Cobalt(III)-catalyzed C-H amidation of arenes using acetoxycarbamates as convenient amino sources under mild conditions

Article abstract of DOI:10.1021/cs501860b

The Co(III)-catalyzed direct C-H amidation of arenes has been developed using O-acylcarbamates as a convenient amino source. This reaction proceeded in high efficiency under external oxidant-free conditions with a broad range of arene substrates, includin

Full text of DOI:10.1021/cs501860b

Research Article
pubs.acs.org/acscatalysis
Cobalt(III)-Catalyzed C−H Amidation of Arenes using
Acetoxycarbamates as Convenient Amino Sources under Mild
Conditions
Pitambar Patel†,‡ and Sukbok Chang*,†,‡
†Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 305-701, Korea
‡Department of Chemistry, Korea Advanced Institute of Science & Technology (KAIST), Daejeon 305-701, Korea
S
* Supporting Information
ABSTRACT: The Co(III)-catalyzed direct C−H amidation of arenes
has been developed using O-acylcarbamates as a convenient amino
source. This reaction proceeded in high efficiency under external
oxidant-free conditions with a broad range of arene substrates,
including 6-arylpurines bearing sensitive functional groups, thus
furnishing synthetically versatile arene N-carbamate products.
KEYWORDS: cobalt catalysis, C−H amidation, acetoxycarbamates, 6-arylpurines, arylamines
1). This protocol was also successfully extended to the selective
amidation of 6-arylpurines.
INTRODUCTION
■
In recent years, transition-metal-catalyzed direct C−H amina-
tion has been extensively investigated as a selective and
straightforward method for the synthesis of (hetero)aryl
amines.1,2 This approach displays certain advantages over the
traditional methods in that arenes are directly employed as
substrates without prefunctionalization. As a result, a list of
metal-catalyzed procedures, including Pd,3 Rh,4 Ru,5 and Ir6
catalyses, have been scrutinized. In this regard, we also have
developed Cp*-based catalytic systems of Rh(III)7a−d and
Ir(III)7e−i for the C−H amination of arenes, alkenes, and
alkanes using organic azides as the amino source. The fact that
the currently available procedures are mediated mainly by
precious late transition metals, albeit being efficient and mild,
has led us to search for an alternative catalytic system
employing earth-abundant first-row transition metals.8 In the
past few years, while copper and iron species have been
scrutinized as catalysts for C−H amination,9 cobalt has
emerged as one of the promising metal species for C−H
bond functionalization, being especially effective in C−C bond
formation.10 In this line, we envisaged that our previous
catalytic systems of Rh or Ir could be extended to their group 9
cousin, cobalt.11 Indeed, we already reported a comparative
study of the Cp*-based group 9 metal-catalyzed C−H
amination using organic azides to reveal that [Cp*CoIII]
showed much poorer catalytic activity in comparison to the
corresponding Rh or Ir systems.12 However, a recent report
from the Kanai group demonstrated that C-2 amidation of
indoles could be highly efficient with CoCp*(CO)I2 catalyst
using sulfonyl azides.13,14 Herein, we report the development of
Co(III)-catalyzed direct C−H amidation using acetoxycarba-
mates as the new and convenient amidating source15,16 to afford
synthetically versatile N-substituted aniline products (Scheme
Scheme 1. Co(III)-Catalyzed C−H Amidation
RESULTS AND DISCUSSION
■
At the outset of our studies, 2-phenylpyridine (1a) was chosen
as a model substrate to react with 1.1 equiv of N-Boc-
substituted benzoylhydroxylamine (2a) for the initial screening
of Co-catalyzed optimal amidation conditions. After a series of
preliminary reactions, we were pleased to observe that the
desired monoamidated product (3a) could be obtained in 72%
yield using an ester-substituted aroyloxy carbamate (2a) as an
amidating reagent when CoCp*(CO)I2 (8 mol %) was
employed as a catalyst in the presence of AgSbF6 (16 mol
%) in acetone at 60 °C (see the Supporting Information for
details). With this promising initial result, we decided to search
Received: November 24, 2014
Revised: December 19, 2014
© XXXX American Chemical Society
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DOI: 10.1021/cs501860b
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Research Article
for additional carbamates to see the effects of leaving groups on
the reaction efficiency (Scheme 2).
