Palladium-Catalyzed Synthesis of N-Cyclohexyl Anilines from Phenols with Hydrazine or Hydroxylamine via N-N/O Cleavage

Article abstract of DOI:10.1002/adsc.201700712

Direct access to amines from biomass-based phenols via deoxygenative transformation remains greatly challenging in organic synthesis. Herein, we present a palladium-catalyzed deoxygenative amination of phenols (and their benzyl ether) with hydrazine as nitrogen source. The hydroxylamine/formic acid can be substituted for hydrazine in some cases. This deoxyamination features the involvement of a complex C?O bond and N?N/O bond-cleavage process and allows for the construction of N-substituted cyclohexyl anilines from an array of phenols by finely controlling the reaction conditions in moderate to good yields. (Figure presented.).

Full text of DOI:10.1002/adsc.201700712

Title: Palladium-Catalyzed Synthesis of N-Cyclohexyl Anilines from
Phenols with Hydrazine or Hydroxylamine via N-N/O Cleavage
Authors: Jiang-Sheng Li, Zihang Qiu, and Chao-Jun Li
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700712
Link to VoR: http://dx.doi.org/10.1002/adsc.201700712

10.1002/adsc.201700712
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium-Catalyzed Synthesis of N-Cyclohexyl Anilines from
Phenols with Hydrazine or Hydroxylamine via N-N/O Cleavage
Jiang-Sheng Li,ab†Zihang Qiua† and Chao-Jun Lia*
a
Department of Chemistry and FQRNT Centre for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke
Street West, Montreal, Quebec H3A0B8, Canada;
Fax: (+1)-5143983797; Phone: (+1)-5143988457; e-mail: cj.li@mcgill.ca
School of Chemistry and Biological Engineering, Changsha University of Science & Technology, Changsha 410114,
b
China
† J.S. Li and Z. Qiu contributed equally
Received:((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
reaction efficiency. Phenols are abundant and
phenols via deoxygenative transformation remains greatly naturally
occurring motifs in renewable
Abstract: Direct access to amines from biomass-based
challenging in organic synthesis. Herein, we present a
palladium-catalyzed deoxygenative amination of phenols
(and their benzyl ether) with hydrazine as nitrogen source.
The hydroxylamine/formic acid can be substituted for
hydrazine in some cases. This deoxyamination features the
involvement of a complex C-O bond and N-N/O bond-
cleavage process and allows for the construction of N-
substituted cyclohexyl anilines from an array of phenols by
finely controlling the reaction conditions in moderate to
good yields.
lignocellulosic biomass, and are important precursors
of aryl or cyclohexyl groups (Scheme 1, b).[15] In
2015, our group developed a Pd-catalyzed reductive
coupling of phenols with amines using sodium
formate as a convenient hydrogen source to produce
anilines or cyclohexylamines.[7,12] Later, Taddei
accomplished this transformation in a flow reactor.[8]
In 2016, Beller reported
a
Pd-catalyzed
deoxygenative coupling of phenols or aryl ethers to
generate alkylated cyclohexylamines with Lewis acid
Hf(OTf)4 as co-catalyst under molecular H2,[16] and
Fu’s group further investigated N-cyclohexylation of
amines with phenols using Al2O3 supported palladium
hydride (PdHx) catalyst.[17]
Keywords: Phenols; Amines; Deoxygenation; N-N/O
cleavage; Hydrazine; Palladium
The N-N bond cleavage of hydrazines has recently
been utilized as a strategy for the formation of C-N
Arylamines serve as important structural units in
pharmaceuticals, pigments and functional materials.[1]
They are also versatile intermediates in numerous
transformations[2] such as C-halo (the Sandmeyer and
Schiemann reactions) or C-N (the Buchwald-Hartwig,
the Chan-Lam and the Ullmann reactions) bond
formations. Very recently, N-cyclohexyl anilines have
been used as ligand to promote dehydrogenative
transformations,[3] and as antioxidant in food
chemistry.[4] To date, a series of elegant methods for
the synthesis of N-cyclohexyl anilines have been
developed, which can be divided into two categories
(Scheme 1, a): one involves the use of anilines as
amino source,[5] with coupling partners including
arylboronic
acids,[6]
phenols,[7]
anilines,[8]
cyclohexanones[9] or cyclohexanols;[10] and the other
relates to the use of cyclohexylamines as precursors,
with coupling partners covering various aryl sources
such as haloarenes,[11] phenol[12] and its derivatives,[13]
arylboronic acids[6] and aryl Grignard reagents.[14]
However, most of these methods require the
prefunctionalization, thereby decreasing the total
Scheme 1. Approaches to access N-cyclohexyl anilines.
