Cationic Cobalt(III)-Catalyzed Aryl and Alkenyl C-H Amidation: A Mild Protocol for the Modification of Purine Derivatives

Article abstract of DOI:10.1002/chem.201503533

A cationic cobalt(III)-catalyzed direct C-H amidation of unactivated (hetero)arenes and alkenes by using 1,4,2-dioxazol-5-ones as the amidating reagent has been developed. This transformation proceeds efficiently under external oxidant-free conditions wit

Full text of DOI:10.1002/chem.201503533

DOI: 10.1002/chem.201503533
Communication
&
CÀH Activation
Cationic Cobalt(III)-Catalyzed Aryl and Alkenyl CÀH Amidation:
A Mild Protocol for the Modification of Purine Derivatives
Yujie Liang,[a] Yu-Feng Liang,[a] Conghui Tang,[a] Yizhi Yuan,[a] and Ning Jiao*[a, b]
employed for chemoselective CÀH functionalization by the
Abstract: A cationic cobalt(III)-catalyzed direct CÀH amida-
tion of unactivated (hetero)arenes and alkenes by using
1,4,2-dioxazol-5-ones as the amidating reagent has been
developed. This transformation proceeds efficiently under
external oxidant-free conditions with a broad substrate
scope. Moreover, 6-arylpurine compounds, which often ex-
hibit high potency in antimycobacterial, cytostatic, and
anti-HCV activities, can be smoothly amidated, thus offer-
ing a mild protocol for their late stage functionalization.
groups of Matsunaga/Kanai,[6] Ackermann,[7] Glorius,[8] Ellman,[9]
and Chang.[10]
With our continuing interest in inexpensive metal-catalyzed
CÀH nitrogenation reactions,[11,12] we have developed a cationic
cobalt(III)-catalyzed CÀH amidation of unactivated (hetero)ar-
enes and alkenes that provides expedient access to diverse
substituted amides in good to excellent yields (Figure 1). The
significances of the present method are fourfold:
Transition-metal-catalyzed directed CÀH bond functionalization
has emerged as a powerful synthetic methodology[1] that
allows for the direct use of nonactivated substrates. Among
the various transition-metal catalysts suitable for the directed
CÀH amidation of arenes, alkenes, and alkanes, Cp*-based (Cp*
= pentamethylcyclopentadiene) catalytic systems of RhIII and
IrIII have been well developed using organic azides as the
amino source.[2] Despite their high catalytic activity and mild
reaction conditions, these protocols suffer from the require-
ment of expensive rhodium and iridium catalysts, especially for
large-scale synthesis. Since the first-row transition metals are
earth abundant and cheaper than their 4d or 5d metal conge-
ners, the development of efficient catalytic systems based on
inexpensive first-row metals as alternatives to the precious
metals is highly attractive.[3] Among the first-row transition
metals, cobalt has emerged as one of the most promising cata-
lysts for the direct CÀH functionalizationthat leads to syntheti-
cally useful and cost-effective transformations.[4] Recently, low-
valent cobalt catalysts were utilized by Nakamura, Yoshikai, Ac-
kermann, and Daugulis for CÀH transformations that were,
until then, the domain of precious Rh, Pd, and Ru catalysts.[5]
More recently, high-valent cobalt(III) catalysts were elegantly
Figure 1. Cobalt-catalyzed CÀH amidation.
1) Cost-effective, user-friendly cobalt catalyst is used under ex-
ternal oxidant-free conditions in the atmospheric environ-
ment. More attractively, compared to the reported Co catal-
ysis, the present cationic cobalt(III) catalysts demonstrate
high efficiency, which may promote the development of CÀ
H functionalization by cationic Co catalysis.
2) 1,4,2-Dioxazol-5-ones are employed as amidating reagent
with CO2 as the single byproduct.
3) External oxidant is not required in this mild protocol.
4) The substrate scope is broad, including versatile arenes,
heteroarenes and alkenes with high selectivity.
Thus, this methodology opens a new way to practical inter-
molecular CÀN bond formation.
6-Arylpurine derivatives, which often exhibit high potency in
antimycobacterial, cytostatic, and anti-HCV activities, are of
high utility in medicinal chemistry. Besides, they also serve as
useful building units in the synthesis of nucleosides.[13] In this
regard, a selective installation of useful functional groups in 6-
arylpurines skeleton for their late stage modification is very im-
portant. However, to date, only a few examples of direct CÀH
functionalization of 6-arylpurines have been reported.[14] We
therefore commenced our studies by using 9-isopropyl-6-
phenyl-9H-purine (1a) as a model substrate to react with
1.1 equivalents of diverse amidating reagents for the optimiza-
tion of cobalt-catalyzed amidation reaction conditions
(Table 1). After a series of preliminary screening, we were
pleased to find that the desired amidation product (3a) could
be obtained in 70% yield using 3-phenyl-1,4,2-dioxazol-5-one
[a] Y. Liang, Y.-F. Liang, C. Tang, Y. Yuan, Prof. Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences
Peking University
Xue Yuan Rd. 38, Beijing 100191 (P. R. China)
Fax: (+86)10-82805297
E-mail: jiaoning@bjmu.edu.cn
Homepage: http://sklnbd.bjmu.edu.cn/nj/
[b] Prof. Dr. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503533.
