Formal Direct Cross-Coupling of Phenols with Amines

Article abstract of DOI:10.1002/anie.201506751

The transition-metal-catalyzed amination of aryl halides has been the most powerful method for the formation of aryl amines over the past decades. Phenols are regarded as ideal alternatives to aryl halides as coupling partners in cross-couplings. An efficient palladium-catalyzed formal cross-coupling of phenols with various amines and anilines has now been developed. A variety of substituted phenols were compatible with the standard reaction conditions. Secondary and tertiary aryl amines could thus be synthesized in moderate to excellent yields.

Full text of DOI:10.1002/anie.201506751

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506751
German Edition: DOI: 10.1002/ange.201506751
Hydrogen Borrowing
Hot Paper
Formal Direct Cross-Coupling of Phenols with Amines
Zhengwang Chen, Huiying Zeng, Simon A. Girard, Feng Wang, Ning Chen, and Chao-Jun Li*
Abstract: The transition-metal-catalyzed amination of aryl
halides has been the most powerful method for the formation
of aryl amines over the past decades. Phenols are regarded as
ideal alternatives to aryl halides as coupling partners in cross-
couplings. An efficient palladium-catalyzed formal cross-
coupling of phenols with various amines and anilines has
now been developed. A variety of substituted phenols were
compatible with the standard reaction conditions. Secondary
and tertiary aryl amines could thus be synthesized in moderate
to excellent yields.
A
ryl amines are ubiquitous building blocks for various
organic molecules and electronic materials and among the
most prevalent structural motifs in pharmaceutical agents.[1]
As a consequence, continuing efforts have been devoted to
developing new and effective approaches for the formation of
aryl amines.[2] Transition-metal-catalyzed amination reac-
tions, such as Chan–Lam amination,[3] intermolecular hydro-
amination,[4] and oxidative aromatization,[5] are examples of
modern methods for aryl amine synthesis.[6] Among the
various methods, the most prominent reactions are the
Buchwald–Hartwig coupling[7] and Ullmann-type amina-
tions,[8] in which aryl halides are frequently used as electro-
philes and coupled with amines (Scheme 1a). However, the
need of pre-synthesizing the aryl halides and the generation of
unwanted halide waste are drawbacks of the aryl halide based
strategies.
Scheme 1. Formal cross-coupling of phenols with amines.
On the other hand, phenols (mostly in their polymeric
forms in lignin and coal) are the second most prevalent
naturally occurring structural units of renewable biomass on
the planet, and have the same oxidation state as haloarenes.[9]
Therefore, the direct coupling of phenols, instead of haloar-
enes, has long been a synthetic aspiration and great scientific
challenge. However, phenols have 1) a very reactive hydroxy
nates[11] (e.g., triflates, tosylates, mesylates, and sulfamates),
2) aryl esters or aryl carbamate derivatives,[12] 3) aryl alkyl
ethers,[13] and 4) phenolic salts[14] (Scheme 1b). Undoubtedly,
taking into account the atom and step economy, the develop-
ment of a direct cross-coupling with non-preactivated phenols
would be highly desirable.
À
group and 2) a C O bond with a high dissociation energy
owing to p–p conjugation. In recent years, great achievements
Recently, we have reported a palladium-catalyzed reduc-
tive coupling of phenols with anilines to afford cyclohexyl-
have been made in developing coupling reactions of phenols
À
by catalytic C O bond cleavage by first transforming them
amines by a reduction/condensation/reduction pathway
into more active derivatives,[10] which include 1) aryl sulfo-
(Scheme 1c).[15] Inspired by the “hydrogen-borrowing strat-
egy” for converting alcohols into amines[16] and oxidative
aromatization,[5] we hypothesized that if we could temporarily
generate a small amount of the ketone form from phenol, the
[*] Dr. Z. W. Chen,[+] Dr. H. Y. Zeng,[+] S. A. Girard, Dr. F. Wang,
Dr. N. Chen, Prof. Dr. C.-J. Li
À
À
conversion of the C O bond into a C N bond with amines
and subsequent dehydrogenative aromatization to form the
desired aryl amine should be feasible. Herein, we present
a highly efficient, palladium-catalyzed formal direct cross-
coupling of phenols with various amines and anilines through
a tandem reduction/condensation/dehydrogenation process
(Scheme 1c).[17]
Department of Chemistry and FQRNT Centre for Green Chemistry
and Catalysis, McGill University
801 Sherbrooke St. W. , Montreal, Quebec H3A 0B8 (Canada)
E-mail: cj.li@mcgill.ca
Dr. Z. W. Chen[+]
School of Chemistry and Chemical Engineering
Gannan Normal University, Ganzhou 341000 (PR China)
[+] These authors contributed equally to this work.
