A ruthenium racemisation catalyst for the synthesis of primary amines from secondary amines

Article abstract of DOI:10.1039/c6dt01525e

A Ru-based half sandwich complex used in amine and alcohol racemization reactions was found to be active in the splitting of secondary amines to primary amines using NH₃. Conversions up to 80% along with very high selectivities were achieved. However, after about 80% conversion the catalyst lost activity. Similar to Shvo's catalyst, the complex might deactivate under the influence of ammonia. It was revealed that not NH₃ but mainly the primary amine is responsible for the deactivation.

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ALTINTAS, M. R. Schaller and D. Vogt, Dalton Trans., 2016, DOI: 10.1039/C6DT01525E.
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A Ruthenium Racemisation Catalyst for Synthesis of
Primary Amines from Secondary Amines
Cite this: DOI: 10.1039/x0xx00000x
Dennis Pingen,a Cigdem Altintas,# Max Schaller,# Dieter Vogt.b*
Received 00th January 2012,
Accepted 00th January 2012
A Ru-based half sandwich complex used in amine and alcohol racemization reactions was found to be
active in the splitting of secondary amines to primary amines using NH3. Conversions up to 80% along
with very high selectivities were achieved. However, after about 80% conversion the catalyst lost activity.
Similar to Shvo’s catalyst, the complex might deactivate under the influence of ammonia. It was revealed
that not NH3 but mainly the primary amine is responsible for the deactivation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
H
R1
N
R2
R1
N
R2
Primary amines are valuable building blocks in industrial
chemistry; they are the main building blocks for a large variety
of polymers, surfactants, corrosion inhibitors and fine-
chemicals.[1,2] For the production of primary amines waste-free,
selective protocols are highly desired with special emphasis on
renewable resources such as bio-alcohols.[3] However, current
industrial (heterogeneously catalysed) syntheses of amines from
alcohols inevitably give mixtures of primary, secondary, and
tertiary amines.[4] Current homogeneously catalysed alcohol
amination reactions were shown to be very selective towards
primary amines, barely producing any secondary amines.
Although the current homogeneous catalysts, developed by
Milstein,[5] Beller[6] and Vogt[7] are very selective towards
primary amines, anticipation on the industrial synthesis of
primary amines also requires the development of catalysts for the
splitting of secondary (and tertiary) amines. Reusing secondary
amines in the synthesis of primary amines will reduce the waste
stream in the total production. At the moment, only one example
of a homogeneous catalyst has been reported that efficiently
catalysed the splitting of secondary and tertiary amines.[8]
H
dehydrogenation
NH3
H
]
2
[Ru]
[Ru
R1 NH2
H
hydrogenation
R2
HN
HHN
R2
R1 R2 alkyl
=
,
Scheme 1: ‘Hydrogen Shuttling’ in the splitting of secondary amines to primary
amines.
The system reported by Beller and coworkers uses Shvo’s
catalyst in the splitting of secondary and tertiary amines. This
catalyst has previously been used in various reactions, one of
which is amine racemization.[9] As the racemization of amines
by Shvo’s catalyst and related systems proceeds via initial
dehydrogenation of the amine,[10] it is anticipated that similar
systems will also be active in the splitting of amines.
Results and discussion
The splitting of secondary amines is expected to proceed via the
‘Hydrogen Shuttling’ concept; dehydrogenation of the secondary
amine forming the dialkylimine. This subsequently undergoes
nucleophilic attack from NH3 resulting in primary amine and
primary imine. The primary imine will then be hydrogenated to
Ruthenium
half
sandwich
complexes
bearing
a
pentaphenylcyclopentadienyl (CpPh5) moiety (Figure
investigated in the splitting of secondary amines with ammonia.
Complexes 1-3 have been employed before in the racemization
of alcohols.[9-12] In addition, complex
racemization of amines.[9-10] Complex
1
) were
produce another equivalent of primary amine (Scheme 1).