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sides, several simple metal salts, such as CoBr2, Co(OAc)2, CuTc
(Tc = thiophene-2-carboxylate), [Fe(acac)3] (acetylacetonate)
were also tested, but they did not work (see the Supporting
Information).
Table 1. Optimization of cobalt-catalyzed CÀH amidation.[a]
With the optimized conditions in hand, the substrate scope
was then investigated. The amidation proceeded smoothly
over a broad range of substrates (Scheme 1). Amidating re-
agent, 1,4,2-dioxazol-5-one, with a methyl group at the 3-posi-
tion can also react smoothly to provide the corresponding
product (3b). Notably, analogous substrates lacking a N7 nitro-
gen were amidated in high yields, which suggested that the
N1 rather than the N7 atom serves as a chelating group to
lead to the amidation (3c–3d). In addition, 6-(4-substituted
phenyl)purine derivatives bearing a pendant N9-(O-acetyl-b-ri-
bofuranosyl) group were also successfully amidated in good
to excellent yields (3e–3j).
To expand the utility of this methodology, we turned our at-
tention to study the reactivity of substrates with different di-
recting groups (Scheme 2). Gratifyingly, this protocol can be
applied for efficient C2 selective CÀH amidation of indole with
only 2.5 mol% catalyst loading (4a). Substrates using oxime
ether as the directing group can also be functionalized (4b)
albeit in low yield. Phenyl derivatives bearing pyrimidine, pyra-
Entry
Amidating
Reagent
Catalyst (mol%)
Solvent
Yield
[%][b]
1
2
3
4
5
6
7[c]
8
A
B
C
D
E
F
F
F
F
F
F
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(CO)I2] (5)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
THF
DCE
N.R.
N.R.
N.R.
N.R.
N.R.
70
30
[Cp*Co(CO)I2] (5)
<5
<1
39
9
10
11
[Cp*Co(MeCN)3][(SbF6)2] (5)
[Cp*Co(MeCN)3][(SbF6)2] (5)
–
N.R.
[a] Reaction conditions: 1a (0.2 mmol), amidating reagent (0.22 mmol), cata-
lyst (2.5–5.0 mol%), solvent (1 mL), stirred at 808C under air for 12 h. [b] Iso-
lated yields; N.R. = no reaction. [c] AgSbF6 (10 mol%) was added.
as an amidating reagent with cationic [Cp*Co(MeCN)3]
[(SbF6)2]—the preparation and catalysis of which were firstly re-
ported by Glorius[8a]—as the catalyst (5 mol%) in DCE under air
(Table 1, entries 1–6). Notably, 1,4,2-dioxazol-5-ones can be
easily prepared from the corresponding hydroxamic acids, and
can be used or stored without a special precaution.[15] Similar
to acyl azides, they are known to generate N-acyl nitrenes
through thermal or photo-initiated decomposition, which
makes them a very good nitrene sources.[16] The direct CÀN
bond formation by using 1,4,2-dioxazol-5-one reagents has
been elegantly developed by the groups of Bolm,[17] DubØ,[18]
and Chang.[19] More recently, Chang and co-workers disclosed
for the first time that these reagents can be used as efficient
amino sources under a Cp*RhIII-catalyzed system.[19] Despite
this progress, the application of 1,4,2-dioxazol-5-one as an ami-
dating reagent in metal-catalyzed CÀH amidation is still highly
desired in organic synthesis. With this simple thought in mind,
further optimization was conducted. It is noteworthy that the
use of [Cp*Co(CO)I2] (5 mol%) combined with AgSbF6
(10 mol%) instead of cationic [Cp*Co(MeCN)3][(SbF6)2]
(5 mol%) only afforded the desired product in 30% yield,
which indicated that cationic [Cp*Co(MeCN)3][(SbF6)2] is much
more active than the in situ generated cationic Cp*CoIII species
in this reaction, and the reaction yield diminishes when the
AgSbF6 is omitted (Table 1, entries 7 and 8). DCE turned out to
be the most effective solvent, while MeCN was found to be to-
tally inhibitory (Table 1, entries 9 and 10). No reaction was ob-
served in the absence of cobalt catalyst (Table 1, entry 11). Be-
Scheme 1. Cobalt-catalyzed CÀH amidation of 6-arylpurine.
