Nickel-catalyzed triarylamine synthesis: Synthetic and mechanistic aspects

Article abstract of DOI:10.1039/c3ob42053a

An improved protocol was described for the amination of chloroarenes with diarylamines under NiCl₂(PCy₃)₂ catalysis in the presence of a Grignard reagent as base. This method fully suits bromo-/iodoarene substrates as well, and even is expanded to certain aryl tosylates. A preliminary investigation into the mechanism suggests that this amination reaction might proceed through Ni^I and Ni^III intermediates rather than via the usually expected Ni⁰-Ni ^II cycle.

Full text of DOI:10.1039/c3ob42053a

Volume 12 Number 8 28 February 2014 Pages 1175–1358
Organic &
Biomolecular
Chemistry
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ISSN 1477-0520
PAPER
Xin-Heng Fan, Lian-Ming Yang et al.
Nickel-catalyzed triarylamine synthesis: synthetic and mechanistic aspects

Organic &
Biomolecular Chemistry
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Nickel-catalyzed triarylamine synthesis: synthetic
and mechanistic aspects†
Xin-Le Li,a,b Wei Wu,a,b Xin-Heng Fan*a and Lian-Ming Yang*a
Cite this: Org. Biomol. Chem., 2014,
12, 1232
An improved protocol was described for the amination of chloroarenes with diarylamines under
NiCl2(PCy3)2 catalysis in the presence of a Grignard reagent as base. This method fully suits bromo-/
iodoarene substrates as well, and even is expanded to certain aryl tosylates. A preliminary investigation
into the mechanism suggests that this amination reaction might proceed through NiI and NiIII intermedi-
ates rather than via the usually expected Ni0–NiII cycle.
Received 14th October 2013,
Accepted 27th November 2013
DOI: 10.1039/c3ob42053a
www.rsc.org/obc
about this special type of catalytic amination, and our efforts
towards this are the subject of this contribution.
Introduction
The nickel-catalyzed aromatic C–N coupling reaction rep-
Extensive work on aromatic aminations under nickel cata-
resents a major advance in catalytic cross-coupling reactions lysis has revealed: that simple anilines are not over-arylated and
and constitutes an essential part of modern amination the reaction proceeds “selectively” to the mono-substitution
methodologies.1–14 Highly attractive aspects of nickel catalysts stage, even in the presence of excess aryl halides; and that a
are their economic cost as well as their ability to effectively diarylamine is not able to be coupled with an aryl halide
aminate cheap, readily available electrophilic substrates, such under “normal” conditions (i.e., a combination of free amines
as aryl chlorides2–4,6–8 and phenolic derivatives,9–13 with no and inorganic bases).18 The reasons were presumably that a
need for specially tailored ligands. Nickel catalysis has dis- diarylamine is usually difficult to coordinate to the Ni centre
played great success in arylations of various aliphatic amines due to its steric bulk and poor nucleophilicity against a rela-
and anilines.2–13 Curiously, this amination method has been tively small Ni centre.3c For metal-catalyzed coupling reactions,
facing a challenge in the synthesis of structurally simple triaryl- this coordination step (i.e., transmetallation) is key in a cata-
amines,15 in striking contrast to the development of Pd-16 and lytic cycle. In view of this situation, we still chose a Grignard
Cu-catalyzed17 aminations. Until now, there have been only a reagent as base for a model reaction between chlorobenzene
very few reports involving triarylamine synthesis under nickel (1a) and diphenylamine (2a). By the Grignard reagent, diphenyl-
catalysis.14 Hence, further investigation in this regard remains amine is readily transformed to a more nucleophilic amide
necessary from both synthetic and mechanistic perspectives.
species, which enhances its ability to bind to the Ni centre. In
addition, the use of highly air-sensitive Ni0 precatalysts is
avoided since easy-to-handle NiII compounds can be reduced
in situ to Ni0 by Grignard reagents.
