The synthesis of N-arylated amides via copper(II) triflate-catalyzed direct oxygenation and N-arylation of benzylamines with aryl iodides

Article abstract of DOI:10.1002/adsc.201200424

An efficient approach for the synthesis of N-arylated amides by copper(II) triflate-catalyzed direct oxygenation and N-arylation reaction of benzylamines with aryl iodides is reported. Various benzylamines and aryl iodides can participate in the reaction, providing a series of N-arylated amides in moderate to good yields.

Full text of DOI:10.1002/adsc.201200424

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DOI: 10.1002/adsc.201200424
The Synthesis of N-Arylated Amides via Copper(II) Triflate-
Catalyzed Direct Oxygenation and N-Arylation of Benzylamines
with Aryl Iodides
Mei Xu,a Xiao-Hong Zhang,a,* Yin-Lin Shao,a Jiang-Sheng Han,a
and Ping Zhonga,b,*
a
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, Peopleꢀs Republic of China
Oujiang College, Wenzhou University, Wenzhou 325035, Peopleꢀs Republic of China
b
Fax : (+86)-0577-8668-9338; phone: (+86)-0577-8668-9338; e-mail: kamenzxh@163.com or zhongp0512@163.com
Received: May 15, 2012; Revised: June 25, 2012; Published online: October 2, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200424.
Abstract: An efficient approach for the synthesis of
N-arylated amides by copper(II) triflate-catalyzed
direct oxygenation and N-arylation reaction of ben-
zylamines with aryl iodides is reported. Various
benzylamines and aryl iodides can participate in the
reaction, providing a series of N-arylated amides in
moderate to good yields.
materials or new catalysts. For example, the Cu- or
Pd-catalyzed N-arylations of amides have been dem-
onstrated by Buchwald,[14] Hartwig[15] and Ma,[16] re-
spectively. Notably, the metal-catalyzed oxidations of
alcohols with amines[17] or ammonia[18] into amide de-
rivatives have been developed by many groups: Mad-
senꢀs[19] or Williamsꢀ,[20] for instance. However, the
metal-catalyzed direct oxygenation of a primary
amine to the amide is very difficult, and there are rel-
atively few reports about this conversion. Mizuno[21]
and Kaneda[22] reported the successful metal-catalyzed
oxygenation of primary amines to primary amides re-
spectively. Recently, Fu and co-workers[23] demon-
strated the copper-catalyzed aerobic oxidative synthe-
sis of primary amides from (aryl)methanamines.
Keywords: aryl iodides; N-arylated amides; benzyl-
amines; copper(II) triflate catalysis; oxygenation
and N-arylation
Amide derivatives are an important class of organic These excellent works urged us to postulate that the
compounds that are present in many natural products N-arylated amides could be synthesized from benzyl-
and medicinal intermediates.[1] Many amides are
ACHTUNGTRENaNGNU mines and aryl iodides by the simultaneous oxygena-
found to exhibit a wide range of biological activities, tion and N-arylation of benzylamines with the aid of
including antifungal,[2] antitumor,[3] antimicrobial,[4] O2. To the best of our knowledge, this is the first
anticancer[5] and anti-inflammatory activities.[6] Owing strategy utilizing Cu
ACTHNUTRGNEN(UG OTf)2-catalyzed oxygenation and
to their great utility, great interest has been directed arylation reaction of benzylamines with aryl iodides
to construction of the amide scaffold for many for the formation of N-arylated amides. Here, we
years.[7]
report the results in detail.
Generally, the traditional syntheses of amides are
The reaction between benzylamine (1a) and iodo-
the reactions of carboxylic acid derivatives, such as benzene (2a) to afford N-phenylbenzamide (3) was
acid chlorides, anhydrides and esters, with amines at chosen to screen the optimal conditions (Table 1). Ini-
high temperatures or with the help of Lewis acids,[8] tially, a series of nitrogen ligands, including 2,2-bipyri-
which are not atom economical and usually generate dine (L1), 1,10-phenanthroline (L2), 4,7-dimethyl-
stoichiometric amounts of by-product.[9] Alternative 1,10-phenanthroline (L3), 2,9-dimethyl-1,10-phenan-
methods, such as the Beckmann rearrangement,[10] hy- throline (L4), l-proline (L5), were tested for the reac-
dration of the nitrile,[11] amidations of aldehydes with tion of substrate 1a when using 10 mol% Cu
ACTHNUTRGNEN(UG OTf)2 as
amines[12] and metal-mediated aminocarbonylations of the catalyst, 2.5 equiv. K2CO3 as base and toluene as
aryl halides[13] have appeared to improve the proce- solvent at 1108C (entries 1–5). It can be seen that the
dures for amide synthesis. Recently, more effort is ligands affected the yields slightly and 1,10-phenan-
being paid to develop other facile, highly efficient and throline (L2) gave the best result. Next, K2CO3 was
economical methods by introduction of new starting replaced with different bases, such as Na2CO3,
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Table 1. Optimization of the reaction conditions.[a]
Entry
Cu source
Ligand
Base
Solvent
Temperature [8C]
Yield[b] [%]
1
2
3
4
5
6
7
8
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
N
L1
L2
L3
L4
L5
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaHCO3
Cs2CO3
KOH
KO-t-Bu
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMF
MeCN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
110
110
110
110
110
110
110
110
110
110
110
110
110
140
140
140
140
140
140
140
37
49
42
33
43
10 (50+6)[c]
<5 (56+6)[c]
<5 (60+7)[c]
<5 (62+9)[c]
<5 (65+8)[c]
trace (70+9)[c]
<5 (65+6)[c]
61
9
10
11
12
13
14
15[e]
16[d]
17[d]
18[d]
19[d]
20[d]
73 (87[d])
–
<5 (72[f])
<5 (65[f])
67
CuO
CuBr2
–
–
–
Cu
[a]
Reaction conditions: 1a (0.45 mmol), 2a (0.3 mmol), [Cu] (10 mol%), base (2.5 equiv.), ligand (20 mol%) and solvent
(3 mL) for 3 days.
