The Rhodium Catalysed Direct Conversion of Phenols to Primary Cyclohexylamines

Article abstract of DOI:10.1002/cctc.201800486

Cyclohexylamines are important intermediates in chemical industry, which are currently produced from petrochemical sources. Phenols, however, are an attractive sustainable feedstock. We here demonstrate the transformation of phenols with ammonia to primary cyclohexylamines. In contrast to previously reported chemistry which used palladium catalysts, we here show that rhodium is an excellent catalyst for the formation of primary cyclohexylamines. Different parameters were studied and it was shown that the reaction is applicable to a scope of phenolic compounds providing high selectivity.

Full text of DOI:10.1002/cctc.201800486

Accepted Article
Title: The Rhodium Catalysed Direct Conversion of Phenols to Primary
Cyclohexylamines
Authors: Patrick Tomkins, Carlot Valgaeren, Koen Adriaensen,
Thomas Cuypers, and Dirk De Vos
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10.1002/cctc.201800486
ChemCatChem
COMMUNICATION
The Rhodium Catalysed Direct Conversion of Phenols to Primary
Cyclohexylamines
Patrick Tomkins, Carlot Valgaeren, Koen Adriaensen, Thomas Cuypers, Dirk E. De Vos*
Abstract: Cyclohexylamines are important intermediates in chemical
industry, which are currently produced from petrochemical sources.
Phenols, however, are an attractive sustainable feedstock. We here
demonstrate the transformation of phenols with ammonia to primary
cyclohexylamines. In contrast to previously reported chemistry which
used palladium catalysts, we here show that rhodium is an excellent
catalyst for the formation of primary cyclohexylamines. Different
parameters were studied and it was shown that the reaction is
applicable to a scope of phenolic compounds providing high selectivity.
prone to secondary amine formation, which makes the formation
of primary amines much more challenging. In patent literature, this
transformation was investigated only to a limited extent, e.g. by
using Pd-based catalysts, resulting in dicyclohexylamine as main
product and up to 30 wt% cyclohexylamine. [12]
Aliphatic amines are an important class of organic compounds.[1]
Of special interest are aliphatic di- or polyamines which can be
used for the production of aliphatic polyurethanes.[2] There are
various methods for the production of primary amines, such as
the hydrogenation of nitriles,[3] hydroamination,[4] reductive
[5]
amination
or alcohol amination via hydrogen borrowing.[6]
However, most of the starting materials are of fossil origin and
sustainable alternatives are needed. Renewable resources are
often rich in oxygen and defunctionalisation is needed before
further processing.[7] Recently, significant effort has been devoted
to convert phenols, or their derivatives, which can be obtained
from lignocellulose, to the corresponding amines. One approach
is the cross-coupling of phenyl pivalate using molecular nickel
catalysts with an N-heterocyclic carbene ligand to form N,N-
disubstituted anilines under inert conditions; however, simpler
phenol derivatives were unreactive.[8] Most recently a strategy
was proposed in which phenol is first reduced to cyclohexanone,
which easily reacts with an alkylamine towards the imine and
further is either hydrogenated towards N-substituted
cyclohexylamines or dehydrogenated towards N-substituted
anilines (Scheme 1).[9] In such reports, sodium formate is typically
used as a reductant, which leads to formation of stoichiometric
CO2 and necessitates neutralisation of the NaOH formed. Only
one report by Beller uses H2;[9c] but besides the Pd/C catalyst, Hf
triflate is needed as a specific co-catalyst which cannot be
recovered intact. Nevertheless, use of H2 would be much
favoured in industrial context.[10] In all these cases the products
are secondary or tertiary amines, which can also be obtained by
reaction of cyclohexylamines or other amines with
cyclohexanone.[11] Primary amine products, however, are highly
Scheme 1. Strategies for the ‘ipso’-amination.
We herein report the direct and selective conversion of phenols to
primary cyclohexylamines. Although palladium based catalysts
were shown to be most suitable for the conversion of phenol to N-
substituted anilines and cyclohexylamines, Pd/C was shown to
perform poorly and Rh/C appeared as the ideal catalyst for this
conversion with 2-propanol as an excellent solvent. High
selectivities were shown to be possible not only for phenol itself,
but also for a range of other phenolic compounds.