With the finding of a readily available amidating reagent in
hand, we subsequently optimized reaction parameters using
tert-butyl acetoxycarbamate (2l) in the amidation of 2-
phenylpyridine (1a; Table 1).
a
Scheme 2. Screen of Amidating Reagents
While the amidation at higher temperature (80 °C; entry 2)
proceeded with similar efficiency, the reaction efficiency
decreased at a lower temperature (entry 3). No reaction was
observed in the absence of cobalt catalyst or silver salt additive
(entries 4 and 5). Among the various silver additives screened,
AgSbF6 turned out to be most effective (entries 6−8), and
NaSbF6 was found to be totally inhibitory (entry 9). The
reaction efficiency decreased to some extent when the catalyst
loading was reduced to 5 mol % (entry 10). Interestingly, a
similar result was obtained with the use of a dimeric cobalt
species [CoCp*I2]2 (4 mol %; entry 11).17 On the other hand,
the observation that sulfonyl azide was totally ineffective (entry
12) under the present Co catalytic conditions is in distinct
contrast to our previous results that organic azides were highly
efficient amidating reagents under the Rh, Ir, or Ru catalyst
systems.5c,7 In addition, C−H amidation using tert-butyl
acetoxycarbamate (2l) did not occur when other cobalt species
such as CoCl2, Co(acac)3, and Co(OAc)2 were used as catalysts
(see the Supporting Information for details).
With the optimized cobalt catalyst system with tert-butyl
acetoxycarbamate (2l), the scope of substrates was investigated
next (Scheme 3). The amidation proceeded efficiently over a
broad range of substrates, irrespective of their electronic and/or
steric variation. In fact, 2-phenylpyridine derivatives bearing
alkyl groups underwent amidation smoothly (3b−e), and this
efficiency was not altered by the position of the substituents.
The reaction of an amino-containing substrate was smooth
(3f), and substrates bearing an ether functionality such as
methoxy (3g) or benzyloxy (3h) were also facile for the
amidation. Synthetically important functional groups such as
acetyl (3i), ester (3j), chloro (3l,n), and bromo (3m) were well
tolerated under the present conditions. Regioselective
amidation took place at the sterically less demanding C−H
bond of substrates bearing meta substituents (3c,d,n). In
addition, it needs to be mentioned that amidation occurred at
the ortho position relative to the 2-pyridyl moiety in the
a
Reaction conditions: 1a (0.1 mmol), 2 (1.1 equiv), CoCp*(CO)I2 (8
mol %), and AgSbF6 (16 mol %) in acetone (0.7 mL) at 60 °C for 16
h. Yields are based on 1H NMR of the crude reaction mixture
(Cl2CHCHCl2 as an internal standard).
It was found that aroyloxy carbamates bearing electron-
withdrawing (2a−c) groups, except for 2d, were slightly more
effective than either that with a unsubstituted benzoyl group
(2e) or those having electron-donating groups (2f,g). In
addition, acyloxy analogues such as phenylacetyl (2h),
methoxycarbonyl (2i), pivaloyl (2j), and isobutyryl (2k) also
participated in the amidation with slightly lower efficiency in
comparison to that of aroyloxy carbamates. Most significantly,
on the other hand, acetoxycarbamate (2l) was found to be a
highly efficient amidating reagent to afford the desired product
(3a) in good yield. It should be emphasized that an easily
removable byproduct (acetic acid) is generated from this
reaction using 2l, thus making this procedure highly convenient
to perform.
Table 1. Variation of Reaction Parameters
a
entry
change from the “standard conditions”
yield of 3a (%)
1
none
88 (84)
2
80 °C instead of 60 °C
50 °C instead of 60 °C
no CoCp*(CO)I2
82
66
0
3
4
5
no AgSbF6
0
6
AgNTf2 instead of AgSbF6
AgOAc instead of AgSbF6
Ag2CO3 instead of AgSbF6
NaSbF6 instead of AgSbF6
68
12
10
0
7
8
9
10
11
12
5 mol % of CoCp*(CO)I2 instead of 8 mol %
4 mol % of [CoCp*I2]2 instead of CoCp*(CO)I2
TsN3 instead of 2l to give 4a
60
74
0
a
1H NMR yield (Cl2CHCHCl2 as an internal standard); isolated yield in parentheses.
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a
Scheme 3. Substrate Scope of Amidation Reaction
a
Reaction conditions: 1a (0.1 mmol), 2l (1.1 equiv), CoCp*(CO)I2 (8 mol %), and AgSbF6 (16 mol %) in acetone (0.7 mL) at 60 °C for 16 h.
TBS = tert-butyldimethylsilyl.
b
a
presence of potential directing groups such as ketone and ester
(3i,j).7g On the other hand, amidation of 2-(3,5-disubstituted
phenyl)pyridines was not successful, presumably due to steric
congestion.
Scheme 4. Scope of Various N-Protecting Groups
Functional group tolerance was again verified in the
successful amidation of compounds bearing free hydroxyl
(3o), acetyl (3p), and silyl (3q) groups. The reaction of fused
heterocyclic arenes such as dibenzofuryl (3t) and dibenzothio-
phenyl (3u) was smooth. In addition, the amidation of 3-
phenylisoquinoline (3v), benzo[h]quinoline (3w), and 2-
phenylpyrimidine (3x) was observed to be facile under the
cobalt-catalyzed conditions. However, the amidation of arenes
containing amide, ester, and oxime directing groups was
sluggish to give low product yields under the present
conditions.