1
was also used in the
did show activity as
2
well, albeit a long induction period was noticed before activity
was observed.[11] Complex
rich variant of complex
racemization.[12]
3
1
was developed as a more electron
but was only active in alcohol
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Table 2: Splitting of dicyclohexylamine with NH3 using complex 1 - detailed
variation of the excess of NH3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
CO
Ph
Cl
PPh3
Ph
Entry
Time
(h)
NH3 (l)
equiv
Conversion
(%)a
Yield
prim.
Prim. Amine
Selectivity
Ru
Cl
CO
Ph
Ru
Ru
Ph
Ph
Ph
1
OC
OC
OC
amine
2
3
(%)a
Figure 1: Half sandwich Ru(CpPh5) complexes investigated in this study.
1
2
3
4
5
6
23.5
21
23.5
24
21
23.5
120
60
40
20
12
4
81.5±5
82.5±3
61±3
51±3
54±3
76.5±4
78±2
58.5±3
47.5±2
53±2
94
94
96
94
98
95
Initial reactions with complexes
1 and dicyclohexylamine as
model substrate were performed employing a catalyst loading of
2 mol% at 150ºC (supporting information Table S5). Under these
conditions only 40% conversion could be achieved (99%
selectivity, see supporting information). Increasing the
temperature to 170ºC significantly improved the conversion
without affecting the selectivity (Table 1). As it can be
anticipated that the excess of NH3 applied will have significant
influence on the amine splitting, the amount of NH3 was varied.
16.5±1
15.5±1
Complex 1 (2 mol%), dicyclohexylamine (1.5 mmol), tert-amyl alcohol (3
ml), NH3(l), 170°C. a) standard deviation over 3 experiments.
The data in Table 2 confirm that a large excess of NH3 is required
(60 - 120 eq.) in order to achieve good conversion. However, it
is worth noting that the selectivity was very high (> 94%) in all
cases, even at very low excess of NH3. Under the optimised
conditions complex
(Table 3).
Table 1: Splitting of dicyclohexylamine with NH3 using complexes 1-3
(Figure 1)
1 was now used for a range of substrates
Entr
y
Com
plex
Time
(h)
NH3(l)
equiv
Conv.
(%)a
Yield
prim.
Amine
(%)a
76.5±4
22.5±2
25±2
0
Prim.
Amine
selectivity
(%)
For amines bearing bulkier secondary alkyl substituents the
conversion and selectivity was generally high (entries 1-3).
Linear secondary amines resulted in lower conversion but also
lower selectivity (entries 4-6). The lower selectivity can be in
part attributed to the formation of nitriles as a side reaction.
Surprisingly, dioctylamine initially gave no primary amine,
mainly secondary imine (entry 5). Increasing the excess of NH3
resulted in lower conversion but gave primary amine in
reasonable selectivity (entry 6). Dibenzylamine also resulted in
low conversion, whereas a higher NH3 loading also appeared to
be beneficial for the selectivity (entries 7 and 8). In case of N-
methylaniline, barely any conversion was observed. A larger
excess of NH3 brought no improvement (entry 9 and 10).
Reacting tertiary amines also leads to fairly low selectivity; the
substrate has to undergo 2 steps before the final product can be
formed. Moreover, it is likely that dehydrogenation of a tertiary
amine is more difficult as it has to proceed either via the
intermediate enamine or iminium species, which makes the
formation of the secondary amine or imine more difficult.
1
2
3
4
1
2b
3
24
24
24
120
120
120
120
81.5±5
22.5±2
26.5±2
0
94
100
94
0
1+K
23.75
OtBu
c
5
6
7
8
1
2
3
21
21
21.5
21.5
12
12
12
12
54±3
13±1
68.5±4
23.5±2
53±3
11±1
62±3
98
85
90
1+K
23.5±2
100
OtBu
c
Complex (2 mol%), dicyclohexylamine (1.5 mmol), tert-amyl alcohol (3
mL), NH3(l), 170°C. a) standard deviation over 3 experiments.b) 0.75 mL
MTBE as co-solvent. c) 2 mol% KOtBu added.