1
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
Table 1 Optimization of the reaction conditions.[a]
bond through the transition-metal-catalyzed C-H
bond functionalization.[18] Indeed, it has earlier been
documented that hydrazines served as a nitrogen
source with carbonyl compounds to prepare amines
via catalytically hydrogenative N-N cleavage.[19] Yet
to our knowledge, there seems to be no report
regarding the synthesis of amines from phenols with
hydrazine via the cleavage of C-O and N-N bonds.
Herein, we present an efficient palladium-catalyzed
direct deoxygenative coupling of phenols with
hydrazine or hydroxylamine as nitrogen atom source
via C-O bond and N-N/O bond cleavages (Scheme 1,
c).
Yield[%]
2a 3a
51 trace
20
45
4
Temp.
[oC]
Entry Hydrazine
Solvent
.
1
N2H4 H2O
N2H4
toluene
toluene
120
120
120
120
120
120
120
2[b]
3
0
5
0
0
0
0
0
0
0
1
1
0
9
N2H4 hydrate toluene
.
4
5
6
7
8
9
N2H4 HCl
toluene
toluene
toluene
THF
.
N2H4 2HCl
4
.
N2H4 H2SO4
38
49
5
.
The initial exploration of this deoxygenative
amination was carried out using phenol 1a and
hydrazine monohydrate as coupling partners in the
presence of Pd/C as the catalyst and HCO2Na as the
hydride donor source. We are delighted to obtain 51%
yield of N-cyclohexyl aniline 2a only with trace
amount of 3a (Table 1, entry 1). Other hydrazine
sources such as its THF solution (1.0 M), its hydrate,
mono or dihydrochloride, and sulfate were also
attempted (Table 1, entries 2-6). It was found that all
other hydrazine reagents tested delivered lower yields
compared with the monohydrate. Then, the solvent
effect was tested. The reaction performed in THF
gave higher yield than in 1,4-dioxane (Table 1, entries
7 vs 8), but still slightly lower than in toluene. When
water was used as the solvent, it resulted in only a
trace amount of the product (Table 1, entry 9).
Changing the amount of Pd/C or using a different
Pd/C catalyst (5 wt% Pd loading) could not improve
the reaction yields (Table 1, entries 10-12). The yield
of 2a stayed nearly constant while 3a was formed in
N2H4 H2O
.
N2H4 H2O
1,4-dioxane 120
.
N2H4 H2O
H2O
120
120
120
120
110
140
160
160
160
160
150
150
150
6
.
10[c]
11[d]
12[e]
13
14
15
16[f]
17[g]
18[h]
19[g]
N2H4 H2O
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
11
44
23
20
52
48 18
56 22
70 12
56 25
75 10
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
N2H4 H2O
.
20[g] [i] N2H4 H2O
90
81
5
7
.
21[g] [j] N2H4 H2O
[a] Reaction conditions: phenol (0.2 mmol), Pd/C (10 wt%
loading) (10 mol%), hydrazine (0.75 equiv), HCO2Na (3.0
equiv), TFA(7.5 µL, 0.5 equiv) in solvent (1.0 mL) under
Ar for 24 h unless otherwise stated. [b] N2H4 in THF (1.0
M). [c] Pd/C (10 wt% loading) (5 mol%). [d] Pd/C (10 wt%
loading) (20 mol%). [e] Pd/C (5 wt% loading) (10 mol%). [f]
HCO2Na (2.5 equiv). [g] HCO2Na (2.0 equiv). [h] HCO2Na
(1.5 equiv). [i] TFA (15 µL, 1.0 equiv). [j] TFA (22 µL, 1.5
o
9% yield when the reaction was conducted at 140 C
(Table 1, entry 14). Further elevating the temperature
1
equiv). Yields are calculated based on H NMR spectra
o
to 160 C decreased the yield of 2a but generated
data.
more 3a (Table 1, entry 15). It was found that the
reaction was much influenced by the amount of
o
HCO2Na. Optimization at 160 C indicated that 2.0
moderate yields, respectively (Table 2, entries 5 & 6),
while ortho-ethylphenol is an ineffective substrate.
Unfortunately, other phenols with bulky alkyl groups
such as iso-propyl, tert-butyl are sluggish under the
optimal conditions. These results suggested that the
deoxyamination efficiency depends greatly on the
steric property of substituents on the phenols. To our
surprise, however, hydroxylbenzoates could proceed
well in such a transformation. For example, methyl
equivalent of HCO2Na gave higher yield of 2a (Table
1, entry 16-18). Changing the reaction temperature to
o
150 C improved the yield slightly (Table 1, entry
19). Changing the TFA amount to 1.0 equiv gave a
better yield (90%) (Table 1, entries 20 vs 21). Thus,
the optimal reaction conditions were established as
follows: phenols (1.0 equiv) reacted with hydrazine
monohydrate (0.75 equiv) in the presence of Pd/C (10
wt% Pd loading, 0.1 equiv) as the catalyst, HCO2Na
(2.0 equiv) as a reductive reagent, and TFA (1.0
para-hydroxylbenzoate
1g
was
successfully
converted into its corresponding cis/trans amines 2g
in 78% total yield. The bulky ethyl ester 1h also gave
the desired products 2h in 80% total yield. However,
the much bulkier phenyl para-hydroxylbenzoate was
found to be partially cleaved with phenol being
detected. When the ester group was anchored on the
meta position, the reaction worked well, and afforded
a 72% total yield of the N-cyclohexylamines 2i
(Table 2, entry 9). However, when the methyl, ethyl
or phenyl salicylates were used instead, negative
o
equiv) as an additive in toluene (0.2 M) at 150 C
under argon for 24 h, providing the 90% yield (85%
isolated yield) of N-cyclohexyl aniline 2a.