Inspired by prior reports,[9a, 9b] we first studied the amination of
phenol in toluene using 10 mol% Pd/C at 140 °C. An excess of
ammonia was added at 2 bar H2 and a short reaction time (30 min)
was chosen to limit the consecutive formation of secondary amine
from the imine (Scheme 2).[13] Although high conversion was
reached (86 %),
a majority of the condensation product
dicyclohexylamine was formed, which was also observed in
literature.[12] Under identical conditions, however, Pt/C and Rh/C
showed higher selectivities toward cyclohexylamine (79 % and
99 %), albeit at lower conversions (38 % and 26 %, respectively).
Ru/C did not appear to be active in this reaction. Higher
conversions (64 % for Pt/C and 98 % for Rh/C) were reached by
increasing the H2 pressure to 6 bar. Selectivity toward
cyclohexylamine decreased in case of Pt/C (63 %) due to
increased secondary amine formation, while it remained excellent
for Rh/C (98 %). Different amounts of Rh/C were employed in the
reaction (Table 1). After 6 h, 1 mol% Rh resulted in a conversion
of 57%, while 2 mol% Rh and 5 mol% Rh gave 90% and 97%
conversion, respectively. All catalyst loadings resulted in excellent
selectivities, with only some secondary amines and
P. Tomkins, C. Valgaeren, K. Adriaensen, T. Cuypers, Prof. Dr. D.
E. De Vos
Centre for Surface Chemistry and Catalysis, Department of
Microbial and Molecular Systems
KU Leuven
Celestijnenlaan 200F, 3001 Leuven, Belgium
E-mail: dirk.devos@kuleuven.be
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201800486
ChemCatChem
COMMUNICATION
cyclohexanone as side products. Hydrogenolysis was not
observed.
Thus, at least the route via cyclohexylamine dehydrogenation
seems to be effectively blocked when using Rh/C.
Different solvents were tested using 2 mol% Rh/C (Table 2). The
solvents must be stable in hydrogenative conditions and at least
slightly polar to sufficiently dissolve ammonia. 1,4-Dioxane did not
show any reaction. Other solvents, such as THF or pure methanol
resulted in lower conversions than toluene (48 % and 69 %
respectively, vs. 90 % for toluene under these conditions), while
maintaining excellent selectivity of 96 %. In contrast, 2-propanol
provided high reactivity, with full conversion at 95 % selectivity
after 6 h (62 % conversion at 0.25 mol% after 2 h). Note that
methanol is present in all cases as co-solvent, and likely
increases ammonia solubility. Using 2 mol% Rh/C in 2-propanol,
the reaction was already completed after 30 min and selectivity
remained high up to the longest investigated reaction time, i.e. 6 h
(Figure S1). The reaction proceeds at temperatures as low as
56 °C (6 h) or 75 °C (30 min) to nearly complete conversion with
≥90 % selectivity (Figure S2). The selectivity decreased at low
temperature due to incomplete conversion of cyclohexanone to
cyclohexylamine, indicating that higher temperatures are needed
for efficient imine formation in the absence of a specific acid
catalyst. At 56 °C, 31 % conversion were reached after 0.5 h
using 2-propanol, whereas a mixture of 2-propanol and toluene
(20/80) resulted in a reduced conversion of 12 %, indicating that
toluene somewhat decreases the reactivity.
Table 1. Reaction of phenol (0.2 mmol) with ammonia (14 eq in 0.4 mL
methanol) in toluene (1 mL) using the defined catalyst (10 mol%) at the given
H2 pressure and 140 °C for 30 min. X = conversion, S(CHA) = selectivity for
cyclohexylamine, based on phenol converted.
Catalyst
p(H2) [bar]
X [%]
S(CHA)
[%]
Pd/C
Pt/C
2
86
38
26
1
59
79
99
-
2
Rh/C
2
Ru/C
2
Pt/C
6
64
98
57
90
97
63
98
98
97
97
Rh/C
6
Rh/C [a]
Rh/C [b]
Rh/C [c]
6
6
6
[a]
[b]
[c]
6 h using 1 mol % Rh/C.
6 h using 2 mol % Rh/C. 6 h using 5 mol %
Rh/C.