Isolated yields.
[b]
[c]
[d]
[e]
[f]
The respective yields (3a+3b) derived from 1a were deteermined by GC-MS analysis.
Using O2 instead of air
Under N2.
Isolated yields of the oxidative product of benzylamine (3a).
NaHCO3, Cs2CO3, KOH and KO-t-Bu, the results in- 3 is derived from the molecular oxygen (entries 14
dicated that the identity of base was critical to the and 15). Under O2, other Cu catalysts, such as
success of the amination reaction (entries 6–10) and CuACHTUNGTRENNUNG
the by-product of benzylimine 3a, resulting from the found to be inferior to CuACHTNUGTRENNUNG
oxidation of benzylamine,[21b,24] was found to be amide product or benzylimine was observed from the
formed associated with a small quantity of 3b as reaction of substrate 1a with 2a in the absence of cat-
shown by GC-MS analysis. Examination of the effects alyst or ligand (entries 19 and 20).
of solvents and reaction temperatures showed that
the reaction proceeded optimally in DMSO at 1408C. electronic and steric effects of substituted benzyl-
The by-products of 3a and 3b were mainly generated amines and aryl iodides were investigated (Table 2).
With the optimal reaction conditions in hand, the
AHCTUNGTRENNUNG
when using DMF or MeCN as solvent (entries 11–14). Initially, a set of aryl iodides derivatives (2a–2h) was
In view of the important roles of O2 as oxidant,[25] by examined by reaction with benzylamine (2a) (en-
bubbling O2 into the reaction mixture, the yield of 3 tries 1–8). The result showed that the reaction seemed
can be increased to 87%, whereas when the reaction to be sensitive to the electronic effects on the aromat-
was performed under N2, no amide product 3 or ben- ic ring of the aryl iodides. For example, aryl iodides
zylimine 3a but only a trace of 3b can be detected by with Cl and F moieties gave higher yields than their
GC-MS, which indicated that O2 was beneficial to the methyl and methoxy equivalents (entries 2–5). The
oxygenation of benzylamines and the oxygen atom of substrate scope was extended to 3-methyl- and 2-me-
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The Synthesis of N-Arylated Amides via Copper(II) Triflate-Catalyzed Direct Oxygenation
Table 2. The investigation of electronic and steric effects on the amination reaction.[a]
Entry
Benzylamine
Aryl iodide
Product (yield [%][b])
1
2
3
4
5
1a
1a
1a
1a
1a
2a
2b
2c
2d
2e
3 (87)
4 (40)
5 (45)
6 (60)
7 (66)
6
7
1a
1a
2f
2g
8 (55)
9 (50)
8
9
1a
1b
2h
2a
10 (32)
11 (55)
10
11
12
1c
1d
1e
2a
2a
2a
12 (58)
13 ( 62)
14 (65)
13
1f
1g
2a
2a
15 (35)
16 (41)
14
[a]
Reaction conditions: 1 (0.45 mmol), 2 (0.30 mmol), Cu
mol%) and DMSO (3 mL) at 1408C for 3 days.
Isolated yield.
ACHTUGNTRNE(UNNG OTf)2 (10 mol%), K2CO3 (2.5 equiv), 1,10-phenanthroline (20
[b]
thoxyphenyl iodides, obtaining the meta- and para- conditions, we obtained no target amide but only the
arylated benzamides in moderate yields (entries 6 and by-products of benzylimine and benzamide.