With the optimized reaction conditions in hand, we
set out to investigate the scope of the phenols.
Generally speaking, less hindered phenols such as
cresols proceeded smoothly under the standard
conditions, furnishing their corresponding cis/trans
isomeric products in good yields (Table 2, entries 2-
4). Para-and meta-ethylphenol 1e and 1f delivered
the corresponding desired amines 2e and 2f in
results
were
obtained.
Likewise,
the
hydroxylphenylacetates
showed
comparable
2
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
Table 2. Pd-catalyzed deoxygenative amination of phenols with hydrazine monohydrate as N source.[a]
.
[a] Reaction conditions: phenol (0.2 mmol), NH2NH2 H2O (8 µL, 0.15 mmol), Pd/C (10 mol% Pd), HCO2Na (27.2 mg, 0.4
mmol), TFA (15 µL, 0.2 mmol), toluene (1 mL) under Ar at 150 oC. Yields were for isolated cis/trans mixtures. The ratio
was determined by 1H NMR analysis of the crude products.
reactivities to the benzoate analogues. For instance,
methyl para- and meta-hydroxylphenylacetates 1j
and 1k reacted with hydrazine to successfully provide
the desired amines 2j and 2k, respectively, but the
efficiency slightly decreased (Table 2, entries 10 &
11). The ortho-hydroxylphenylacetate analogues gave
complex mixtures, along with the formation of the
cyclization product benzofuran-2(3H)-one. It is
worthwhile to note that, in all cases of the esters, no
hydrazination or amidation of the ester group
occurred under the current conditions. Furthermore,
pure cis- and trans-isomers of these secondary
amines except 2f could be isolated readily by
chromatography
method,
and
individually
3
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
characterized by NMR and MS analysis (see
Supporting Information). Besides mono substituted
phenols, disubstituted analogues are also suitable for
this protocol. The use of 3,5-dimethylphenol 1l gave
rise to its corresponding amines 2l in 65% total yield
with three stereo-isomers, the two major of which
were purified by preparative thin layer
chromatography.
As expected, dehalo reduction occurs under
“hydrogenation conditions”. When 3-fluorophenol
was used as a substrate, the C-F bond was completely
cleaved and phenol was obtained as main product,
except with traces of N-cyclohexyl aniline and N,N-
diphenylamime being detected by GC-MS analysis.
Ortho- or para-methoxylphenol suffered from partial
demethoxygenation, producing a complex mixture
containing its desired amines, while the meta-
analogue underwent complete C-OMe cleavage,
yielding mainly N-cyclohexyl aniline.
Scheme 3. Reductive amination of benzyl phenyl ether
with hydrazine monohydrate.
Table 3. The use of hydroxylamine as N source.[a]
In addition, phenols such as 1m, 1n and 1o were
also studied (Scheme 2). Subjecting phenol 1m (5-
indanol) to the standard conditions produced 46% of
2m (cis/trans isomeric mixture) and 40% of 3m. 2-
Naphthanol 1n was reactive enough for such
transformation to generate a complex mixture under
standard conditions. After reducing the amount of
sodium formate to 1.0 equivalent, diaryl amine 3n
was obtained in 50% yield. Interestingly, 8-
hydroxyquinoline 1o underwent the transformation
smoothly, furnishing three aminoquinolines in a total
isolated yield of 65%. Here it is noteworthy that the
products di(quinolin-8-yl) amine 3o and 8-amino-
[a]Conditions: phenol (0.2 mmol), NH2OH.HCl (0.15
mmol). Yields and cis/trans ratio were determined by H
NMR analysis of the crude product, using 1,3,5-
trimethoxybenzene as an internal standard.[b]The cis-
isomer overlapped with other peaks in either CDCl3 or
CD3OD.
1
Hydroxylamine and its derivatives have been
extensively utilized as amino source in the reductive
amination and C-H amidation through an N-O bond-
cleavage step.[20] We are curious about the possibility
of using hydroxylamine in this deoxyamination.