Table 2. Results for the reaction of phenol (0.2 mmol) with ammonia (14 eq in
0.4 mL methanol) using Rh/C (2 mol%) in the given solvent (1 mL) at 6 bar
hydrogen and 140 °C for 6 h. X = conversion, S(CHA) = selectivity for
cyclohexylamine.
Solvent
1,4-Dioxane [a]
THF
Time [h]
X [%]
0
S(CHA) [%]
6
6
-
48
96
96
95
96
97
91
94
Methanol
6
69
2-Propanol
2-Propanol
2-Propanol[b]
2-Propanol[c]
6
100
100
62
Scheme 2. Steps in the amination of phenol to cyclohexylamine.
A Lewis base (LB) assists as a proton acceptor, while a proton
donor is needed as well. Both functions can be combined in
alcoholic solvents.
0.5
2
0.5
0.5
31
The surprisingly high selectivity of Rh/C compared to that of Pd/C
can either be due to faster hydrogenation of the reactive imine
intermediate to the desired cyclohexylamine (Scheme 2, Step 5)
and/or to a decreased availability of reactive imine species at the
catalyst surface. Note that the imine could also be formed by
dehydrogenation of the cyclohexylamine product, even under a
small pressure of H2. While aniline was not detected, N-
phenylcyclohexylamine was indeed observed, though only in
trace amounts (0.5 % for Pd/C, ≤ 0.2 % for Rh/C). This indicates
that dehydrogenation and re-aromatization are not very significant
under the reaction conditions employed. When employing
cyclohexylamine as the substrate instead of phenol in the reaction,
Pd/C converted 9 % cyclohexylamine to dicyclohexylamine, while
this was not observed at all for Rh/C after 6 h of reaction. This
indicates that dehydrogenation is indeed kinetically more feasible
on Pd/C than on Rh/C, even under identical hydrogen pressure.[14]
2-Propanol/
12
Toluene (20/80)[c]
tert-Butanol[c]
0.5
30
95
[a] 5 mol% Rh/C, [b] 0.25 mol% Rh/C, 7 eq ammonia, [c] The reaction was
carried out at 56 °C.
Some additional solvents were tested, in order to better
understand the role of 2-propanol. tert-Butanol gave an almost
identical yield compared to 2-propanol. This proves that 2-
propanol is not assuming itself the role of hydrogen transfer agent.
Related alcoholic solvents resulted in high reaction rates at low
temperature (Table S2), while aprotic polar solvents, such as THF
or dioxane (Table 2) did not provide such high activity. Thus, the
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10.1002/cctc.201800486
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COMMUNICATION
proton donating and accepting properties of the alcoholic solvents
must be responsible for this behaviour. In fact, steps 2-4 (Scheme
2) require either proton donation or abstraction, in which the
alcohol solvents are likely involved. Methanol did not result in a
high yield, which may be explained by insufficient basicity as
evidenced by the lower basicity according to Catalán (SB=0.545
vs. > 0.65 for the other investigated alcohols).[15] Therefore, it
poorly accepts H-bonds in cases where a proton needs to be
removed, e.g. in the imine formation (Scheme 2, step 4).
The amount of ammonia has a large effect on the selectivity
(Figure 1). When only small amounts of ammonia (1.75 eq) were
added to the reaction mixture in 2-propanol as solvent,
condensation towards dicyclohexylamine and incomplete
conversion of intermediately formed cyclohexanone were
observed rather than cyclohexylamine (CHA) formation. With
substituent generally decreases the reaction rate, its position has
only a minor effect on the rate, as is also observed for
hydrogenation of phenols on the more conventional Pd
catalysts.[17] Logically, p-tert-butylphenol is severely sterically
hindered and only converted to a small extent, though selectively
(entry 8). Ammonia is a small reactant; thus the imine formation
from the corresponding cyclohexanone should not be rate limiting.