Complex
conversion and a selectivity of 94% to the primary amine was
achieved (entry 1). Complexes and both showed low activity
1 is the most active with 120 eq. of NH3. Up to 80%
2
3
under these conditions (entries 2-3). The situation was different
at a 10-fold lower excess of NH3 (12 eq., entries 5-7). Now
complex
still gave reasonable conversion (54%). Complex
least active under both conditions (entries 2 and 6). The addition
of base has been shown to be beneficial in activating complex
3
showed the highest conversion of 68% and complex
1
2
was the
1
in the alcohol racemization.[13] However, addition of KOtBu with
120 eq. of NH3 completely deactivated the catalyst (entry 4),
while with 12 eq. of NH3 still some conversion was achieved;
though lower than without the addition of base (entry 8).
The results reported in Table 1 confirmed the expected strong
effect of the excess amount of ammonia on the performance in
catalysis. Complex
1 appeared to be the most active and
therefore, the effect of the NH3 excess was investigated in more
detail for this complex (Table 2).
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Table 3: Substrate scope screening employing complex 1
Entry
Substrate
NH3 (l) (eq.)
Conv. (%)a
Prim. Amine (%)/selec.
Other (imine/secondary imine/tertiary amine/nitrile)
(%)a
1
2
Dicyclohexylamine
N-methyl
60
60
82.5±3
73±2
78±2 / 94
58±2 / 79
5% dicyclohexylimine
15.5 dicyclohexylamine
cyclohexyl-amineb
N-isopropyl
3
60
81±5
74±4 / 91
7.5% dicyclohexylamine
cyclohexylamineb
Dihexylamine
Dioctylamine
4
5
60
60
43.5±1
80±4
25±1 / 56
0 / 0
11% trihexylamine, 8.5% hexanenitrile
43.5% dioctylimine, 20% dioctylenamine, 16%
octanenitrile,
6
Dioctylamine
120
50±4
30±3 / 60
4.5% octylimine, 12.5% dioctylimine, 3%
octanenitrile
7
8
Dibenzylamine
Dibenzylamine
120
60
25±3
23±2
19±4 / 76
0 / 0
3% dibenzylimine, 2.5% benzonitrile,
2.5% benzylimine, 17.5% dibenzylimine, 3%
benzonitrile,
1.5% methyleneaniline
0.5% methyleneaniline
43% dioctylamine, 3.5% octanenitrile
2.5% hexylimine, 41.5% dihexylimine, 8%
dihexylamine, 3.5% hexanenitrile
9
N-methylanilinec,d
N-methylanilinec,d
Trioctylamined
120
60
120
120
2.5
2
60
1 / 39
1.5 / 75
13.5 / 22
12 / 17
10
11
12
Trihexylamined
67.5
Complex 1 (2 mol%, 0.03 mmol), substrate (1.5 mmol), t-amylalcohol (3 mL), NH3 (2.5 mL for 60 eq., 5 mL for 120 eq.), 170ºC, 23.5 h. a) standard deviation
over 3 experiments. b) based on cyclohexylamine. c) based on aniline. d) single experiments, not in triplo
As the results show, the maximum conversion reached was
limited in all cases, which might be due to catalyst deactivation.
In fact Beller and co-workers have shown earlier that the related
100
90
80
Shvo’s catalyst was inhibited by the coordination of NH3 as well
as by primary amines.[14] They showed that this inhibition
70
A
became partially reversible at higher temperature. However,
even at 150ºC or higher, conversions were generally lower than
60
50
40
30
20
10
0
B
85%.[8] In our case for complex
1 increasing the reaction
B
temperature to 170ºC did increase the conversion, though never
reaching more than 80%. Hence we conclude that we are dealing
with a different cause of deactivation and decided to study this
in more detail. In order to see if we are dealing with product
inhibition by primary amines, 20 mol% of cyclohexylamine were
added at the start and the reaction was monitored in time (Graph
A
0
5
10
15
20
25
30
35
40
45
50
Time (h)
1). The reaction was significantly slower and gave Graph 1: Catalyst inhibition by primary amine (A) compared to the reaction
without additional primary amine (B). Conditions: Complex 1 (2 mol%, 0.09 mmol),
dicyclohexylamine (4.5 mmol), tamylalcohol (9 mL), NH3(l) (2.5 mmol, 90 mmol, 60
approximately 15-20% lower conversion.
eq.), 170ºC. Cyclohexylamine (20 mol%, 0.9 mmol) added (A).
dicyclohexylamine, ●= cyclohexylamine, = dicyclohexylimine. Data points for A
have been normalized to the actually produced cyclohexylamine.
