the stirring
solution of the dimer forming a black solution immediately,
with the formation of a brown precipitate after a few min-
utes. This mixture was stirred for 1 h at room temperature.
A solution of NaBPh4 (69 mg, 0.20 mmol) in MeOH (3 mL)
was then added dropwise, forming a black solution. This was
stirred for an additional 1 h at room temperature. After the
allotted time the solvent was removed followed by the addi-
tion of MeOH (5 ml). The product was then recovered via
fltration and washed with MeOH (10 mL) (119 mg, 77%).
IR (ATR) ʋ (C=N) 1614, 1590 cm−1. 1H NMR (300 MHz,
Acetone-d6) δH: 2.31 (s, 6 H, Ar–(CH3)2, 2.51 (m, 4 H,
COD), 2.88 (broad s, 4 H, COD), 4.23 (broad s, 4 H, COD),
6.67 (s, 2 H, Ar–H), 6.77 (m, 4 H, BPh4), 6.91 (m, 8 H,
BPh4), 7.33 (m, 8 H, BPh4), 7.80 (m, 1 H, pyr–H), 8.06
(comp, 2 H, pyr, CH=N, pyr–H), 8.22 (m, 1 H, pyr–H),
8.45 (d, J = 2.9 Hz, 1 H, CH=N). 13C NMR (75 MHz,
Acetone-d6) δC: 17.7, 30.1, 87.1, 87.3, 114.9, 121.4, 125.1,
129.3, 130.2, 130.3, 136.2, 136.8, 141.7, 150.4, 154.8,
156.4, 163.7, 175.3 ppm. HRMS (ESI+mode, m/z): Calcd
for [M]+ 437.1100, found 437.1099. Anal. Calcd (%) for
C46H46BN2ORh.0.25Dichloromethane: C, 71.42; H, 6.03; N,
3.60; found: C, 71.31; H, 6.67; N, 3.54. Decomposed with
melting: 139–141 °C.
1
IR (ATR) ʋ (C=N) 1637, 1585 cm−1 (pyridyl). H NMR
(600 MHz, CDCl3) δH: 2.10 (s, 6 H, Ar–(CH3)2), 6.55 (s,
2 H, Ar–H), 6.76 (s, 1 H, Ar-OH), 7.43 (m, 1 H, pyr–H),
7.86 (m, 1 H, pyr–H), 8.29 (d, J = 7.9 Hz, 1 H, pyr–H),
8.34 (s, 1 H, CH=N), 8.71 (d, J=4.6 Hz, 1 H, pyr–H). 13
C
NMR (151 MHz, CDCl3) δC: 18.6, 115.3, 121.4, 125.5,
129.0, 137.2, 143.3, 149.4, 152.8, 154.5, 163.5 ppm. HRMS
(ESI + mode, m/z): Calcd for [M + H]+ 227.1184, found
227.1181, Calcd for [M+Na]+ 249.1004, found 249.1000.
Anal. Calcd (%) for C14H14N2O.0.2EtOH: C, 73.45; H, 6.51;
N, 11.90; found: C, 73.37; H, 6.49; N, 12.35. Melting point:
179–181 °C.
4.4 Synthesis and Characterization of Rh(I)
Imino‑pyridine Complexes (C1–C2)
4.4.1 Synthesis and Characterization of C1
The [RhCODCl]2 dimer (50 mg, 0.10 mmol) was dissolved in
DCM (5 mL) forming a bright yellow solution. 4-{[pyridin-
2-ylmethylidene]amino}phenol (40 mg, 0.20 mmol) was then
added as a solid to the stirring solution of the dimer, leading
to an immediate colour change from bright yellow to black.
This was allowed to stir for 1 h at room temperature. After
1 h, the solvent was removed and the resulting dark-brown
residue dissolved in MeOH (5 mL). Sodium tetraphenylbo-
rate (68 mg, 0.20 mmol) was then added as a solid, with the
immediate precipitation of a brown solid. This was stirred for
15 min at 0 °C. The precipitate was recovered via fltration
4.5 General Method
for the Hydroaminomethylation Reaction
4.5.1 Hydroaminomethylation of 1‑Octene and Piperidine
The reactor was charged with Toluene (5 mL) and the
appropriate amount of the catalyst precursor with the
1 3

J. October, S. F. Mapolie
Acknowledgements The authors acknowledge the fnancial support
by the National Research Foundation of South Africa (Grant No.
CPRR160408161799) and the c*change Centre of Excellence in Catal-
ysis, the Department of Science and Technology (DST), South Africa.
The Division of Research Development at Stellenbosch University is
also thanked for their fnancial support.
catalyst precursor suspended in toluene. 1-Octene (179 mg,
0.25 mL, 1.6 mmol) and piperidine (136 mg, 0.158 mL,
1.6 mmol) was added resulting in a Rh:1-octene:piperidine
ratio of 1:1000:1000. The reactor was fushed with syngas
three times after which the reactor was pressurised to the
required pressure and the heating started. At the conclu-
sion of the reaction, the reactor was cooled and the reac-
tor carefully vented and then opened. The reaction mixture
was fltered using a syringe flter and dried over MgSO4. A
sample (0.95 mL) of the reaction mixture was transferred to
a GC vial containing 0.05 mL p-xylene (internal standard).
The mixture was analysed using GC-FID. The components
of the reaction mixture were identifed using authentic GC
standards (where available).
Compliance with Ethical Standards
Conflict of interest The authors declare no confict of interest.
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