Based on our previous work, we first used phenol and
para-toluidine to optimize the reaction conditions. We found
that a higher temperature (1208C), a smaller amount of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506751.
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Table 2: Palladium-catalyzed coupling of phenols with amines.[a]
sodium formate (1.5 equiv), and acid additives were advanta-
geous to form the aryl amine product. However, the
formation of cyclohexylamine side products could not be
avoided (see the Supporting Information, Table S1). Fortu-
nately, when aliphatic amines were used to replace toluidine,
the corresponding products could be obtained in good yields
without any cyclohexylamine side products. Based on these
results, the reaction of phenol (1a) with n-octylamine (2a)
was chosen as the model reaction, and a variety of factors,
including acid, base, catalyst, ligand, solvent, oxidant, and
temperature, were evaluated. To our delight, the reaction
gave the desired coupling product in 52% yield when Pd/C
was used as the catalyst with sodium formate as the temporary
hydrogen source (Table 1, entry 1). Then, various additives,
Table 1: Optimization of the reaction conditions.[a]
Entry
Catalyst
Additive
T [oC]
Yield[b] [%]
1
2
3
4
5
6
7
8
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd(OH)2/C
[PdCl2(dtbpf)]
Pd(NO3)2·2H2O
[Pd(acac)2]
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
–
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
130
140
150
52
n.p.
8
60
65
47
50
36
50
n.p.
n.p.
trace
n.r.
73
81
84
86 (81)
86
ZnCl2
HOAc
PhCOOH
TFA
NaOtBu
Na2CO3
DABCO
TFA
TFA
TFA
TFA
TFA
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.28 mmol), Pd/C
(10 mol%), HCO2Na (1.5 equiv), and TFA (0.5 equiv) in toluene (1 mL)
for 12 h under an argon atmosphere; yields of isolated products are
given. [b] 4-Chlorophenol was used as the substrate. [c] 24 h.
9[c]
10
11
12
13[d]
14[e]
15[f]
16[f]
17[f]
18[f]
With the optimized reaction conditions in hand, the
substrate scope was explored. As shown in Table 2, different
amines reacted efficiently with phenol. Various alkyl aryl
amines were obtained in good to high yields in all cases
(Table 2; 3a–3d). The use of the sterically hindered amine
cyclohexylamine also led to the corresponding product in high
yield (3e). The reaction of cyclic secondary amines with
phenol under the optimized standard conditions furnished the
corresponding tertiary aniline derivatives also in good yields
(3 f and 3g). Various anilines bearing electron-withdrawing or
electron-donating groups were all suitable substrates for this
transformation, and the corresponding products were
obtained in moderate to good yields (3h–3k). A tertiary
aniline was obtained in moderate yield by reacting a secon-
dary aniline with phenol (3l). As expected, the corresponding
dechlorinated aryl amine was obtained when 4-chlorophenol
was used as the substrate (3e).
TFA
TFA
TFA
TFA
Pd/C
TFA
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), catalyst
(10 mol%), HCO2Na (1.5 equiv), and an additive (0.5 equiv) in toluene
(1 mL) for 12 h under argon atmosphere. [b] Yields were determined by
GC analysis with mesitylene as the internal standard; yields of isolated
products are given in parentheses. [c] 20 wt% Pd(OH)2/C. [d] Without
HCO2Na. [e] 2a (0.24 mmol) was used. [f] 2a (0.28 mmol) was used.
acac=acetylacetonate, DABCO=1,4-diazabicyclo[2.2.2]octane,
dtbpf=1,1’-bis(di-tert-butylphosphino)ferrocene, n.p.=no product,
n.r.=no reaction, TFA=trifluoroacetic acid.
On the other hand, as shown in Table 3, different phenols
reacted with n-octylamine (3m–3x), cyclohexylamine (3y–
3ad), and para-methoxyaniline (3af and 3ag) to give the
corresponding cross-coupling products in moderate to high
yields. The presence of a substituent in the para or meta
position of the phenol does not have a significant effect on the
reaction (3m–3s), nor does a substituent group in the ortho
position (3t). When biphenol was used as the starting
such as acids and bases, were evaluated, and trifluoroacetic
acid gave the best results (entries 2–8). Others palladium
catalysts were also tested, but only Pd(OH)2/C afforded the
desired product in moderate yield (entries 9–12). A control
experiment showed that sodium formate was essential to the
transformation (entry 13). Subsequently, the effects of the
ligand, solvent, and oxidant were also explored, which did not
lead to any improvements (see Table S5–S7). With a slight
excess of n-octylamine, the product yield was further
improved to 81% (entry 15). Finally, by changing the reaction
temperature to 1408C, the desired product was obtained in
86% yield (yield of isolated product: 81%; entries 16–18).
