Copper-catalyzed, ceric ammonium nitrate mediated N-arylation of amines

Article abstract of DOI:10.1039/c8nj06145a

Cu-Catalyzed, ligand- and base-free cross-coupling of aryl boronic acids with primary and secondary amines has been reported. This ‘Chan-Evans-Lam' reaction has revealed that at room temperature, with a catalytic amount of copper(ii) acetate and ceric ammonium nitrate (CAN) as an oxidant, N-arylation can result in an effective C-N bond formation. This air stable, practical, robust protocol enables tolerance towards a variety of functional groups on both boronic acid and amine partners.

Full text of DOI:10.1039/c8nj06145a

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Copper-Catalyzed, Ceric Ammonium Nitrate Mediated N-Arylation
of Amines†
Uma Maheshwar Gonela,a and Seth Y Ablordeppey*a
Received 00th January 20xx,
Accepted 00th January 20xx
Cu-Catalyzed, ligand and base free cross-coupling of aryl boronic acids with primary and secondary amines has been
reported. This ‘Chan-Evans-Lam’ reaction has revealed that at room temperature, catalytic copper(II) acetate and ceric
ammonium nitrate (CAN) as an oxidant, N-arylation can result in an effective C-N bond formation. This air stable, practical,
robust protocol enables a variety of functional group tolerance on both boronic acid and amine partners.
DOI: 10.1039/x0xx00000x
www.rsc.org/
The formation of C-N bonds has been one of the most important aryl boronic acids. Overall, these reports were usually
transformations in organic synthesis for several decades. Its associated with a ligand, a base, molecular sieves, elevated
widespread applications can be found in a variety of fields temperatures or poor yields.
including the syntheses of pharmaceuticals, agrochemicals, Ceric ammonium nitrate (CAN) with its low cost, low toxicity, air
dyes and natural products.1 Transition-metal catalysis has made and moisture stable properties, has attracted widespread
this transformation more general and practical. In particular, applications in organic synthesis.21,22 In particular, this greener
Buchwald, Hartwig and others have made great contributions oxidant is attractive because of its high reduction potential,
through the use of Pd and Cu catalyzed C-N bond formation Lewis acid nature and especially its role as a single-electron-
reactions involving the coupling of electrophilic aryl halides and transfer oxidant.23 With these advantages, we have
nucleophilic amines.2
hypothesized CAN could be utilized as an oxidant in N-arylation
In the late 1990’s, Cu-mediated cross-coupling of arylboronic of amines in the presence of a Cu-catalyst.24
acids with amines and phenols was reported by Chan, 3 Evans,4
and Lam5 independently and involved stoichiometric amounts Figure 1. Outline of Preparation of N-Aryl amines
of Cu(II) catalyst and an amine base. Collman et al.
demonstrated a limited application of a catalytic version
(CuOH.TMEDA) of this reaction in the synthesis of substituted
indoles.6 Buchwald has reported the role of the ligand, meristic
acid in Cu-catalyzed arylation of anilines, however, inferior
results for aliphatic amines were observed.7
Recent reports have extended the scope of cross-coupling
reactions to include potassium aryl trifluoroborate salts,8
boronic acid pinacol esters,9 arylboroxines10, diarylboronic
acids11 with amines, phenols12, thioethers13 and phosphonate
diesters as coupling partners.14 15 It is noteworthy that other This study reports Cu(OAc)2 catalyzed cross-coupling of
,
transition metals such as Ni,16,17,18 Mn/Cu bimetallic arylboronic acids with primary/secondary amines in the
conditions19 have exhibited efficacy within this protocol. absence of a base and a ligand. To the best of our knowledge,
Another report by Kobayashi et al. described a visible light there is no report employing CAN mediated Cu-catalyzed cross-
mediated Cu(II)-catalyzed oxidative cross-coupling reaction coupling of boronic acids with amines.
(myristic acid/fac-[Ir(ppy)3]/2,6-lutidine) between arylboronic Our investigation was initiated with cross-coupling of
acids and anilines.20 This report was limited to aryl amines and phenylboronic acid (1a) with p-anisidine (2a) using a catalytic
amount of Cu(OAc)2 and CAN in CH2Cl2 at room temperature
(Table 1, entry 1). Gratifyingly, the desired product resulted in
a.Dr. Uma Maheshwar Gonela, and Dr. Seth Y Ablordeppey
Division of Basic Pharmaceutical Sciences, Florida A&M University, College of
Pharmacy and Pharmaceutical Sciences, Tallahassee, FL 32307, United States.
E-mail: seth.ablordeppey@famu.edu.
good yield. Subsequently, optimization of the catalyst was
evaluated. Cu(I) gave low yield, whereas Cu(II) salts resulted in
better yields with Cu(OAc)2 being the most desirable (entries 2-
8). As depicted in Table 1, in the absence of Cu-catalyst or CAN,
low or no cross-
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Scheme 1: Scope of Substituted Anilinesa
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DOI: 10.1039/C8NJ06145A
Table 1: Optimization of Reaction Conditionsa
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9
entry
catalyst
additive
solvent
yieldb
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1
2
Cu(OAc)2
CuI
CAN
CAN
CAN
CAN
CAN
CAN
CAN
-
CH2Cl2
CH2Cl2
77
nd
45
nd
nr
3
