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Catalysis Science & Technology
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DOI: 10.1039/C8CY00193F
COMMUNICATION
Journal Name
In a first set of exploratory reactions, various carbon- [a] Reaction conditions: phenol (2 mmol) in toluene (20 mL), 5 mol% catalyst, 0.5
2 3
bar H and 2 bar NH for 24 h. [b] Conversions (X) and yields (Y) were determined
supported noble metal catalysts (5 mol% metal relative to
phenol) were screened for the amination of phenol in toluene
by GC-FID analysis with n-nonane as external standard. [c] Benzene yield of 24%.
at 140 °C, 0.5 bar H
). Solubility experiments showed that at this ammonia partial
pressure, about 0.9M of NH was dissolved in the phenol-
2 3
and 2 bar NH for 24 h (Table 1, entry 1-
Next, the use of other solvents was explored for the
amination of phenol at 200 °C (Table 2). Solvents must provide
sufficient capacity to dissolve ammonia and must remain
stable under the employed reaction conditions. Hence,
solvents of interest lack reactive functional groups – such as
reactive carbonyl group(s) – or are unable to form such groups
under employed reaction conditions, for instance, via
dehydrogenation of primary or secondary alcohols. The
presence of these groups in the reaction mixture could render
the solvent unstable and/or induce side product formation
through, for example, reductive amination with ammonia and
4
3
toluene mixture at room temperature (Figure S1). We
hypothesized that this excess of ammonia (i.e., 9 equivalent to
phenol) would be necessary to suppress formation of
secondary amines. At 140 °C, all carbon-supported catalysts
showed an incomplete conversion of phenol (≤ 44%).
However, aniline yield varied considerably with the transition
metal in following order: Ru < Rh < Pt < Pd. Both Ru/C and
Rh/C performed very poorly for the phenol-to-aniline
conversion, as the deeply hydrogenated cyclohexylamine was
the predominant reaction product in relatively high selectivity
2
0–23
amines.
Both γ-valerolactone and propylene carbonate
were tested as possible solvents. However, undesired ring-
2
4
(up to 81% for Rh/C) and aniline was only formed in very low
opening occurred in the presence of ammonia, making these
solvents unsuitable for the direct phenol-to-aniline amination.
For that reason, tert-amyl methyl ether (TAME), tert-amyl
alcohol (TAA) and 2-methyltetrahydrofuran (MeTHF) were
selected; these solvents are not readily dehydrogenated and
are stable in the presence of ammonia. Moreover, these
solvents have been proposed as greener alternatives for
yields (≤ 1 %). As outlined in Scheme 2, this can either be due
to fast imine hydrogenation to cyclohexylamine, decreased
formation of the imine intermediate from cyclohexylamine
and/or slow dehydrogenation to aniline, even if only a low H
2
partial pressure was applied to the reaction mixture. Pt/C and
Pd/C, on the other hand, are more suitable for this
transformation as considerable amounts of aniline were
observed with promising selectivities of 65% and 60%,
respectively. In view of the endothermic nature of the
dehydrogenation, the reaction temperature was increased for
the latter catalysts in an attempt to improve phenol
conversion and to shift the selectivity towards aniline (Table 1,
entry 5-8). As a result, aniline yield increased gradually with
increasing temperature up to 88% for Pd/C at 200 °C, while for
Pt/C it only improved slightly to 29%. This result for Pt/C can
be explained by a moderate phenol conversion and a
significant benzene side product formation (24% yield), via
cyclohexene as an intermediate (Scheme S1). The latter is
quite remarkable as benzene yield consistently remained
below 4% for the Pd-catalyzed reactions. The kinetic profile for
Pd/C revealed that the conversion of phenol steadily increased
over time from 24% to 94% at 1 h and 24 h, respectively
2
5,26
toluene.
Unfortunately, both phenol conversion and
aniline selectivity were insufficient in TAME. In contrast, a high
aniline yield of 82% was obtained for the direct phenol-to-
aniline amination in TAA. The difference in reactivity between
these solvents can be attributed to the protic nature of TAA,
which could help the reaction steps in which protons are
transferred (e.g., partial hydrogenation of phenol and
formation of the imine from cyclohexanone). Another
important factor is the higher solubility of NH
3
in TAA
compared to TAME (Figure S1). Lastly, MeTHF did not improve
aniline yield (61%) compared to the result in toluene.
In Scheme 2 a tentative mechanism is proposed for the
formation of the main reaction products and intermediates in
the direct phenol-to-aniline amination using Pd/C. First, phenol
(1) is partially hydrogenated to cyclohexanone (2), which
reacts quickly via a nucleophilic attack of ammonia and
subsequent dehydration of the formed hemiaminal into
(Figure S2). Moreover, aniline selectivity increased with time
up to 95% after only 4 h of reaction, whereafter it remained
constant. As a result, cyclohexylamine could only be observed
in significant yields shortly after the start of the reaction, e.g.,
cyclohexanimine (
very reactive and is either (in)directly dehydrogenated to
aniline ( ) or hydrogenated to cyclohexylamine ( ). However,
3). This imine intermediate is known to be
4
5
5
% after 1 h.
both cyclohexanimine and cyclohexanone are able to
participate in undesired condensation with either
cyclohexylamine or aniline into secondary amines, such as
[
a]
Table 1 Screening of noble metal catalysts for the amination of phenol to aniline.
[b]
[b]
Entry
Catalyst
Ru/C
Rh/C
Pt/C
T [°C]
140
140
140
140
160
180
200
200
X [%]
Y [%]
<1
1
dicyclohexylamine (
subsequent dehydrogenation – diphenylamine (
noted that, even in the
of H
6), N-cyclohexylaniline (7) and – after
1
2
3
4
5
6
7
8
12
8
). It should be
presence
16
33
22
25
54
78
88
2
, the imine could be formed through dehydrogenation of
Pd/C
Pd/C
Pd/C
Pd/C
Pt/C
43
cyclohexylamine. This resulted in the formation of significant
69
92
94
[
c]
53
29
2
| J. Name., 2012, 00, 1-3
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Cuypers, Thomas