■ =
▲
In order to reveal the inhibiting effect of the primary amine, the
reaction of cyclohexylamine with complex was monitored by
1
13C NMR. The Cp ring carbon atoms give a characteristic carbon
shift and can probe a change at the Ru centre (106 ppm, Figure
2A). The resonance at about 197 ppm corresponds to the CO
ligands, which can also be used as a probe for changes in the
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coordination sphere of Ru. The signals between 127 – 132 ppm cause of catalyst deactivation. However, a small new peak
belong to the phenyl groups on the Cp ring.
emerges at 97 ppm (Figure 3A).
Figure 3: 13C NMR (125.73 MHz, 298K, CDCl3) spectrum of complex
1 after addition
Figure 2: 13C NMR (125.7 MHz, 298K, CDCl3) spectrum of complex 1 showing the
distinctive Cp carbon shift at 106 ppm (upper spectrum, A), and the resulting
complex mixture after treatment with 10 equivalents of cyclohexylamine after 18
h at r.t. (lower spectrum, B)
of cyclohexylamine and heating to 60ºC for 1 h (upper spectrum, A) and spectrum
of complex 1 after reaction with NH3 at 170ºC for 1 h (lower spectrum), due to a
low concentration, the carbonyl signals could not be observed.
A similar treatment of complex
instead of cyclohexylamine. A solution of complex
1
was repeated with ammonia
was placed
The addition of primary amine slowly led to an upfield shift of
the Cp ring carbon atoms in the 13C NMR. After 18 h at r.t., the
peak at 106 ppm had completely disappeared and a new peak at
101 ppm was observed (Figure 2B). There are now 2 carbonyl
1
in a 10 mL stainless steel autoclave, which was subsequently
charged with NH3. After stirring the solution for 1 h at 170°C the
autoclave was opened and the excess of NH3 was released. The
solution was transferred to a Wilmad-Young NMR tube and a
spectrum was recorded (Figure 3B).
Figure 3B reveals that a similar reaction occurred; again, a shift
of the Cp carbons was observed, now from 106 to 102 ppm.
When then 10 eq. of cyclohexylamine were added to the NH3
adduct complex, the NH3 was immediately replaced for the
cyclohexylamine. If this type of coordination was indeed the
cause of deactivation, one would expect this process to be
irreversible.
signals observed, one for the monocarbonyl complex
5 in which
one CO of the original complex is replaced by the amine[15] and
the other one for the cationic dicarbonyl complex
chloride has been replaced by the amine.[16]
4, in which the
Investigating the reversibility of the amine coordination and
substitution, the mixture consisting of complex and
1
cyclohexylamine was warmed to 60ºC for 1 h. After this time,
the peak at 106 ppm is observed again, indicating that
cyclohexylamine coordination is indeed reversible (Scheme 2).