Results and discussion
Our screening experiments were focused primarily on
nickel catalysts that may be pre-formed or in situ generated,
with i-PrMgCl as base and dioxane as solvent. Several common
NiII systems were surveyed (Table 1). It was found that the type
and nature of ligands binding to the Ni centre are closely
related to the catalytic activity. Apparently, phosphine-ligated
nickel precursors perform better than those bearing other
types of ligands (entries 1–3 vs. entries 4–6). Among phosphine
ligands, tricyclohexylphosphine (PCy3) offers the relatively
highest activity (entry 6). The activation ability of PCy3 was con-
firmed by the following observations. Those NiII precatalysts
which performed poorly and even did not work at all dramati-
cally promote the reaction once associated with additional
PCy3 (entries 7–10 vs. entries 1, and 3–5). Note that an
N-heterocyclic carbene (NHC) ligand (entry 2), although it is
Our previous communications14 disclosed the first examples
of nickel-catalyzed triarylamine synthesis via the amination of
bromo-/iodoarenes with metal diarylamides produced in situ-
from diarylamines with a Grignard reagent or sodium hydride
as the base. We have since desired to expand the substrate
scope of this reaction to chloroarenes or even phenolic deriva-
tives as well as to uncover more mechanistic information
aBeijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of
Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
P. R. China. E-mail: xinxin9968@iccas.ac.cn, yanglm@iccas.ac.cn;
Fax: +8610-62559373
bGraduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ob42053a
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Table 1 Screening of catalyst systemsa
Table 2 Effects of basesa
Base (equiv)
Relative yieldb (%)
Entry
[Nickel(II)] (mol%)/L (mol%)
Isolated yield (%)
1
2
3
4
5
6
i-PrMgCl (1.6)
t-BuOM (2) (M = Na, K)
NaH (2)
n-BuLi (1.6)
EtMgBr (1.6)
PhMgBr (1.6)
100
0
0
35.2
95.8
98.6
1
2
3
4
5
6
7
8
Ni(acac)2 (5)
Ni(acac)2 (5) + IPr·HCl (10)
NiCl2(bpy) (5)
NiCl2(PPh3)2 (5)
NiCl2(dppe) (5)
Trace
0b
0
19
35
78 (66c)
76
a A set of parallel reactions performed on a 0.5 mmol scale. b The
values of yield calculated relative to the yield obtained using i-PrMgCl.
NiCl2(PCy3)2 (5)
Ni(acac)2 (5) + PCy3 (10)
NiCl2(bpy) (5) + PCy3 (10)
NiCl2(PPh3)2 (5) + PCy3 (10)
NiCl2(dppe) (5) + PCy3 (10)
NiCl2(PCy3)2 (5) + PCy3 (10)
NiCl2(PCy3)2 (10)
53
71
73
9
10
11
12
13
14
78
75b
65d
70e
(entries 1, 5, and 6 vs. entry 4). Also, aryl or alkyl Grignard
reagents may actually provide the same outcome, taking into
account possible errors of the isolated yields in individual
cases (entries 1, 5, and 6).
NiCl2(PCy3)2 (20)
NiCl2(PCy3)2 (3)
a Reactions performed on a 1 mmol scale. b 1.7 equiv. of i-PrMgCl
used. c In toluene. d 1.9 equiv. of i-PrMgCl used. e 1.56 equiv. of
i-PrMgCl used.
To examine the applicability of this protocol, the coupling
reaction of representative diarylamines (1a–e) with a series of
aryl chlorides (2a–h) was carried out under the above-
mentioned standard conditions. Meanwhile, some typical aryl
bromides/iodides or tosylates were selected as electrophiles in
this reaction. The results are summarized in Table 3. Regard-
ing the diarylamines, electronic factors did not interfere
notably with the reaction, where good yields could be provided
(3aa, 3ba, 3ca, and 3da); but the reaction was highly sensitive
to their steric hindrance, as in the case of N-(1-naphthyl)-
aniline (3ea). As for aryl chlorides, both electronic and steric
effects had considerable impact on this reaction. Generally,
those chloroarenes which are electron-neutral (3aa–da, 3ag,
and 3bg), with weak electronic effects (3ac and 3bc; 3ad and
3bd), as well as with weak steric effects (3ag and 3bg), are suit-
able substrates, which could be aminated smoothly to give the
triarylamine products with satisfactory yields. Either sterically
demanding (3ab and 3bb) or highly electron-rich (3ae and 3be)
aryl chlorides impeded this reaction. Surprisingly, strongly
electron-withdrawing groups, which usually activate electro-
philic substrates in metal-catalyzed cross-couplings, did not
seemed to be very favourable for this amination reaction (3af
and 3bf). Also, two other electron-poor chloroarenes, p-chloro-
benzophenone and p-chlorobenzonitrile, were subjected to
this amination reaction with diphenylamine. However, these
chlorides quenched the reaction and no expected product was
observed. Additionally, it was shown that various aryl bro-
mides/iodides, including electron-neutral (3aa), -rich (3ae and
3be) and -poor (3af), and sterically hindered (3ab and 3bb),
underwent smooth transformations under the standard con-
ditions in good to excellent yields. Of particular note, a par-
ticular tosylate, 2-naphthyl tosylate, was found to be suitable
for this reaction, and coupled with diarylamines to afford good
yields of the triarylamine products (3ah, 3bh, and 3ch),
although most aryl tosylates, such as phenyl and 1-naphthyl
also highly electron-donating and has been utilized widely
in nickel-catalyzed aminations, was totally ineffective for
this reaction. For the best precatalyst NiCl2(PCy3)2, the use of
excess PCy3 seems to be unnecessary since this does not help
to further enhance its catalytic efficiency (entry 11 vs. entry 6).
