Para-Selective C-H Amidation of Simple Arenes with Nitriles

Article abstract of DOI:10.1002/adsc.201500651

A para-selective C-H amidation of simple arenes with nitriles has been developed. By increasing the amount of arenes, a further meta-selective C-H arylation of the produced amides occurred. Both steric and electronic effects are utilized to control the selectivity, resulting in only para-selective amidation products. The readily available nitriles as amidation reagents instead of amides makes the synthesis of N-arylamides more accessible.

Full text of DOI:10.1002/adsc.201500651

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DOI: 10.1002/adsc.201500651
À
para-Selective C H Amidation of Simple Arenes with Nitriles
Shi-Kai Xiang,a,* Jin-Mei Li,a He Huang,a Chun Feng,a Hai-Liang Ni,a
Xiao-Zhen Chen,b Bi-Qin Wang,a,* Ke-Qing Zhao,a Ping Hu,a and Carl Redshawa,c
a
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Peopleꢀs Republic of China
E-mail: xiangsk@hotmail.com or wangbiqin1964@126.com
Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, Peopleꢀs Republic of China
Department of Chemistry, University of Hull, Cottingham Rd, Hull, HU6 7RX, U.K.
b
c
Received: July 7, 2015; Revised: August 25, 2015; Published online: November 17, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500651.
À
Abstract:
A
para-selective C H amidation of
simple arenes with nitriles has been developed. By
increasing the amount of arenes, a further meta-se-
À
lective C H arylation of the produced amides oc-
curred. Both steric and electronic effects are uti-
lized to control the selectivity, resulting in only
para-selective amidation products. The readily
available nitriles as amidation reagents instead of
amides makes the synthesis of N-arylamides more
accessible.
Keywords: amidation; arenes; copper; nitriles
The ubiquitous presence of N-arylamides in natural
products, biologically active molecules and organic
functional materials makes them valuable synthetic
targets.[1,2] The Goldberg reaction[2] is one of the most
common methods for the synthesis of N-arylamides,
but it requires prefunctionalization of arenes to aryl
halides. Therefore, extensive efforts have been devot-
À
ed to the synthesis of N-arylamides via C H amida-
tion of arenes. Over the last few years, there have
À
been tremendous advances in the field of C H amina-
tion and amidation of arenes.[3–10] Highlights include
the amidation of arenes bearing directing groups
which has been achieved by Li,[5a] Shen,[5b] Yu and
Che,[5c,7a,b] Ribas and Stahl,[5d,e] Chen,[6a,b] Daugulis,[6c]
Buchwald,[6d] Chang,[7c] Glorius,[7d] and Zhu[7e] et al.
(Scheme 1a), and the amidation of activated arenes
such as heterocycles and polyfluoroarenes as devel-
oped by Schreiber and co-workers (Scheme 1b).[8] In
À
contrast, the C H amidation of simple arenes remains
a challenge because of poor reactivity and selectivity.
In 2011, the metal-free oxidative amidation of simple
arenes using solely PhI(OAc)2 as oxidant was report-
ed independently by the Cho and Chang,[9a] De-
Boef,[9b] and Antonchick[9c] groups [Scheme 1c, Eq. Scheme 1. C H amidation of arenes.
À
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Table 1. Optimization of the reaction conditions.[a]
(1)]. However, most of the reactions resulted in a mix-
ture of several isomers. Recently, the Hartwig[10a] and
DeBoef[10b] groups have achieved the regioselective
amidation of simple arenes using respectively
Pd(OAc)2 and Au(I) as the catalyst [Scheme 1c, Eq.
(2)].
However, in all of the above-mentioned amidation
reactions of simple arenes, only substrates with
À
strong-acidic N H bond have been employed as ami-
dation reagents, such as N-arylamides and imides. The
weak nucleophilicity of N-unsubstituted amides under
acidic or neutral conditions leads to their poor reac-
tivity in the amidation reactions.[5e] Therefore, it is
very difficult to synthesize N-monosubstituted amides
À
by C H amidation of simple arenes using amides as
the amidation reagents.
Nitriles are employed widely as nitrogen nucleo-
philic reagents under acidic conditions in Ritter reac-
tions,[11] which means nitriles may have a better reac-
tivity than N-unsubstituted amides in amidation reac-
tions of simple arenes under acid conditions. Further-
more, our and othersꢀ recent work[12] also indicated
that the use of nitriles as nitrogen nucleophiles in-
stead of amides can make the synthesis of N-aryl-
amides more accessible, because many amides are
readily prepared from nitriles by hydrolysis. Such re-
À
sults stimulated us to explore the feasibility of C H
amidation of simple arenes with nitriles. Herein, we
À
[a]
report a para-selective C H amidation of simple
Reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol), cata-
lyst, MesI(OAc)2 (0.45 mmol), additive, H2O (0.9 mmol),
DCE (2.0 mL, contains 0.1% v/v H2O), reflux, 15 h,
under argon. The yields are of isolated products.