[a] [Cp*Co(MeCN)3][(SbF6)2] (10 mol%) was used 24 h.
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groups, offers ample opportunity for further derivatization. Ad-
ditionally, when meta-substituted 2-phenylpyridines were used,
a high regioselectivity favoring activation at the less hindered
CÀH bond was observed with the formation of the products
6m–6o. The high efficiency was not affected even when the
pyridine rings had additional substituents (6p and 6q). In addi-
tion, the amidation of benzo[h]quinoline was almost quantita-
tive (6r). A substrate bearing substituents at both the phenyl
and the pyridine ring was also tolerated (6s). A variation on
the amidating reagent was subsequently examined. Substitu-
ents (chloro and bromo) at the phenyl moiety, or 1,4,2-dioxa-
zol-5-one with an alkyl substituent at the 3-position afforded
the corresponding products in good yields (6t–6v). Notably,
amidation of a thiophene derivative bearing a pyridine direct-
ing group is also feasible, leading to the corresponding prod-
uct in 87% yield (6w).
Scheme 2. Cobalt-catalyzed CÀH amidation with different directing groups.
[a] [Cp*Co(MeCN)3][(SbF6)2] (2.5 mol%) was used.
Delightfully, the amidation reaction worked not only with
(hetero)arenes, but also with olefins through alkenyl CÀH acti-
vation. 2-(Propenyl)pyridine (5x) displayed a good reactivity in
this reaction, selectively furnishing the desired product 6x in
86% yield [Eq. (1)].
zole, and pyridine as directing groups were effective and gave
the corresponding products in good to excellent yields.
To display the excellent functional group tolerance of the
present method, we therefore extended our protocol utilizing
diverse 2-phenylpyridines as substrates. As depicted in
Scheme 3, various 2-phenylpyridines regardless of electron-do-
nating or -withdrawing groups on the aromatic ring reacted
smoothly to furnish the products in good to excellent yields
(6a–6l). The valuable functional groups tolerance, including
methyl, methoxyl, aldehyde, borate, fluoro, bromo, ester
To gain insights into the mechanism, further experiments
were conducted (Scheme 4). A significant level of deuterium
incorporation (63%) was observed at the ortho position of 2-
phenylpyridine when it was subjected to the present cobalt-
catalyzed system in CD3OD in the absence of amidating re-
agent, which indicated that the cobalt-mediated CÀH bond
cleavage is reversible (Scheme 4a). On the other hand, low
level of primary kinetic isotope effects were observed (KH/KD =
1.1–1.5; Scheme 4b). The observed results suggested that the
CÀH bond cleavage may not be involved in the rate-limiting
step.
Based on the mechanistic studies and the relevant re-
ports,[6–10] a plausible catalytic cycle of the present cobalt-cata-
lyzed CÀH amidation reaction is proposed in Scheme 5. The re-
action initiates with the cobalt-catalyzed CÀH bond cleavage
Scheme 3. Cobalt-catalyzed CÀH amidation of 2-phenylpyridines.
[a] [Cp*Co(MeCN)3][(SbF6)2] (2.5 mol%) was used.
Scheme 4. Mechanistic studies.
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Scheme 5. Proposed catalytic cycle.
to furnish the five-membered metallacyclic intermediate I,
which is subsequently coordinated by 3-phenyl-1,4,2-dioxazol-
5-one to form the intermediate II with the release of CO2. Sub-
sequently, migratory insertion of intermediate II affords the in-
termediate III. Finally, protodemetalation of intermediate III lib-
erates the desired product along with the regeneration of the
active catalyst.
In summary, we have demonstrated an efficient cobalt-cata-
lyzed CÀH amidation of unactivated (hetero)arenes and al-
kenes by using 1,4,2-dioxazol-5-ones as amidating reagents.
This reaction proceeded efficiently under external oxidant-free
conditions with ample substrate scope, as well as excellent
chemo- and regioselectivity. The employment of cationic co-
balt(III) catalysts enables the high efficiency of this transforma-
tion, which may promote the development of CÀH functionali-
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Acknowledgements
Financial support from National Basic Research Program of
China (973 Program) (grant No. 2015CB856600), National Sci-
ence Foundation of China (Nos. 21325206, 21172006), and Na-
tional Young Top-notch Talent Support Program are greatly ap-
preciated. We thank Yiqun Zhang in this group for reproducing
the results of 3e and 4d.
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Keywords: amidation · CÀH functionalization · CÀN bond
formation · cobalt · purine compounds
´
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Revised: September 4, 2015
Published online on September 29, 2015
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