Rhodium-catalyzed direct C-H amination of benzamides with aryl azides: A synthetic route to diarylamines

Article abstract of DOI:10.1002/anie.201205723

No muss, no fuss: A rhodium-catalyzed direct intermolecular C-H amination of benzamides and ketoximes using aryl azides as the amine source has been developed. The reaction exhibits a broad substrate scope with excellent functional-group tolerance, requires no external oxidants, releases N ₂ as the only by-product, and produces diarylamines in high yields. Copyright

Full text of DOI:10.1002/anie.201205723

Angewandte
Chemie
DOI: 10.1002/anie.201205723
À
C H Activation
À
Rhodium-Catalyzed Direct C H Amination of Benzamides with Aryl
Azides: A Synthetic Route to Diarylamines**
Jaeyune Ryu, Kwangmin Shin, Sae Hume Park, Ji Young Kim, and Sukbok Chang*
Dedicated to Professor Chang Kiu Lee
Diarylamines are widely present in biologically active natural
products, and are also an important synthetic unit in
numerous pharmaceuticals, agrochemicals, dyes, and func-
tional materials.[1] Among the diarylamines, derivatives bear-
ing an ortho-carbonyl group serve as important pharmaco-
phores and are key components in a wide range of bioactive
compounds (Scheme 1a).[2,3] Furthermore, they are easily
converted into various heterocycles of high synthetic utility.[4]
Conventional methods for the preparation of diarylamines
rely on the Cu-mediated coupling between aryl halides and
anilines,[5] and generate stoichiometric amounts of copper
waste. The subsequent development of catalytic methods for
this N-arylation has been predicated upon the choice of
suitable ligands (Scheme 1b).[6,7]
In recent years, new research has been focused on metal-
À
catalyzed direct C H amination (or amidation) of (hetero)-
arenes without the need for pre-functionalized aryl halides.[8]
Although this approach is highly promising, it still generates
stoichiometric amounts of by-products from the external
oxidants,[9] halide salts, or bases employed;[10] the substrate
scope is also often rather limited.
In our continuing efforts to develop highly efficient
amination reactions,[11] we recently reported a Rh-catalyzed
À
direct arene C H amidation using sulfonyl azides as the
amine source.[12] It proceeds without external oxidants and
releases N2 as a single by-product. This result led us to attempt
the direct amination of arenes with aryl azides, which can be
readily prepared with wide diversity, to afford diarylamines.[13]
In particular, we were interested in the direct installation of
an amino group onto benzamides to afford 2-anilinobenz-
amides (Scheme 1c). Although some elegant examples of
À
catalytic C H amination were previously reported using aryl-
or vinyl azides, they were mainly limited to intramolcular
conversions.[14,15] In contrast, only a few examples of inter-
molecular reactions are known, primarily for the amination of
3
[14,16,17]
À
allylic or benzylic sp C H bonds.
It is in this context
that we herein describe a highly efficient and selective Rh-
À
catalyzed direct intermolecular C H amination of benza-
mides and ketoximes using aryl azides.
À
At the outset of our study, we searched for optimal C H
amination conditions (Table 1). All reactions were catalyzed
by a cationic RhIII species, which was generated in situ by
treating [{RhCp*Cl2}2] with a silver salt. No conversion was
observed with primary or tertiary benzamides (entries 1 and
2) or with N-pivaloyloxy benzamide (entry 3), whereas
amination of N-methylbenzamide with 4-nitrophenyl azide
(2a) was found to proceed, albeit with moderate yield
(entry 4). We were pleased to observe that high yield was
obtained when N-tert-butylbenzamide (1.8 equiv relative to
2a) was applied, even with a lower loading of the rhodium
catalyst (2.5 mol%), when conducted at 858C in 1,2-dichloro-
ethane (1,2-DCE; entry 5). Reactions at lower temperatures
resulted in decreased reactivity (entries 6 and 7). Other
catalytic systems tested were largely ineffective under these
conditions (entries 8 and 9). Other carbonyl groups, including
ketone, ester, or carboxylic acids, were not found to be viable
for direct amination (entries 10–12).
Scheme 1. Utility and synthesis of diarylamines.
[*] J. Ryu, K. Shin, S. H. Park, J. Y. Kim, Prof. Dr. S. Chang
Department of Chemistry and Molecular-Level Interface Research
Center, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 305–701 (Korea)
E-mail: sbchang@kaist.ac.kr
[**] This research was supported by the Korea Research Foundation
Grant (KRF-2008-C00024), MIRC (NRF-2011–0001322), and
a Global Ph.D. Fellowship (NRF-2011-0008452) to J.R.
With the optimal conditions established, we next inves-
tigated the substrate scope of various aryl amides in the
reaction with 4-nitrophenyl azide (Scheme 2). The reaction
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205723.