7). Gratifyingly, a-iodonaphthalene was also tolerated
Subsequently, the scope of the benzylamines for
in this process, providing the corresponding benza- this reaction with iodobenzene was evaluated (en-
mide in 32% yield (entry 8), demonstrating that the tries 9–14), giving similar results with aryl iodides.
amination is possible even in cases where steric hin- Benzylamines bearing electron-withdrawing groups,
drance from the aryl iodide is increased. Nevertheless, such as F and Br, showed slightly higher reactivity
when we atempted to react iodomethane or bromo- than those with electron-donating groups, like methyl
benzene with benzylamine (1a) under the optimal and methoxy moieties (entries 9–12). When the sub-
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Table 3. The CuACHTUNGTRENNUNG
(OTf)2-catalyzed amidation of benzylamine (1) with aryl iodide derivatives (2).[a]
Entry
1
Benzylamine
Aryl iodide
Product (yield%)[b]
trace
1b
1b
1b
1b
2b
2c
2d
2e
2
3
4
17 (57)
18 (65)
19 (70)
5
6
1b
1b
2f
2g
20 (48)
21 (36)
7
1b
2h
22 (39)
8
1e
1e
1e
1e
2b
2c
2d
2e
23 (69)
24 (75)
25 (85)
26 (89)
9
10
11
12
13
1e
1e
2f
2g
27 (73)
28 (38)
14
1e
2h
29 (43)
[a]
Reaction conditions: 1 (0.45 mmol), 2 (0.30 mmol), Cu
(20 mol%) and DMSO (3 mL) at 1408C for 3 days.
Isolated yield.
ACHTUGNTNERN(UNG OTf)2 (10 mol%), K2CO3 (2.5 equiv.), 1,10-phenanthroline
[b]
strate scope was extended to 3- or 2-fluorobenzyl- zylamine (1b) and 4-fluorobenzylamine (1e) were ex-
ACHTUNGTRENNUNGamine, the corresponding products were obtained in amined with respect to various aryl iodides to synthe-
lower yields (entries 13 and 14). During our experi- size N-arylated amide derivatives (Table 3). As in pre-
ments, the direct dehalogenated by-products from vious studies, 4-fluorobenzylamine (1e) (entries 8–14)
benzylamines and aryl iodides can be detected by generated its corresponding amides in higher yields
GC-MS, resulting in the relatively low yields.
than 4-methoxybenzylamine (1b) (entries 1–7). Simi-
Excited by the facile synthesis of amides from sub- larly, the aryl iodides having electron-withdrawing
stituted benzylamines and aryl iodides, 4-methoxyben- groups (entries 3, 4, 10, 11) favored the amination
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The Synthesis of N-Arylated Amides via Copper(II) Triflate-Catalyzed Direct Oxygenation
Scheme 1. The reaction pathway.
more than those with electron-donating groups (en- Experimental Section
tries 1, 2, 8, 9). When both the amine and aryl iodide
possessed an electron-rich group, such as a methoxy Typical Experimental Procedure for the CuACHTUNRGTNEUNG(OTf)2-
moiety, only a trace of the desired product can be ob- Catalyzed Direct Oxygenation and N-Arylation of
tained (entry 1). On the contrary, 4-fluoro-N-(4-fluo- Benzylamines with Aryl Iodides
rophenyl)benzamide and 4-fluoro-N-(4-chlorophenyl)-
Iodide
(10 mol%),
(0.3 mmol),
K2CO3
amine
(2.5 equiv.),
(0.45 mmol),
CuACHTUNGTRENNUNG(OTf)2
benzamide were produced with higher yields of 89%
and 85%, respectively (entries 10 and 11). Moreover,
3-methyliodobenzene and 2-methoxyiodobenzene
were also good substrates, giving the corresponding
amides in moderate yields (entries 5, 6, 12, 13). Al-
though the yields of 39% and 43% were not so good,
a-iodonaphthalene was also applicable for this reac-
tion (entries 7 and 14).
The reaction process was monitored by GC-MS and
the reaction mixture was sampled every two hours. It
was found that the benzylimine A was first produced
by the Cu-catalyzed oxidative reaction of 1a with the
aid of O2,[24] then disappeared slowly to give the inter-
mediate B,[26] which was reacted with iodobenzene to
give the amide product 3 by removing HI. The possi-
ble reaction pathway is proposed in Scheme 1. The in-
teresting conversion of primary amine (1a) to inter-
mediate B was also demonstrated by Mizuno through
the sequence of oxidative dehydrogenation and suc-
cessive hydration.[21b] A controlled reaction of benzyl-
amine (1a) was carried out in the absence of iodoben-
zene, giving B as a major product with a small quanti-
ty of A by GC-MS analysis.
1,10-phenanthroline
(20 mol%) and DMSO (3 mL) were added to a Schlenk
tube. The tube was then charged with O2, and the reaction
mixture stirred at 1408C for 3 days. After the reaction was
finished, the reaction mixture was cooled to room tempera-
ture, diluted with ethyl acetate, and washed with brine. The
combined organic extracts were dried over Na2SO4, concen-
trated under vacuum, and the resulting residue was purified
by silica gel column chromatography to afford the desired
product.
Acknowledgements
The authors thank the National Natural Science Foundation
of China (No. 20972114) and the Zhejiang Provincial Natural
Science Foundation of China (Y4100578).
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ˇ
ˇ
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