As the next step, we were interested in the introduction of
different amido groups in addition to the N-Boc moiety by
examining various carbamates (Scheme 4). We were pleased to
see that a range of amidating reagents was successfully
employed under the presently developed Co catalytic system,
a
Reaction conditions: 1a (0.1 mmol), 2 (1.1 equiv), CoCp*(CO)I2 (8
mol %), and AgSbF6 (16 mol %) in acetone (0.7 mL) at 60 °C for 16
h.
although the reaction efficiency was slightly lower in
comparison to that of the corresponding N-Boc. It is
noteworthy that those installed N substituents can readily be
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a
Scheme 5. Direct C−H Amidation of 6-Arylpurine Derivatives
a
Reaction conditions: 8 (0.1 mmol), 2l (1.1 equiv), CoCp*(CO)I2 (8 mol %), and AgSbF6 (16 mol %) in acetone (0.7 mL) at 60 °C for 16 h.
removed under varied conditions: acidic (N-Boc), reductive
(N-Cbz; 5a,b), or basic (N-Fmoc; 7).18
subjected to the Co-catalytic system in CD3OD in the absence
of amidating reagents (Scheme 6a) to suggest that the cobalt-
6-Arylpurine compounds are biologically interesting in that
they often exhibit high potency in antimycobacterial, cytostatic,
and anti-HCV activities.19 Whereas purine is an important
building unit in the synthesis of nucleosides, it also serves as an
effective directing group in the direct C−H functionalization of
6-arylpurines in Ru,20a Pd,20b,c or Rh catalytic systems.20d In
particular, we recently demonstrated that the purinyl N1 atom
plays a key role in the Rh-catalyzed amidation in generating a
rhodacycle intermediate, while N7 promotes an intramolecular
H bonding of amidated products.20d In this aspect, we were
curious whether similar directing effects can also be displayed
by the purine moiety even in the present cobalt catalyst system.
We were pleased to observe that the amidation indeed occurs
over a wide range of 6-arylpurines in the Co-catalyzed system
by using tert-butyl acetoxycarbamate (2l) as the amino source
(Scheme 5). While its efficiency was found to be little
influenced by the type of N9 substituent, a series of 6-
phenylpurines bearing an N9-isopropyl or readily removable
N9-benzyl group was amidated selectively at the desired
position (9a,b and 9c−f, respectively). In addition, substrates
bearing a pendant N9-(O-acetyl-β-ribofuranosyl) group were
also amidated in synthetically acceptable yields (9g,h), thus
demonstrating an excellent level of functional group tolerance.
It was also noteworthy that an analogous substrate lacking an
N7 nitrogen was amidated in good yield (9i), suggesting that
the N1 rather than the N7 atom serves as a chelating group to
lead to the present amidation, as in the case of the
corresponding Rh catalyst system using organic azides.20d
To obtain mechanistic insights into the present Co-catalyzed
direct C−H amidation, preliminary experiments were carried
out. A significant level of deuterium incorporation (50%) was
observed at the ortho position of 2-phenylpyridine when it was
Scheme 6. Preliminary Mechanistic Studies: (a) Deuterium
Scrambling and (b) KIE Study
mediated C−H bond cleavage is reversible. Whereas a low level
of primary kinetic isotope effects (KIE = 1.1) was measured in
parallel experiments (Scheme 6b), more significant effects (KIE
= 1.6) were observed in intermolecular competition reactions in
the same vessel. Although it is not conclusive at the present
stage, these rather low KIE values may imply that the C−H
bond cleavage may not be involved in the rate-limiting stage.21
On the basis of the above mechanistic studies and relevant
reports on Rh- or Ir-catalyzed amidation reactions,15,16b,22
a
plausible catalytic cycle of the present Co(III)-catalyzed direct
C−H amidation is depicted in Scheme 7 using 2-phenyl-
pyridine as a representative substrate. First, the treatment of the
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Scheme 7. Plausible Catalytic Cycle
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.C.: sbchang@kaist.ac.kr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Professor Youngkyu Do (KAIST) for his
honorable retirement. This work was supported by the Institute
for Basic Science (IBS-R10-D1) in Korea. We thank Dr. Jeung
Gon Kim and Dr. Amit B. Pawar for valuable discussions.
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CONCLUSION
■
In summary, we have developed the direct C−H amidation of
arenes using acetoxycarbamates as a new and convenient amino
source with a cobalt catalyst system. This amidation does not
require external oxidants or bases and releases acetic acid as a
readily removable byproduct. A range of arenes, including 6-
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ASSOCIATED CONTENT
* Supporting Information
The following file is available free of charge on the ACS
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