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Table 1: Optimization studies.[a]
lower yields (numbers in parentheses), but still at a syntheti-
cally acceptable level, thus the present method is highly
flexible. Substrates having a meta-substituent (1e) underwent
À
the amination only at the sterically more accessible C H
bond. Likewise, 2-naphthamide was aminated exclusively at
the 3-position (3 f). The present amination was highly
compatible with various functional groups. For example,
substrates bearing a chloro (1g), bromo (1h), or ester group
(1i) were all smoothly aminated in high yields. Notably, a free
hydroxy group was tolerated under the present conditions,
producing product 3j. A highly functionalized arene com-
pound was also obtained in excellent yield (3k). Steric bulk
on the N-alkyl portion of the secondary amides was found to
have a dramatic influence on the amination efficiency. For
instance, reactions of benzamides bearing bulkier N-alkyl
groups such as adamantyl (1l) or cyclohexyl (1m) provided
higher product yields than those bearing isopropyl (1n) or
methyl (1o) groups.[18]
Entry
R
Catalyst (mol%)
T
[8C]
Yield
[%][b]
1
2
3
4
5
6
7
8
NH2
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (2.5)/AgSbF6 (10)
[{RhCp*Cl2}2] (2.5)/AgSbF6 (10)
[{RhCp*Cl2}2] (2.5)/AgSbF6 (10)
[Rh2(O2CCF3)4] (4)
85
85
85
85
85
70
50
85
85
85
85
85
<5
<5
<5
40
85(80)
73
NMe2
NHOPiv
NHMe
NHtBu
NHtBu
NHtBu
NHtBu
NHtBu
Me
49
<5
<5
<1
<1
<1
9[c]
10
11
12
[{Ru(p-cymene)Cl2}2] (4)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
[{RhCp*Cl2}2] (4)/AgSbF6 (16)
OMe
OH
The scope of aryl azides was then examined in the
amination of N-tert-butylbenzamide 1a (Scheme 3). Phenyl
[a] Conditions: 1 (0.36 mmol) and 2a (0.2 mmol) in 1,2-dichloroethane
(0.5 mL). [b] Yield determined by 1H NMR spectroscopy; yield of isolated
product shown in parentheses. [c] KPF6 (16 mol%) as an additive.
Cp*=pentamethylcyclopentadiene, Piv=pivaloyl.
was effective irrespective of variation in the electronic nature
of the benzamides (3a–3d).
It should be mentioned that the use of benzamides as the
limiting reagent (1/2a = 1:1.5) afforded products in slightly
Scheme 3. Aryl Azide substrate scope. Conditions: 1a (0.36 mmol), 2,
(0.2 mmol) in 1,2-DCE (0.5 mL); yields given are of isolated products.
[a] Reaction run for 48 h.
azides substituted with trifluoromethyl (2p), sulfonyl (2q),
ester (2r), ketone (2s), or chloro (2t) groups in the para
position were all viable for this direct amination, although
only moderate product yields were obtained in reactions with
2s and 2t. Phenyl azides bearing meta substituents (2u and
2v) also readily participated in the amination reaction,
providing products in satisfactory yields. The best result was
obtained with 3,5-bis(trifluoromethyl)phenyl azide to afford
the corresponding aminated product (3w) in excellent yield.
As in the case of benzamides, a broad range of functional
groups on the aryl azides were tolerated, thus allowing for
high diversity in the synthesis of diarylamines.
Scheme 2. Aromatic amide substrate scope. Conditions:
1 (0.36 mmol) and 2a (0.2 mmol) in 1,2-DCE (0.5 mL); yields given
are of isolated products. Yields in parentheses were obtained from
reactions with 1/2a=1:1.5. [a] Reaction run for 48 h.
Encouraged by the successful results in the amination of
benzamides, we turned our attention to additional substrates
2
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Scheme 5. Proposed reaction pathways.
will then bind to aryl azides, leading to II. Subsequent
formation of a rhodium(III) amido species IV from II can
take place by either a concerted migratory insertion or,
alternatively, a stepwise nitrenoid pathway in which a high
valent rhodium(V) species III is involved.[23,24] Finally, the
desired aminated product 3 is delivered by protonolysis of IV.
Scheme 4. Aromatic Ketoxime substrate scope. Conditions: 4
(0.2 mmol), 2 (0.3 mmol) in 1,2-DCE (0.5 mL); yields given are of
isolated products.
À
Diarylamines accessible by the present direct C H
bearing synthetically useful directing groups (Scheme 4). We
were pleased to observe that ketoximes also worked well to
facilitate direct amination with aryl azides. Electronically
modified aryl ketoximes were smoothly aminated with
4-nitrophenyl azide (2a) to give 5a–5c. A bicyclic ketoxime
derived from a-tetralone also successfully underwent amin-
ation to give 5d. Variation with regard to aryl azides was also
feasible in the present amination. In particular, it was noted
that phenyl azides bearing not only electron-withdrawing
substituents, but also electron-neutral and even electron-
donating groups were all viable for this amination, emphasiz-
ing the generality of this method.
amination have high synthetic utility. Since they are known
to be redily converted into 9H-carbazoles,[25] our method was
combined with a Pd-catalyzed oxidative cyclization procedure
to afford functonalized carbazoles 6a–6c in respectable yields
(Scheme 6).