When using hydroxylamine hydrochloride/formate in
lieu of the hydrazine, the reaction of phenol 1a
resulted in the formation of 2a in excellent yield of
90%, but cresols 1b-d are ineffective under the same
1,2,3,4-tetrahydroquinoline
5o
are
potential
polydentate ligands for catalysis chemistry.
The aryl ethers are more abundant structural units
in biomass resources such as lignin. We hope this
chemistry can be applied to these model ethers. When
benzyl phenyl ether was subjected to this protocol, N-
cyclohexyl aniline was obtained in 82% yield, as
phenol delivered (Scheme 3).
conditions
(Table
3).
Surprisingly,
both
hydroxylbenzoates and hydroxylphenylacetate gave
moderate yields. The catalyst recycling experiment
was also performed; unfortunately, the NMR yield
significantly dropped from 90% to 16% with the
recycled Pd/C as the catalyst under the optimized
conditions using phenol as the substrate.
To probe the mechanism of this chemistry, possible
intermediates instead of phenol were used for a series
of control experiments. For instance, the use of
cyclohexanone and cyclohexenone reacting with
hydrazine can lead to the formation of the desired
amine 2a in moderate to excellent yields (Scheme 4,
a). Noteworthy, under standard conditions, 2-
cyclohexenone gave a quantitative yield. When both
ketones reacted with cyclohexylamine under the
standard conditions in the presence of 1.0 equiv of
HCO2Na, the amine 2a was obtained in 94% and
75% yields, respectively (Scheme 4, b). Interestingly,
cyclohexylamine can dimerize to form the desired
product 2a under modified standard conditions either
with or without HCO2Na (Scheme 4, c). Furthermore,
when using the hydrazone or azine for this
transformation, the desired amine can also be
obtained, albeit in moderate yields (Scheme 4, d). All
these experimental results indicated that all the
Scheme 2. Other examples.
4
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
intermediates tested in the control experiments, such
as cyclohexanone, cyclohexenone, cyclohexylamine
and even hydrazone or azine are likely involved in
the present transformation.
Based on these results and previous literatures,[7, 12,
21] a possible mechanism was proposed (Scheme 5).[22]
Firstly, phenol 1 was reduced to cyclohexanone or
cyclohexenone A, which then condensed with
hydrazine to form hydrazone or azine B. The
intermediate B was further reduced to access the key
intermediate cyclohexylamine C. Then C reacted
with A to generate D, subsequently giving the desired
amine 2 via dehydrogenation. Alternatively, C
reacted with the imine derived from itself to produce
the target amine 2.
In summary, we have disclosed the catalytic
deoxgenative amination of phenols with hydrazine as
N source using commercial Pd/C. Benzyl phenyl
ether is suitable for this amination, and
hydroxylamine/formic acid is an alternative to
hydrazine in some cases. This chemistry involves a
complex C-O bond and N-N or N-O bond-cleavage
process and enables access to a variety of N-
substituted cyclohexyl anilines from lignin-derived
phenols. In most cases, the cis-/trans-isomer can be
isolated using flash chromatography. These amines
have various applications in pharmaceuticals,
pigments and functional materials. Further
deoxygenative transformations of biomass are under
way in our lab.
Experimental Section
General procedure for the palladium-catalyzed
synthesis of N-cyclohexyl anilines
An oven-dried screw cap test tube was charged with a
magnetic stir-bar, phenol 1 (0.2 mmol), Pd/C (10 wt%,
21.2 mg, 10 mol% based on Pd content) and sodium
formate (27.2 mg, 0.4 mmol). The tube was then evacuated
and backfilled with argon for three times. Toluene (1 mL,
pretreated by three cycles of evacuation-refill with argon),
hydrazine monohydrate (8 μL, 0.15 mmol), TFA (15 μL,
0.2 mmol) were sequentially added by syringe under argon
flow. Then the tube was screwed and placed in a preheated
o
oil bath at 150 C with vigorous stirring for 24 h. The
reaction system was cooled to room temperature and
filtered through the pad of silica gel. The filtrate was
concentrated and the resulting residue was purified via the
column chromatography using hexane: ethyl acetate (20:1-
4:1) as eluent.
Scheme 4. Control experiments.
Acknowledgements
The authors acknowledge the Canada Research Chair
Foundation (to C.J.L.), the CFI, FQRNT Center for Green
Chemistry and Catalysis, NSERC and McGill University for
financial support.
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Scheme 5. Proposed mechanism.
5
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
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6
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10.1002/adsc.201700712
Advanced Synthesis & Catalysis
UPDATE
Palladium-Catalyzed Synthesis of N-Cyclohexyl
Anilines from Phenols with Hydrazine or
Hydroxylamine via N-N/O Cleavage
Adv. Synth. Catal. 2017, Volume, Page – Page
Jiang-Sheng Li,ab†Zihang Qiua† and Chao-Jun Lia*
7
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