The process of amine coordination is apparently reversible at
already fairly low temperature, and is therefore unlikely to be the
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Cl-
Cl-
very stable and does not react back (catalyst deactivation). The
remaining Ru complex remains in solution though most likely in
some dimeric form, which is typical for these types of ruthenium
complexes.[22]
Ph
Ph
Ph
Ru
Ph
Ph
Ph
Ph
Ph
Ph
NH3
NH3
Ph
Ph
Ph
Ph
Ph
NH3
Ph
Cl
NH2R
Ru
Ru
CO
OC
OC
NH2R
CO
OC
CO
13C NMR: 106
13C NMR: 101
13C NMR: 97
13C NMR: 97
ppm
ppm
ppm
Ph
ppm
Ph
Cl-
Cl-
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
=
R
alkyl
Ph
Ph
Scheme 2: Reversible deactivation of Ru(CpPh5)(CO)2Cl by NH3 and primary amine,
similar to the deactivation of Shvo’s catalyst.[14]
Ph
Ph
Ph
Ph
Ru
Ph
Ru
Ph
Ph
Cl
Ph
Ru
Ru
OC
NH2R
NH
R
OC
OC
NH2R
OC
NH2R
NH
R
OC
CO
CO
OC
H
NH2R
196
ppm
To exclude deactivation by the starting material in catalysis,
dicyclohexylamine, a reaction with 10 eq. of dicyclohexylamine
was performed. Because conversion up to 80% is observed, it is
not expected that this deactivates the catalyst. In addition, the
bulkiness of the substrate might even hamper coordination to the
complex if not under the exact reaction conditions. Figure S8
(supporting information) shows the complex after 18 h at room
temperature in the presence of dicyclohexylamine. However,
after addition of 10 eq. cyclohexylamine to this mixture and
heating for 4 h at 60ºC, the peaks at 101 and 97 ppm were
observed again. In addition, a peak at 170 ppm was observed
(Figure 4). The large downfield shift indicates that this originates
from a carbonyl compound. However, the only carbonyl source
in the mixture are the carbonyl ligands on the complex itself. It
might be possible that one of the carbonyls undergoes
nucleophilic attack by cyclohexylamine.[17,18] This is in
agreement with previous other carbonyl complexes, which were
used in the carbonylation of amines.[19] Moreover, complexes
similar to those used in this study have been shown to be
susceptible for nucleophilic attack by even less nucleophilic
compounds.[20]
R
and
208
170
ppm
ppm
HN
R
Cl- HNHR2
=
R
cyclohexyl
Scheme 3: Attack of primary amine on CO in the complex, followed by
deprotonation of (secondary) amine.
In the 13C NMR spectra, distinct signals of the carbon in the CN
bond of cyclohexyl groups were observed. Compared to free
cyclohexylamine, this showed a downfield shift (see supporting
information, Figure S11). In addition, the reaction was also
monitored by in situ IR. The carbonyl vibrations at 2010 and
2050 cm-1 seem to disappear equally. This indicates that there is
no difference between the carbonyls in terms of reactivity for the
nucleophilic attack of the amine, and that the original starting
complex disappears. Upon heating the mixture to 40ºC,
upcoming peaks at 1710 and 1620 cm-1 are observed over longer
reaction times. These regions are typical for amide/formamide
vibrations. The increase of these peaks over time indicates the
formation of carbamoyl and formamide derivatives from CO
(Figure 5). The band at 1700 cm-1 increases fast in the beginning
and remains strong, also indicating the formation of an amide
that most likely remains coordinated to Ru. The band at 1620 cm-
1 confirms this as well.
Figure 4: 13C NMR (125.73 MHz, 298K, CDCl3) spectrum of Ru(CpPh5)(CO)2Cl in the
presence of 10 eq. dicyclohexylamine and 10 eq. cyclohexylamine heated to 60ºC
for 4 h.
Upon nucleophilic attack of the primary amine on one of the CO
ligands, a carbamoyl complex is formed.[21] The carbamoyl
ligand neutralizes the cationic complex. The remaining proton in
the amide bond can be removed by the excess of amines present
in solution (Scheme 3). The carbamoyl complex appears to be
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Figure 5: in situ FT-IR spectra of the reaction of Ru(CpPh5)(CO)2Cl with 10 eq.
cyclohexylamine at 40ºC in the region of 2150 to 1550 cm-1 (upper spectrum), and
in the region of 2600 to 2200 cm-1 (lower spectrum).
only starts at 90ºC after longer reaction times compared to 60ºC
for complex 1. This suggests that the complex is more stable,
though just less active (see supporting information for further
details).