H2O was added after 2 h.
arenes using nitriles as the amidation reagents.[10c]
When the amount of arene was increased to four
À
equivalents, a further meta-selective C H arylation of
[b]
[c]
[d]
the produced amides occurred, resulting in tandem
0.36 mmol of 2a was used.
The reaction was carried out at 708C.
À
À
C N and C C bond formation (Scheme 1d).
À
We began to investigate the C H amidation of o-
xylene 1a with benzonitrile 2a using Cu(OTf)2 as a cat-
alyst, MesI(OAc)2 as an oxidant and DCE as the sol- loading of catalyst or benzonitrile 2a led to a lower
vent under reflux conditions. The variation of the yield of 3aa (entries 15 and 16, Table 1). An attempt
acids indicated that TfOH could effectively promote to reduce the temperature to 708C resulted in a signif-
the reaction to give the product 3aa in a yield of 64% icantly decreased yield of 3aa (entry 17, Table 1). It is
(entries 1–3, Table 1). Use of TMSOTf as an additive notable that only the para-selective isomer was ob-
1
instead of TfOH could increase the yield to 74% served in H NMR of the crude reaction mixture.
(entry 4, Table 1). The screening of copper salts re-
To explore the substrate scope of simple arenes,
vealed that Cu(OTf)2 was superior to all other tested benzonitrile 2a was selected to react with a range of
salts (entries 4–8, Table 1). No desired product was substituted arenes 1 under standard conditions. Both
observed in the absence of a copper catalyst (entry 9, mono- and disubstituted simple arenes could undergo
Table 1). In consideration of the decomposition of the amidation reaction to generate the corresponding
some of the TMSOTf into HOTf in the presence of N-arylamides in moderate to high yields with high re-
H2O, we decided to add H2O to the reaction after two gioselectivities (35–93%, Table 2). The amidation re-
hours, because at that time TMSOTf had been con- actions occurred para to the activating ortho-/para-di-
sumed to generate the diaryliodonium salts 5 instead recting substituents, when monosubstituted benzenes
of being decomposed into HOTf based on the reac- were deployed in this reaction (3da, 3ea, 3fa and 3ga,
tion mechanism in Scheme 3. As expected, the best Table 2). Stronger ortho-/para-directors dominated
yield of 83% was obtained if H2O was added after and directed the regioselectivities when disubstituted
two hours (entry 10, Table 1). Adjustment of the benzenes were employed as the substrates. The ami-
amount of MesI(OAc)2 and TMSOTf did not improve dation reactions occurred para to the stronger ortho-/
the yield of 3aa (entries 11–14, Table 1). A lower para-directing substituents (3ba and 3ca, Table 2).
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para-Selective C H Amidation of Simple Arenes with Nitriles
Table 2. Substrate scope of simple arenes.[a]
Table 3. Substrate scope of nitriles.[a]
[a]
Reaction conditions:
Cu(OTf)2 (0.03 mmol),
1
(0.3 mmol), 2a (0.45 mmol),
MesI(OAc)2 (0.45 mmol),
TMSOTf (0.6 mmol), H2O (0.9 mmol), DCE (2.0 mL,
contains 0.1% v/v H2O), reflux, 15 h, under argon. H2O
was added after 2 h. The yields were isolated yields.
The reaction was carried out for 24 h.
[b]
[a]
Reaction conditions: 1a (0.3 mmol),
2
(0.45 mmol),
(0.45 mmol),
Cumene 1f and tert-butylbenzene 1g gave low yields,
maybe because of Friedel–Crafts dealkylation of the
substrates (3fa and 3ga, Table 2). Use of m-xylene 1h
as a substrate afforded a low yield, indicating that the
reaction is sensitive to steric effects (3ha, Table 2).