À
material, only one C N bond formation was observed
À
whereas two C N bonds were formed when resorcinol was
employed (3t vs. 3u). Different naphthols also worked well
under the standard conditions, and the corresponding prod-
ucts were obtained in good to excellent yields (3v–3x).
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Table 3: Reactions of various substituted phenols and amines.[a]
Scheme 2. Tentative mechanism for the reaction between phenol and
amines.
A tentative mechanism for this novel coupling reaction is
shown in Scheme 2. Initially, sodium formate reacts with the
palladium catalyst to generate the HPdIIH species.[18] Sub-
sequently, reduction of phenol generates intermediate A,
which undergoes further reduction immediately followed by
dehydration with the amine to give intermediate C.[19]
Substoichiometric amounts of acid can accelerate the con-
densation of cyclohexanone and the amine. A sequential
dehydrogenation process of intermediate C leads to inter-
mediate D, which is similar to a mechanism proposed by Liu
et al. (path a).[18] Alternatively, it is also possible that dehy-
dration of intermediate A with the amine gives intermediate
D (path b). Finally, the second dehydrogenation of inter-
mediate D, a process similar to the first dehydrogenation,
results in the aryl amine products and regenerates the HPdIIH
species. When a large excess of sodium formate is used,
intermediates C and D can be reduced to cyclohexylamine
derivatives, the products obtained in our previous work.[15]
Further control experiments suggest that both cyclohexanone
or cyclohexenone could be the reaction intermediates, as
good yields were obtained when they were used as the starting
materials under the standard reaction conditions (Sche-
me 2a,b).
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.28 mmol), Pd/C (10 mol%),
HCO2Na (1.5 equiv), and TFA (0.5 equiv) in toluene (1 mL) for 12 h
under argon atmosphere; yields of isolated products are given. [b] 2
(0.48 mmol). [c] 2-Allylphenol was used as the substrate. [d] Catechol
and trans-1,2-diaminocyclohexane were used as the substrates. [e] 24 h.
Moderate to high yields were achieved when various phenols
reacted with cyclohexylamine (3y–3ad). For a phenol bearing
an allyl group at the ortho position, the product was formed in
moderate yield with reduction of the allyl group to a propyl
group (3y). Furthermore, 4-propylphenol and 2-methoxyphe-
nol (as the most prevalent structural units in lignin) provided
the products in good yield (3aa and 3ab). It is noteworthy that
the corresponding product was obtained in high yield without
ester–amide exchange when the substrate included an ester
In conclusion, we have developed a highly efficient direct
coupling of phenols with various amines and anilines cata-
lyzed by palladium. Both primary and secondary amines are
suitable substrates for this novel coupling process. A wide
range of important phenols and phenolic lignin model
compounds, such as 4-propylphenol and 2-methoxyphenol,
could be coupled efficiently. Compared to the formation of
À
group (3z). A double C N bond formation occurred in high
yield when catechol was reacted with cyclohexylamine (3ad).
A nitrogen-containing heterocycle was formed when catechol
and 1,2-diaminocyclohexane were used as the substrates
(3ae). Good yields were also achieved when phenol deriva-
tives reacted with para-methoxyaniline (3af and 3ag).
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Experimental Section
An oven-dried screw-cap test tube was charged with a magnetic stir
bar, Pd/C (10 wt%, 21.2 mg, 10 mol% based on Pd content) and
sodium formate (20.4 mg, 0.3 mmol). The tube was then evacuated
and backfilled with argon. The evacuation/backfilling sequence was
repeated two additional times. Under a counter-flow of argon,
a solution of the phenol (0.2 mmol) and the amine (0.28 mmol) in
toluene (1 mL) was added, followed by the addition of trifluoroacetic
acid (8 mL, 0.1 mmol) by syringe. The tube was placed in a preheated
oil bath at 1408C, and the mixture was stirred vigorously for 12 h. The
reaction mixture was cooled to room temperature and filtered
through a pad of silica gel. The filtrate was concentrated, and the
resulting residue was purified by column chromatography to give the
desired aryl amine.
Acknowledgements
We thank the CSC (China Scholarship Council) for a post-
doctoral fellowship (Z.W.C.) and NSERC, FQRNT, CFI, and
the Canada Research Chair (to C.J.L.) for their support of our
research.
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Keywords: aryl amines · dehydrogenation ·
hydrogen borrowing · palladium · phenols
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Received: July 21, 2015
Revised: August 18, 2015
Published online: November 4, 2015
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