Another indication for the nucleophilic attack of the amine to the
carbonyl is seen in the 2400 cm-1 region. Here a strong band is
seen almost from the start, which later becomes less intense. This
region indicates the presence of ammonium ions. The decrease
in intensity indicates that the ammonium is deprotonated,
forming the inactive complex as stable species. The conversion
of secondary n-alkylamines was found to be lower. It is likely
that n-alkylamines react even more readily with the carbonyl
moieties due to less steric hindrance. Performing the same
reactions with n-hexylamine, revealed it was indeed fast, as the
13C-signal at 106 ppm was not observed at all (Figure S11 and
S12 in SI). In addition, the resulting carbamoyl peak after heating
has a slightly different shift (164.7 ppm), indicating that this is a
product of a reaction of the amine with a carbonyl ligand. So far,
the spectroscopic techniques used suggest a carbamoyl complex,
though not certain in what form exactly. Therefore we performed
mass spectrometry on the complex used in the in situ IR
experiments. In this higher mass complexes (mass higher than
the monometallic complex) were found, showing that indeed
multimetallic, mostly bimetallic, species are formed upon
Conclusions
We have shown that a catalyst previously applied in amine and
alcohol racemization is also an active catalyst in the splitting of
secondary and tertiary amines with ammonia. However, full
conversion could not be achieved, which is most likely due to
deactivation of the catalyst. In this, it seems that deactivation is
mainly caused by the primary amine product. One possibility is
that the deactivation proceeds via nucleophilic attack of the
primary amine to a carbonyl moiety on the metal complex. The
resulting carbamoyl species has been identified by NMR, in situ
FTIR and mass spectrometry.
Acknowledgements
We would like to thank Ton Staring for his technical assistance. This
research has been in part funded by the Netherlands Ministry of Economic
Affairs and the Netherlands Ministry of Education, Culture, and Sciences
within the framework of the CatchBio program.
degradation. Also, when complex
1 was refluxed in toluene in
Experimental
the presence of 40 eq. cyclohexylamine, the resulting complex
displayed no carbonyl signals anymore, and only a single peak at
170 ppm was found (Figure 6).
All work was carried out under standard Schlenk conditions
under argon; all solvents were dried, degassed and purged with
Ar prior to use. All used glassware was pre-dried at 120°C and
heated with a heat gun while purged with Ar prior to use.
Chemicals were purchased from Sigma-Aldrich and Strem and
were used as received. All reactions were performed in a
magnetically stirred home-made stainless steel autoclave
equipped with manometer, temperature controller and sampling
unit (50μL samples). 1H, 13C, 31P NMR spectra were recorded on
a Bruker Avance 400 MHz with broad band inverse detection
probe and a Bruker Avance 500 MHz with dual channel cryo
probe optimized for 13C/1H (DCH). Chemical shifts are reported
in ppm using TetraMethylSilane (TMS) as reference. All NMR
experiments were performed under inert atmosphere in airtight
Wilmad Young NMR quartz tubes. GC-analyses were performed
on a Shimadzu GC-2010 equipped with an Ultra-2 column.
Ammonia of purity grade N4.7 was used and was introduced to
the autoclaves using
controller (MFC). FT-IR spectra were recorded in situ on a
Shimadzu 8300 equipped with a home-made autoclave with a
a Bronkhorst liquiFlow mass-flow
200
180
160
140
120
100
Chemical Shift (ppm)
Figure 6: 13C NMR (125.73 MHz, 298K, CDCl3) spectrum of Ru(CpPh5)(CO)2Cl (1)
after refluxing in toluene for 24
cyclohexylamine.
built-in ZnS path length window. Complexes
1,
2[10c] and
h in the presence of 40 equivalents of
complex 3[12] were synthesized according to literature
procedures.
In addition, complex
experiments. Complex
3
was also subjected to the deactivation
showed initially less activity, though
General procedure for the splitting of secondary amines:
3
A 75 ml stainless-steel autoclave is charged with 2 mol% of the
appropriate complex and purged with Ar. Secondary amine and
t-amylalcohol were added via syringe. The autoclave is closed
and liquid NH3 was added via a mass flow controller (MFC). The
mixture was heated to 170°C for the 23.5 h time.