The direct amination of indole[19] was also performed with
the same catalyst system [Eq. (1)]. Using a pyrimidyl group as
an easily installable and readily removable directing group,[20]
the reaction inserted an anilino unit selectively at the C2
position of the indole skeleton.
Scheme 6. Synthesis of multi-functionalized carbazoles.
Facile manipulation of the functional groups present in
2-anilinobenzamides offers an additional synthetic opportu-
nity. For example, N-tert-butyl-(3-trifluoromethyanilino)-
benzamide 3v, which was produced in 63% yield
(Scheme 3), can serve as a precursor to flufenamic acid,
a nonsteroidal anti-inflammatory drug (NSAID).[26] More-
over, its analogues, which exhibit diverse bioactivity,[3] can
also be envisioned to be obtained by our approach. We were
successful in preparing an N-nitrosoamino derivative of
flufenamic acid, 3vv, which is also an anti-inflammatory
agent[27] and a precursor of 1-aryl indazoles,[4b] from 3v in
acceptable yield from a literature procedure [Eq. (2)] (see the
To gain insight into the reaction mechanism, preliminary
isotopic experiments were conducted. Analysis of the ami-
nated products and recovered starting material showed
negligible H/D-scrambling, thus implying the irreversible
À
nature of the C H activation step. A competition reaction
between N-tert-butylbenzamide 1a and [D5]1a showed
a kinetic isotopic effect (KIE) of 2.6, which indicates that
À
the C H bond cleavage is most likely the rate-limiting step
(see the Supporting Information for details).[21]
The proposed pathway for this amination reaction is
À
shown in Scheme 5. A cyclometalation process through C H
bond activation delivers 5-membered rhodacycle intermedi-
ate I.[12,22] Irrespective of the exact structure of rhodacycle I, it
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Supporting Information for details).[28] Furthermore, the
N-dealkylation of 3a was readily carried out upon treatment
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with
tert-butyldimethylsilyl
trifluoromethanesulfonate
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benzamides, a pharmacophore exhibiting novel biological
activity, such as SIRT1-inhibitory action.[2]
In summary, we have presented the first example of
À
a rhodium-catalyzed direct C H amination of benzamides
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using aryl azides to deliver 2-anilinobenzamides, which have
versatile synthetic and medicinal utility. The substrate scope
was quite broad with respect to both benzamides and aryl
azides, and it was readily extended to the direct aminaton of
ketoximes with high functional group tolerance. The reaction
does not require any external oxidants and releases only N2 as
a by-product, thus providing an environmentally benign
amination process.
Experimental Section
Representative synthetic procedure: N-tert-butyl benzamide (1a,
64 mg, 0.36 mmol), 4-nitirophenyl azide (2a, 33 mg, 0.2 mmol),
[RhCp*Cl2]2 (3.1 mg, 2.5 mol%), AgSbF6 (6.9 mg, 10 mol%) and
1,2-dichloroethane (0.5 mL) were added under atmospheric condi-
tions to a screw cap vial equipped with a spinvane triangular-shaped
Teflon stir bar. The reaction mixture was stirred in a pre-heated oil
bath at 858C for 18 h, then cooled to room temperature, filtered
through a plug of celite, and washed with CH2Cl2 (10 mL ꢀ 3). The
crude reaction mixture was purified by chromatography on silica gel
(n-hexane/EtOAc = 10:1) to give the desired product, 3a (52 mg,
83%).
[11] a) J. Y. Kim, S. H. Cho, J. Joseph, S. Chang, Angew. Chem. 2010,
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Yoon, S. Chang, J. Am. Chem. Soc. 2011, 133, 5996.
Received: July 18, 2012
Published online: && &&, &&&&
[12] J. Y. Kim, S. H. Park, J. Ryu, S. H. Cho, S. H. Kim, S. Chang, J.
Am. Chem. Soc. 2012, 134, 9110.
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À
Keywords: amination · aryl azides · C H activation ·
diarylamines · rhodium
.
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Communications
À
C H Activation
J. Ryu, K. Shin, S. H. Park, J. Y. Kim,
S. Chang*
&&&& — &&&&
À
Rhodium-Catalyzed Direct C H
Amination of Benzamides with Aryl
Azides: A Synthetic Route to
Diarylamines
No muss, no fuss: A rhodium-catalyzed
substrate scope with excellent functional
group tolerance, requires no external
oxidants, releases N2 as the only by-
product, and produces diarylamines in
high yields.
À
direct intermolecular C H amination of
benzamides and ketoximes using aryl
azides as the amine source has been
developed. The reaction exhibits a broad
6
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!