By a rough analysis of the reaction mixture in entry 6, we
found that the starting chlorobenzene was not consumed com-
pletely, with small amounts (<5%) of biphenyl as a detectable
by-product. Thus, we attempted to increase the catalyst load-
ings so as to achieve a high conversion and yield. However, the
use of double the amount of the Ni precursor did not improve
the outcome (entry 12 vs. entry 6), and rather, over-loading of
the catalyst led to a significantly decreased yield (entry 13).
This phenomenon differs completely from the situation in
most metal-catalyzed coupling reactions, and the reason may
be related to the reaction mechanism (see below). On the other
hand, decreasing the catalyst loading, as expected, gave the
desired product in a reduced yield (entry 14). As a result, our
standard conditions for this reaction were chosen as in entry 5
of Table 1.
The type of bases employed was examined as well (Table 2).
Indeed, those bases commonly used in metal-catalyzed amin-
ations do not function (entry 2). Sodium hydride, although it
is able to convert diphenylamine into the more nucleophilic
diphenylamide, is also ineffective (entry 3). Generally, organo-
metallic reagents are the base of choice by which this reaction
could be promoted (entries 1, and 4–6). As we know, organo-
metallic reagents function both as a strong base (deprotonat-
ing the N–H of diphenylamine) and as a reducing agent
(transforming the NiII species to the catalytically active Ni0). In
contrast, the Grignard reagent is superior to the organolithium
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Table 3 Substrate scopea
actually the catalytic species and the reaction proceeds
through nickel(I) and nickel(III) intermediates.
According to the mechanistic pathway of a Ni0–NiII cycle,
arylnickel(II) halides may be regarded as the oxidative adducts
of haloarenes to the Ni0. Some arylnickel(II) halide complexes
are readily available and isolable compounds.19 To examine
the intermediacy of arylnickel(II) complexes, we conducted stoi-
chiometric reactions of metal diphenylamides with arylnickel(II)
halides, as shown in eqn (1) and (2). As a result, no triaryl-
amine was produced using sodium diphenylamide as nucleo-
phile (eqn (1)), and only a trace of the desired product
was detected under more favorable conditions (eqn (2)).
These observations led us to conclude that the arylnickel(II)
complex might not be a potential intermediate on the catalytic
pathway, and thus it would be reasonable to rule out the
Ni0–NiII mechanism for this nickel-catalyzed triarylamine
synthesis.
ð1Þ
ð2Þ
Accordingly, the NiI species might be actually responsible
for the catalytic cycle in this transformation. Kochi et al.20
have investigated in detail the oxidative addition of Ni0 to aryl
halides, and proposed a route to in situ formation of the NiI
species via a single-electron transfer from Ni0 to a haloarene.
Based on their studies, we proposed a route to generating the
NiI intermediate in this reaction (Scheme 1). The Ni0 (B), pro-
duced from the reduction of a NiII precursor (A) by the
Grignard reagent, undergoes a single-electron transfer to an
aryl halide to afford the NiI anion radical (C) which may
decompose into the ArNi(II)XLn oxidative adduct (D) or
(Ph)2NNi(I)Ln (E) and an aryl radical. (Ph)2NNi(I)Ln (E) is the
catalytically effective species and ArNi(II)XLn (D) the “null and
void” for this Ni-catalyzed reaction.
a Reaction conditions: 1 (1.5 mmol), 2 (1.0 mmol), dioxane (5 mL),
NiCl2(PCy3)2 (0.05 mmol), i-PrMgCl (0.8 mL, 2 M in THF), 95 °C, 12 h;
Isolated yield.
As implied in Scheme 1, since products D and E are derived
from a common intermediate C, the two decompositions must
compete against each other and a selective conversion can be
achieved depending upon the conditions specifically given. It
was presumed that under our reaction conditions, E might be
tosylates, have not yet been established as applicable sub-
strates (generally, <30% yields obtained).
Currently two mechanistic pathways have been popularly
accepted for nickel-catalyzed aromatic amination.1,2,3c,5 One,
likely more preferred, claims that Ni0 serves as the catalytically
active species and the coupling reactions follow a Ni0–NiII
cycle involving sequential oxidative addition, transmetallation
and reductive elimination; the other suggests that NiI, result-
ing from a single-electron transfer of Ni0 to an aryl halide, is
Scheme 1 A plausible route to in situ generating the nickel(I) species.