Rather, the hydrogenation of the phenol to cyclohexanone seems
slower with increasing size of substituent, which can be
rationalized by the hindered adsorption of the substituted phenol
ring on the Rh metal, which is needed for its hydrogenation. For
all alkylphenols, the cis-substituted alkylcyclohexylamines were
formed predominantly, which is a similar trend as observed in the
alkylaniline hydrogenation using Rh/SiO2.[18]
Guaiacol, as a more functionalized bio-based molecule was
transformed to the corresponding 2-methoxycyclohexylamine
with 72 % conversion at 80 % selectivity, without loss of the
methoxy group (entry 9). In the milder conditions, the selectivity
of the guaiacol conversion is even very high.[17] Inspired by a
recent report,[9c] benzyl phenyl ether was employed in the reaction
(entry 10); it gave cyclohexylamine in high yield (88 %); toluene
was formed as co-product. The amination of bisphenol F (entry
11) resulted, in the initial mild conditions, in the asymmetric 4-((4-
aminocyclohexyl)methyl)phenol at a conversion of 30 % and with
91 % selectivity. Under somewhat optimized reaction conditions
(5 mol% Rh/C and 16 bar H2 at 110 °C for 16 h), the fully
hydrogenated diamine H12-MDA (4,4’-diaminodicyclohexyl
methane) was obtained in 68 % yield.
increasing
ammonia
concentration,
the
amount
of
cyclohexylamine formed gradually increased. The maximum
cyclohexylamine yield was reached at the highest investigated
ammonia concentration, i.e. 14 eq with respect to phenol. Note
that in an industrial setting, it would be easy to evaporate and
recycle the excess ammonia. Compared with the reaction in
toluene (Figure S3), more CHA is formed in the reaction in 2-
propanol, at the same, low, ammonia concentration (Figure 1). At
1.75 eq ammonia, 38 % CHA were formed in case of isopropanol
as solvent, while none was detected in case of toluene. This can
be explained by the excellent solubility of ammonia in alcoholic
solvents.[16]
100
80
Table 3. Results for the reaction of phenolics (0.2 mmol) with ammonia (14 eq
in 0.4 mL methanol) in the given solvent (1 mL) at 6 bar H2 and 140 °C for 6 h.
X = conversion, S = selectivity towards the product.
Phenol conversion
60
Cyclohexanone
Cyclohexylamine
40
Dicyclohexylamine
20
0
Substrate
Product
X [%]
S [%]
0
2
4
6
8
10
12
14
16
Ammonia added [eq]
1
2
Figure 1. Influence of the ammonia concentration on the reductive amination of
phenol. The reaction was carried out for 6 h using Rh/C (2 mol%), phenol
(0.2 mmol), ammonia (14 eq in 0.4 mL methanol) in 2-propanol (1 mL) at 6 bar
H2 and 140 °C for 6 h.
>98[a]
54[a]
(c:t 71:29)[b]
Different phenols were tested in the reaction to investigate the
effect of substituents on the reactivity and to prove the generality
of the Rh-catalyzed approach (Table 3). The three cresol isomers
were converted to the corresponding methylcyclohexylamines in
54 % to 66 % yield with excellent selectivity (entries 1-3; 5 mol%
Rh/C, 16 bar H2, 16 h). Under the same conditions, high
conversions of 75 % to 77 % were reached for 2- and 4-
ethylphenol, and for the bio-based 4-n-propylphenol, at ≥ 98 %
selectivity (entries 4, 6 and 7). Reactions with 2-, 3- and 4-
ethylphenol were also performed under milder conditions (2 mol%
Rh/C, 6 bar H2, 6 h). These conditions would yield 95 %
cyclohexylamine from unsubstituted phenol. For the 2-, 3- and 4-
ethylphenols, conversions amounted to 26 %, 20 % and 27 %,
respectively (entries 4-6). This shows that while the alkyl
>98[a]
61[a]
(c:t 71:29)[b]
3
>98[a]
66[a]
(c:t 73:27)[b]
4
>98
26
(c:t 80:20)[b]
>98[a]
75[a]
(c:t 75:25)[a, b]
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10.1002/cctc.201800486
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COMMUNICATION
hydroxycyclohexylamine in 89 % yield via the formation of 4-
hydroxycyclohexanone.
The
corresponding
Substrate (cont.)
Product
X [%]
S [%]
4-aminocyclohexylamine was only formed in 5 % yield. This
compound is likely formed by intermediate dehydrogenation of the
secondary alcohol and subsequent amination, since the
nucleophilic substitution at the alcohol is unlikely. 4-Nitrophenol
was converted under identical conditions as hydroquinone (entry
13). The nitro group was hydrogenated; 68 % was converted to
the diamine, while 31 % of the 4-hydroxycyclohexylamine was
formed. No nitro intermediates were observed. This indicates that
the hydrogenation of 4-nitrophenol proceeds via the
4-hydroxycyclohexylimine, which is further transformed to
4-hydroxycyclohexylamine. The diamine is formed via
4-aminocyclohexanone as intermediate.