1,2,3,4-Tetrahydronaphthalene 1i underwent this
Cu(OTf)2
(0.03 mmol),
MesI(OAc)2
TMSOTf (0.6 mmol), H2O (0.9 mmol), DCE (2.0 mL,
contains 0.1% v/v H2O), reflux, 15 h, under argon. H2O
was added after 2 h. The yields are of isolated products.
transformation to generate the para-selective amida- not affected by steric effects of the nitriles (3ab, 3ac
tion product 3ia in a yield of 66% (3ia, Table 2). All and 3ad, Table 3). Substituted benzonitriles with F, Cl
of the above findings demonstrate that the regioselec- or I smoothly led to the corresponding amides with
tivities of such amidation reactions are clearly con- the halogen substituents intact (3af, 3ag and 3ah,
trolled by both steric and electronic effects, resulting Table 3). The heterocyclic aromatic nitriles 2i, 2j also
in the isolation of only para-selective amidation prod- successfully underwent the reaction to afford the de-
ucts. Other isomers were not observed in each case by sired products 3ai, 3aj in yields of 68%, 53%, respec-
1H NMR of the crude reaction mixtures. It is regretful tively (3ai and 3aj, Table 3). High yields were ob-
that electron-deficient arenes could not undergo this tained when aliphatic nitriles 2k, 2l and 2n were se-
reaction to get the desired products, such as chloro- lected as the substrates (3ak, 3al and 3an, Table 3).
benzene and nitrobenzene.
Pleasingly, the aliphatic nitrile 2m smoothly under-
To investigate the substrate scope of the nitriles, o- went this transformation, revealing that the halogen
xylene 1a was selected as the partner to react with substituents were compatible under such reaction con-
different nitriles. The results in Table 3 show that ditions (3am, Table 3). In addition, other isomers
both aryl nitriles and aliphatic nitriles could undergo were not observed except the para-selective product
1
this transformation to generate the desired products in each case by H NMR of the crude reaction mix-
in moderate to excellent yields (40–84%). Substituted tures.
benzonitriles bearing both electron-donating and elec-
By increasing the amount of arene to four equiva-
tron-withdrawing groups smoothly underwent this re- lents, we were surprised to find that further arylation
action generating the desired products 3 in good yield of the produced N-arylamides occurred selectively at
(3aa–3ah, Table 3). It is notable that the reaction was the meta-position of the aromatic ring (Table 4).[13]
A
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Shi-Kai Xiang et al.
Table 4. para-Selective amidation of o-xylene 1a with nitriles
2, followed by meta-selective arylation of the produced N-ar-
ylamides with another molecule of o-xylene 1a.[a]
Figure 1. Yield of 3aa as a function of time in the reaction
of diaryliodonium salt 5a with benzonitrile 2a catalyzed by
&
*
CuOTf and Cu(OTf)2. =CuOTf, =Cu(OTf)2. Reaction
conditions: diaryliodonium salt 5a (0.3 mmol), benzonitrile
2a (0.45 mmol), [Cu] (0.03 mmol), H2O (0.9 mmol), DCE
(2.0 mL, contains 0.1% v/v H2O), reflux, under argon. The
yields are of isolated compounds.
[a]
Reaction conditions: 1a (1.2 mmol),
Cu(OTf)2 (0.06 mmol), MesI(OAc)2
TMSOTf (1.5 mmol), H2O (0.9 mmol), DCE (3.0 mL,
contains 0.1% v/v H2O), reflux, 24 h, under Ar. H2O was
added after 3 h. The yields are of isolated products.
2
(0.3 mmol),
(0.9 mmol),
variety of aryl nitriles was selected to react with o- lar mechanism reported recently by us.[12a,b] The
xylene 1a. The results revealed that benzonitriles diaryliodonium salt 5a was employed to react with
bearing both electron-donating and electron-with- benzonitrile 2a to get the corresponding product 3aa
drawing groups smoothly underwent the multi-step in a yield of 81% [Eq. (2), Scheme 2]. It meant that
transformation to generate the corresponding prod- the diaryliodonium salt 5a may be an intermediate of
ucts 4 in moderate yields (4a–4f, Table 4).
this transformation.[14] In addition, the isotopic label-
Some control experiments were carried out to un- ling experiment with use of H218O revealed that the
derstand the mechanism of the amidation reaction water was indeed involved in this reaction [Eq. (3),
(Scheme 2 and Figure 1). It was found that no product Scheme 2].
was detected when using benzamide to react with o-
However, an attempt to prepare the Cu(III) inter-
xylene 1a instead of benzonitrile according to our mediates B in Scheme 3 to evaluate the following
standard condition [Eq. (1), Scheme 2]. It excluded transformations failed maybe because of the instabili-
the possibility of amide being involved in the reaction ty of it [Eq. (4), Scheme 2]. The literature indicated
through hydrolysis from nitriles according to the simi- that only a few stable organocopper(III) complexes
Scheme 2. Mechanistic experiments.