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produced predominantly under the aid of a bulky diarylamide solution in THF) was added slowly at room temperature via
ligand. This hypothetical route to generation of the active NiI syringe. After stirring for 5 minutes, an aryl halide (1.0 mmol)
species may find supporting evidence from the recent study on (if liquid) or its solution in a minimum volume of dioxane (if
the bulky ligand-controlled conversion selectivity of nickel(0) solid) was added via syringe. The mixture was stirred at room
into nickel(I) or nickel(II),21 and is also consistent with our temperature for another 10 minutes, and then placed in an oil
experimental observations: (1) small amounts of biaryl by-pro- bath of 95 °C for 12 h. The reaction mixture, after being cooled
ducts, which may result from the self-coupling of aryl radicals, to ambient temperature, was poured into 20 mL of saturated
are detected in almost all cases; (2) generally the efficacy of the aqueous NH4Cl solution and then extracted with ethyl acetate
Ni-based catalyst in this transformation is not high enough (10 mL × 3). The combined organic phases were concentrated
because a competitive reaction producing the catalytically in- under reduced pressure and the residue purified by column
active NiII species may not be fully suppressed under our current chromatography.
conditions; (3) the reactivity of electrophilic substrates
increases in the order chloride ≪ bromide < iodide, with a
Typical experiment for the reaction of chloromagnesium
diphenylamide with arylnickel(II) halides
decrease in the strength of halogen–CAr bonds; and (4) the
reaction is intolerant of functionality like carbonyl and cyano
groups which may likely interfere with a single-electron trans-
fer process of the Ni0 species.
An oven-dried 25 mL three-necked flask was charged with
diphenylamine (256 mg, 1.55 mmol), PCy3 (560 mg, 2 mmol)
and dioxane (10 mL). To the mixture was slowly added
i-PrMgCl reagent (0.75 mL, 1.5 mmol, 2.0 M solution in THF)
via syringe, and stirring of the resulting mixture was continued
at room temperature for 5 minutes. Then the arylnickel(II)
halide (1 mmol) was added into the flask by a solid-addition
adapter, and the mixture was heated in an oil bath of 95 °C for
12 h. The reaction mixture was poured to 20 mL of saturated
NH4Cl aqueous solution and then extracted with ethyl acetate
(10 mL × 3). Solvents were removed in vacuo to afford the
residue which was subjected to normal analyses.
Hillhouse et al.22 have reported examples of rapid C–N
coupling reductive eliminations from oxidation of the nickel(II)
amido alkyl complex to the nickel(III). Combining their studies
with our viewpoint that the NiI may be the active species that
initiates the reaction, a plausible mechanism is proposed that
might follow a catalytic cycle of a NiI–NiIII shuttle involving
sequential oxidative addition, transmetallation, and reductive
elimination (Scheme 2). Overall, efficient production of the NiI
species must be the most important factor for this reaction,
and oxidative addition, by our current recognition, may be
regarded as the rate-determining step in the catalytic cycle.
Conclusions
Experimental
General procedure for the nickel-catalyzed synthesis of
triarylamines
In summary, we have first demonstrated the possibility of the
nickel-catalyzed amination of chloroarenes for triarylamine
synthesis, and more clearly suggested the mechanism of
this reaction which might be different from that of a normal
Ni0–NiII cycle. The relative drawback of this protocol may be
easily overcome by appropriate choice of the coupling partner
pattern of the aryl halide and the diarylamine. Therefore, this
simple, inexpensive method for triarylamine synthesis can
become a powerful alternative to the corresponding palladium
or copper catalysis. Studies are underway to improve the
Ni catalyst efficiency by, for example, directly using potential
nickel(I) complexes23 as precatalysts, to overcome the problem
of functional group capability by modifying reaction con-
ditions, as well as to further clarify the mechanistic details by
kinetic studies, and/or isolating and identifying certain nickel
intermediates during the reaction.
An oven-dried 25 mL three-necked flask was charged with
NiCl2(PCy3)2 (5 mol% relative to the aromatic chlorides,
34 mg), and a diarylamine (1.5 mmol). The flask was evacuated
and backfilled with nitrogen, with the operation being
repeated twice. Anhydrous dioxane (5 mL) was added via
syringe. Then i-PrMgCl reagent (0.8 mL, 1.6 mmol, 2.0 M
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Project No. 20872142 and 21102150) for financial
support of this work.
Scheme 2 Proposed mechanism for the nickel-catalyzed amination of
haloarenes with diarylamines.
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15 Triarylamines are important building blocks of organic
optoelectronic materials utilized as components in organic
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