CuCl2
CH2Cl2
4
CuBr2
CH2Cl2
5
Cu2O
CH2Cl2
6
CuSO4
CH2Cl2
nd
trace
7
7
Cu(acac)2
Cu(OAc)2
-
CH2Cl2
8
CH2Cl2
9
CAN
CAN
CAN
CAN
CAN
CAN
CAN
CH2Cl2
nr
10
11
12
13
14
15
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CH3CN
PhMe
nd
93
nd
nd
nr
PhMe/H2O
THF
PhMe/50 oC
Scheme 2: Scope of Substituted Boronic acidsa
PhMe
61c
aReaction conditions: ArB(OH)2
(1, 1.2 mmol), ArNH2 (2, 1
mmol), catalyst (0.1 mmol), additive (1.5 mmol), solvent (5.0
mL), 12 h. bIsolated yield. C1 Gram scale. nr = No reaction,
starting materials recovered. nd = No desired product.
coupling occurred (entries 8,9). Various solvents including
CH2Cl2, CH3CN, THF, toluene, toluene/H2O were examined.
Apparently, anhydrous nonpolar solvents such as CH2Cl2 and
Toluene are preferred with toluene providing the highest yield
of the desired product at room temperature (entries 10-14). It
was also observed that extended reaction time resulted in
homo-coupling of aryl boronic acid.
With optimized reaction conditions in hand, we proceeded to
explore the scope of substrate coupling reaction between
substituted anilines with phenylboronic acid (Scheme 1).
Electron donating and withdrawing substituted anilines were
evaluated. Electron withdrawing substituted anilines were
generally good-yielding (4a-g; 43-73%) however, the electron-
rich anilines proved to be greater cross-coupling partners (3a,
4h-k; 56-93%). Evaluation of the substituted anilines also
revealed that meta-substituted electron withdrawing anilines
gave the cross-coupled product in high yields (4d-e; 67-71%),
ortho-substituted electron withdrawing groups produced lower
yields (4f-g; 49-54%), and the ortho-substituted methoxy aniline
gave an excellent yield (4h; 78%). Finally, the electron deficient
boronic acids proceeded well (6a, b). In fact, there was no clear
difference between electron-rich or electron withdrawing
substitution on aryl boronic acid and the corresponding N, N-
diaryl amine products were obtained in good yields (6c-h).
Further, electron deficient boronic acids were investigated, with
halo-substituted boronic acids resulting in excellent yields (6g
and 6h).
heterocyclic amine afforded the desired product 4l
Encouraged by the aforementioned results, various substituted
aryl boronic acids were also examined (Scheme 2). Aromatic
.
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Scheme 3: Scope of Primary and Secondary Aminesa
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Scheme 5. Proposed Reaction Mechanism
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This protocol was further extended to N-arylation of aliphatic
amines (Scheme 3), a procedure we have often employed in our
lab.25 Gratifyingly, the aliphatic amines were also effective between Cu(II) with an amine. The second step could be
under the cross-coupling conditions (8a-d) employed. The transmetalation of
coupling of benzylamine with substituted aryl boronic acids our hypothesis, CAN could oxidize the Cu(II) (
afforded the desired products in good yields. However, subsequent reductive elimination offering the desired N-aryl
secondary amines 8b 8c, and 8d resulted in poor yields. product and Cu(I). Oxidation of Cu(I) in the presence of
A
with boronic acid to form
B
. According to
B) to Cu(III) (
C
) with
,
Finally, we further evaluated the utility of the protocol by atmospheric-O2 would then complete the catalytic cycle.
carrying out a gram scale reaction which resulted in the desired In summary, we have developed a useful and practical
coupled product 3a in 61% (table 1, entry 15).
Cu(II)/CAN mediated protocol for the N-arylation of aryl and
aliphatic amines. Conventional Chan-Evans-Lam protocol
requires stoichiometric amount of catalyst and base, whereas
this simple and easy reaction setup at room temperature could
produce desired products efficiently in the absence of any base,
ligand and catalytic amounts of Cu(OAc)2. This base free
condition could also offer arylation of substrates with base-
sensitive functional groups.
Scheme 4. Controll Experiment with TEMPO
To understand the possible reaction mechanism, control
experiment was carried out using the free radical trap (Scheme
4). In the presence of an oxidant, boronic acids have been
reported to form the aryl radicals through a single-electron
transfer.26 When the reaction has carried out under standard
condition and in the presence of 2,2,6,6-tetramethyl-
piperidine-1-oxy (TEMPO), arylated amine product formation
was observed. This observation appears to suggest that, this
protocol is not proceeding through a free radical mechanism.
Following previous reports by Lam and Watson et al.27 we have
opined a mechanism by which this protocol proceeds as shown
in Scheme 5. The cycle could be initiated with coordination
Acknowledgements
This work would not have been possible without the continuing
financial support of the NIH/NIGMS SCORE grant number
2SC1GM116724, NIMHD RCMI grant number G12 RR03020 and
a Title III Grant to Florida A&M University. The work was also
supported in part by the Pharmaceutical Research Center
NIH/NCRR grant number 1C06RR12512.
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This journal is © The Royal Society of Chemistry 2016
New J. Chem., 2018, 00, 1-3 | 5
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