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para-Selective C H Amidation of Simple Arenes with Nitriles
have been obtained.[15] Instead, diaryliodonium salt 5a C C bonds formation. Both steric and electronic ef-
(recovery rate of 50%), 2-iodomesitylene (yield of fects were utilized to control the selectivity, resulting
36%) and 3,4-dimethylphenyl trifluoromethanesulfo- in only para-selective amidation products. N-Mono-
nate (yield of 14%) were acquired. In order to prove substituted amides can be synthesized with this ap-
the possible active copper catalyst, we monitored the proach, which both complements and extends the pre-
kinetics of the reaction (Figure 1). The result indicat- vious scope of applicable substrates. Given the use of
ed that CuOTf was more active than Cu(OTf)2, which readily available nitriles as amidation reagents (in-
À
implied that CuOTf may be the active catalyst.
stead of amides), this route allows for the facile syn-
On the basis of the experimental results, a possible thesis of N-arylamides. Further studies on other reac-
catalytic mechanism for this transformation is illus- tions of nitriles and the applications are now ongoing
trated in Scheme 3.[13b,16] Initially, the TMSOTf-pro- in our laboratory.
moted electrophilic aromatic substitution of arenes
with MesI(OAc)2 generates the diaryliodonium salts
5. The active catalyst CuOTf, formed from Cu(OTf)2 Experimental Section
through the disproportionation or reduction, is oxi-
dized to the intermediates B by the diaryliodonium Typical Procedure for the Synthesis of N-(3,4-
salts 5. The intermediates B undergo a ligand ex- Dimethylphenyl)benzamide (3aa)
change with nitriles 2 to afford the intermediates C.
The proposed Cu(III) species promote the hydrolysis
of coordinated nitriles into intermediates D, which
then undergo a reductive elimination to yield the
final products 3 and release the catalyst CuOTf to
complete the catalytic cycle.
Cu(OTf)2 (10.8 mg, 0.03 mmol) and MesI(OAc)2 (163.8 mg,
0.45 mmol) were placed into
a 25-mL Schlenk tube
equipped with a magnetic stir bar. To this mixture was
added in sequence DCE (2.0 mL, contains 0.1% v/v H2O),
o-xylene 1a (36 mL, 0.3 mmol), benzonitrile 2a (46 mL,
0.45 mmol), TMSOTf (108 mL, 0.6 mmol) with an injection
syringe under stirring and an argon atmosphere. After the
reaction mixture had been stirred for 2 h at reflux, water
(16 mL, 0.9 mmol) was added with an injection syringe and
the mixture was stirred for additional 13 h at reflux under
an argon atmosphere. The solution was cooled to room tem-
perature and was quenched by the addition of 20 mL water
and extracted with dichloromethane (3”10 mL). The organ-
ic layer was dried over anhydrous MgSO4, and concentrated
under vacuum. The residue was purified by column chroma-
tography on silica gel (eluent: petroleum ether/ethyl ace-
tate/dichloromethane=10:1:1) to afford 3aa; yield: 56.0 mg
In summary, we have developed a copper-catalyzed
À
highly para-selective C H amidation of simple arenes
using nitriles as the amidation reagents. When the
amount of arenes was increased to four equivalents,
À
a further meta-selective C H arylation of the pro-
duced amides occurred, resulting in tandem C N and
À
(83%), IR (KBr): nmax =3304, 1649, 1591, 1528, 820 cmÀ1
;
1H NMR (400 MHz, CDCl3): d=7.87–7.84 (m, 2H), 7.76 (s,
1H), 7.57–7.44 (m, 4H), 7.35 (dd, J1 =8.0 Hz, J2 =2.4 Hz
1H), 7.12 (d, J=8.4 Hz, 1H), 2.27 (s, 3H), 2.25 (s, 3H);
13C NMR (100 MHz, CDCl3): d=165.6, 137.3, 135.5, 135.1,
133.0, 131.7, 130.0, 128.7, 126.9, 121.5, 117.7, 19,9, 19.2; HR-
MS (ESI): m/z=226.1226, calcd. for C15H16NO [M+H]+:
226.1226.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (No. 21202109 and 21402187), Special Funds
of Sichuan Normal University for Sharing the Large Preci-
sion Equipments (Nos. DJ2014-26 and DJ2014-24) and Si-
chuan Normal University is greatly appreciated. CR thanks
the EPSRC.
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Adv. Synth. Catal. 2015, 357, 3435 – 3440