showed fairly good conversion at low NH3 loading. As the reason
of deactivation is clearer now, it might be that this complex just
deactivates much faster. On the other hand, it might still be
possible that the complex is just less active. In this case, also the
PPh3 can be monitored by means of 31P NMR. Again it was found
that a reaction occurs upon addition of primary amine. However,
it is also observed now that PPh3 remains coordinated (free PPh3
would show up at -5 ppm). The deactivation still occurs at a fairly
low temperature relative to the reaction temperature, though it
Procedure for the NMR reactions:
Reaction of complex
1 with cyclohexylamine:
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ARTICLE
Into a Wilmad-Young NMR tube, complex
mg) was weighed and dissolved in CDCl3. H and 13C NMR
1 (0.073 mmol, 46.6
1
Notes and references
a
Dr. D.L.L. Pingen, Chemical Materials Science, Department of
Chemistry, University of Konstanz, Universitätsstrasse 10, 78457
Konstanz, Germany
spectra were recorded and after that, 10 eq. cyclohexylamine
1
(0.73 mmol, 63 μL) were added. Again, both H and 13C NMR
b
were recorded and the mixture was left at room temperature for
18h. After this time, both 1H and 13C were recorded. The mixture
was heated to 60ºC for 1 h.
Prof. Dr. D. Vogt, EaStCHEM, School of Chemistry, University of
Edinburgh, King’s Buildings, Joseph Black Building, West Mains Road,
Edinburgh EH9 3JJ, Scotland, UK.
[∗] Prof. Dr. D. Vogt, E-mail: D.Vogt@ed.ac.uk
[#] Authors contributed equally to the work
Reaction of complex
amine:
1
with secondary amine and primary
(0.09 mmol, 32.7
Supporting information for this article is available on the WWW under
http://www.xxx.yyy or from the author
In a Wilmad-Young NMR tube, complex
mg) was weighed and dissolved in CDCl3. H and 13C NMR
spectra were recorded and after that, 10 eq. dicyclohexylamine
(0.5 mmol, 99.4 μL) were added. Again, both H and 13C NMR
were recorded and the mixture was heated to 60ºC for 4h. Spectra
were recorded again and the mixture was continued to heat at
60ºC for 4 days. 1H and 13C NMR revealed no change.
1
1
1
2
R. J. Palmer, in Encyclopedia of Polymer Science and Technology,
John Wiley & Sons, Inc., 2002
.
1
K. Eller, E. Henkes, R. Rossbacher, H. Höke, in Ullmann's
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Co. KGaA, 2000
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A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411-2502.
a) K. S. Hayes, Appl. Catal., A 2001, 221, 187-195; b) S. A. Lawrence,
Amines: Synthesis, Properties and Applications., Amines: Synthesis,
Properties and Applications, 2004; c) A. Simplício, J. Clancy, J.
Gilmer, Molecules 2008, 13, 519-547.
Reaction of complex
1
with ammonia and cyclohexylamine:
(0.09 mmol, 57
In a 15 mL stainless steel autoclave, complex
1
mg) was placed. CDCl3 was added to dissolve the mixture and
NH3 (2.5 mL, 90 mmol) was subsequently added via a Mass
Flow controller. The autoclave was closed and heated to 170ºC
for 1.45 h. After cooling to room temperature, the resulting
5
6
C. Gunanathan, D. Milstein, Angew. Chem. Int. Ed. 2008, 47, 8661-
8664.
a) S. Imm, S. Bähn, L. Neubert, H. Neumann, M. Beller, Angew. Chem.
Int. Ed. 2010, 49, 8126-8129; b) S. Imm, S. Bähn, M. Zhang, L.
Neubert, H. Neumann, F. Klasovsky, J. Pfeffer, T. Haas, M. Beller,
Angew. Chem. Int. Ed. 2011, 50, 7599-7603.
1
mixture was transferred to a Wilmad-Young NMR tube and H
and 13C NMR were recorded. After that, 10 eq. cyclohexylamine
(0.9 mmol, 89 mg, 0.1 mL) were added. Again, both 1H and 13
C
7
a) D. Pingen, C. Müller, D. Vogt, Angew. Chem. Int. Ed. 2010, 49,
NMR were recorded and the mixture was left at room
temperature for 18h. After this time, both H and 13C were
recorded.