In conclusion, the amination of phenol towards cyclohexylamine
was first investigated over different metal catalysts. Prior studies
for related reactions focused on Pd-based catalysts, which
showed much lower selectivity towards primary amines in this
reaction. Surprisingly Rh/C showed remarkable activity and
selectivity. A parametric study revealed that isopropanol was an
outstanding solvent because of its protic character and polarity.
Combined with a sufficient amount of ammonia, high yields of
cyclohexylamine are possible at low temperature and after short
reaction time. Further, the applicability to a range of phenols was
proven, whereby excellent selectivity was reached. These results
do not only allow for the selective production of cyclohexylamines,
but will also inspire the investigation of new catalysts for related
reactions.
5
>98
20
(c:t 90:10)[b]
6
>98
27
(c:t 90:10)[b]
>98[a]
76[a]
(c:t 80:20)[a,b]
7
8
>98
(c:t 85:15)[b]
98[a]
21
77[a]
(c:t 77:23)[a,b]
>98
6
(c:t n.d.)[b]
9
>98
(c:t 84:16)[b]
80[a]
11
72[a]
(c:t 87:13)[a, b]
10
11
97 [d]
91 [d]
91 (A)[c]
(c:t n.d.)[c,b]
2 (B)[c]
Experimental Section
30[c]
In a typical experiment the catalyst, phenol (0.2 mmol), hexadecane
(10 µL), solvent (1 mL) and NH3 in methanol (7M, 0.4 mL, 14 eq) were
added into an 11 mL stainless steel autoclave, which was subsequently
purged with N2, and to which 4 bar of N2 and 6 bar of H2 were added. The
reaction mixture was stirred at an internal temperature of 140 °C and the
final reaction mixture was analysed by GC-FID and GC-MS after filtration.
31 (A)[d]
(c:t n.d.)[d]
68 (B)[d]
(cc:ct:tt
>99[d]
69/26/5)[d, e]i
12
X=OH
89 [f]
(c:t 86:14)[b, f]
X=NH2
5[e]
Acknowledgements
>99[e]
(c:t n.d.)[b, f]
P.T and T.C. thank Fonds Wetenschappelijk Onderzoek (FWO)
for doctoral fellowships, D.E.D.V. acknowledges the IWT and
FWO for research project funding, the Flemish government for
long-term structural funding through the Methusalem programme,
and EoS (FWO-FNRS) for financial support.
13
X=OH
31[e]
>99[e]
(c:t 85:15)[b, f]
X=NH2 68[e]
(c:t 44:56)[b, f]
Keywords: Phenol • Amination • Cyclohexylamine • Rhodium •
Reduction
[a]
5
mol% Rh/C (with respect to phenol moieties), 16 bar H2, 16 h.
[b] diastereomer ratio expressed as cis:trans [c] 0.1 mmol substrate. [d] 0.1 mmol
[e]
substrate, 110 °C, 5 mol% Rh/C, 16 bar H2, 16 h.
diastereomer ratio
expressed as cis-cis:cis-trans:trans-trans. [f] 3 h reaction with 7 eq ammonia.
[1]
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hydroxycyclohexylamine. The high reactivity allowed for full
conversion within the reaction time of 3 h, with production of 4-
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Patrick Tomkins, Carlot Valgaeren, Koen
Adriaensen, Thomas Cuypers, Dirk E.
De Vos*
Page No. – Page No.
The Rhodium Catalysed Direct
Conversion of Phenols to Primary
Cyclohexylamines
Phenols are an attractive sustainable feedstock for cyclohexylamine synthesis. We
here demonstrate the transformation of phenols with ammonia to primary
cyclohexylamines. In contrast to previously reported chemistry which used
palladium catalysts, we here show that rhodium is an excellent catalyst for the
formation of primary cyclohexylamines. Different parameters were studied and it
was shown that the reaction is applicable to a scope of phenolic compounds.
This article is protected by copyright. All rights reserved.