8130-8133; b) D. Pingen, O. Diebolt, D. Vogt, ChemCatChem 2013
5, 2905-2912.
,
1
8
9
S. Bähn, S. Imm, L. Neubert, M. Zhang, H. Neumann, M. Beller,
Chem. Eur. J. 2011, 17, 4705-4708.
Reaction of complex
3 with cyclohexylamine:
J. Paetzold, J. E. Backvall, J. Am. Chem. Soc. 2005, 127, 17620-17621.
In a Wilmad-Young NMR tube, complex
mg) was weighed and dissolved in CDCl3. H and 13C NMR
3
(0.05 mmol, 39.1 10 a) G. Csjernyik, K. Bogár, J.-E. Bäckvall, Tetrahedron Lett. 2004, 45,
1
6799-6802. b
) B. Martín-Matute, M. Edin, K. Bogár, J.-E. Bäckvall,
Angew. Chem. Int. Ed. 2004, 43, 6535-6539; c) B. Martín-Matute, M.
spectra were recorded and after that, 11.4 eq. cyclohexylamine
1
Edin, K. Bogár, B. Kaynak, J.-E. Bäckvall, J. Am. Chem. Soc. 2005
127, 8817-8825.
,
(65 μL, 0.57 mmol) were added. Both H and 13C NMR were
recorded and the mixture was left at room temperature for 18h.
After this time, both 1H and 13C NMR were recorded. The
mixture was heated to 90ºC for 6h. Again, both 1H and 13C NMR
were recorded. The mixture was heated again to 90ºC for 2 days
before recording the spectra again.
11 M. C. Warner, J.-E. Bäckvall, Acc. Chem. Res. 2013, 46, 2545-2555.
12 S.-B. Ko, B. Baburaj, M.-J. Kim, J. Park, J. Org. Chem. 2007, 72,
6860-6864.
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14 D. Hollmann, H. Jiao, A. Spannenberg, S. Bähn, A. Tillack, R. Parton,
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Organometallics 2002, 21, 2101-2106. b) M. Kanthak, A. Aniol, M.
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215-229. c) E. A. Nyawade, H. B. Friedrich, B. Omondi, Inorg. Chim.
Acta, 2014, 415, 44-51.
In situ FT-IR monitoring
In a home-made stainless steel autoclave equipped with a ZnS
path length cell, 6 mL CHCl3 (dry degassed) was placed and a
background was recorded for further use. At the same time,
complex 1 (0.1 mmol, 63.8 mg) was weighed into a Schlenk tube
and dissolved in dry degassed CHCl3. The autoclave was
emptied and dried before purging it with Ar again. The solution
16 A. Scherer, T. Mukherjee, F. Hampel, J. A. Gladysz, Organometallics
2014, 33, 6709-6722.
of complex
1 was transferred to the autoclave and a spectrum
was recorded again. After this, cyclohexylamine (1 mmol, 115
μL) was added and the autoclave was sealed and heated to 40ºC.
Spectra were recorded with 15 minutes time intervals.
17 A. Aballay, G. E. Buono-Core, F. Godoy, A. H. Klahn, A. Ibañez, M.
T. Garland, J. Organomet. Chem. 2009, 694, 3749-3752.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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DOI: 10.1039/C6DT01525E
Dalton Transactions
Page 8 of 9
ARTICLE
Journal Name
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,
8 | J. Name., 2012, 00, 1-3
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Page 9 of 9
Dalton Transactions
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DOI: 10.1039/C6DT01525E
A Ruthenium Racemisation Catalyst for Synthesis of Primary Amines from Secondary
Amines
Highly selective splitting of secondary amines to primary amines with ammonia was achieved
with pentaphenyl-Cp complexes of Ru, previously reported as racemization catalysts for alcohols
and amines.
H
R1
N
R2
R1
N
R2
Ph
Ph
H
dehydrogenation
Ph
NH3
Cl
CO
Ph
Ru
Ph
[Ru]
[RuH2]
OC
R1 NH2
H
hydrogenation
R1, R2 = alkyl
R2
HN
HHN
R2