Cobalt- and manganese-catalyzed direct amination of azoles under mild reaction conditions and the mechanistic details

Article abstract of DOI:10.1002/anie.201005922

A bonding moment: A new cobalt- or manganese-catalyzed amination of azoles has been developed using peroxide and an acid additive to couple various types of azoles with ammonia, and primary or secondary amines (see scheme). The catalyst loadings are low, the optimal reaction conditions are mild, and the substrate scope is broad. The product azoles are an important pharmacophore of high biological activity. Copyright

Full text of DOI:10.1002/anie.201005922

Angewandte
Chemie
DOI: 10.1002/anie.201005922
À
C N Bond Formation
Cobalt- and Manganese-Catalyzed Direct Amination of Azoles under
Mild Reaction Conditions and the Mechanistic Details**
Ji Young Kim, Seung Hwan Cho, Jomy Joseph, and Sukbok Chang*
À
Transition metal catalyzed C N bond formation is highly
important in synthetic chemistry since it can lead to nitrogen-
containing molecules that are of great interest in biological,
pharmaceutical, and materials science.[1] A range of C N
À
bond-forming reactions have been reported utilizing organic
(pseudo)halides such as aryl iodides, bromides, chlorides,
triflates, and sulfonates reacting with amines or their pre-
cursors. In particular, since Ullmann and Goldberg pioneered
the copper-mediated N-arylation of aryl halides,[2] Pd-, Cu-,
À
and Rh-catalyzed C N bond formation has been developed
with suitable ligands.[3,4] Organometallic compounds includ-
ing aryl boronic acids, stannanes, or siloxanes were also
employed in the metal-mediated cross-couplings of ami-
nes.[2a,5] Despite the significant progress, only limited exam-
À
ples of direct C N bond formation of unfunctionalized arenes
or heterocyclic compounds with amines have been reported.[6]
For instance, oxidative nitrenoid insertion of amido groups
Scheme 1. Transition-metal-mediated amination.
À
into double bonds or saturated C H bonds were recently
developed.[7] In addition, Pd- and Cu-catalyzed site-selective
amination of directing-group-containing arenes has also been
Although the substrate scope of the reaction turned out to be
broad under relatively moderate conditions, the most critical
disadvantage of our amination procedure was the use of
stoichiometric amounts of a silver species; such amounts were
required to “noncatalytically” oxidize the 2-aminoazolidine
intermediates. This aspect has led us to search for a new
catalytic amination reaction of azoles that employs amines
under mild reaction conditions and has a broad substrate
scope. Herein, we describe our efforts on the development of
the new catalyst systems, and their mechanistic details are also
presented (Scheme 1B).
On the basis of the proposed mechanism of the silver-
mediated amination of benzoxazoles,[15] we envisioned that
stoichiometric amounts of a silver species could be replaced
by other catalytic systems in either of two ways. Although
regeneration of the + 1 oxidation state of the silver species,
probably by using an external oxidant, from Ag0 [16] after the
amination reaction would be the most straightforward route,
our extensive efforts for this approach were unsuccessful
(Table 1, entries 1–3).
actively investigated.[8,9]
Since the elegant example of copper-catalyzed amidation
of alkynes with amides by Stahl and co-workers,[10a] direct
À
oxidative C H amination of heteroarenes has been partic-
ularly scrutinized in recent years.[10b,11] For instance, Mori
et al.,[12a] and Wang and Schreiber[12b] independently devel-
oped a copper-catalyzed amination and amidation of azoles,
respectively, and they proceed under rather harsh reaction
conditions (Scheme 1A, a). More recently, Miura et al.
successfully developed a new and milder catalytic protocol
for azole amination using chloroamines instead of the parent
amines albeit with a rather limited substrate scope with
respect to the amine (Scheme 1A, b).[13]
During the course of our studies on the metal-catalyzed
[14]
À
À
C H bond functionalization,
we found that C N bond
formation of azoles could be achieved with the use of either
formamides or parent amines when a silver salt was employed
in the presence of certain Brønsted acids (Scheme 1A, c).[15]
This led us to search for another procedure wherein
transition metals other than Ag salts were employed as a
catalyst in combination with suitable oxidants. We were
pleased to find that certain transition-metal species can
catalyze the azole amination when they are used in combi-
nation with proper oxidants and Brønsted acids. For example,
cobalt(II) acetate (2 mol%) efficiently catalyzed the amina-
tion reaction of benzoxazole to afford the desired 2-aminated
product in high yield when aqueous tert-butyl hydroperoxide
solution (T-HYDRO) was employed in the presence of a
benzoic acid additive (1.2 equiv of each relative to morpho-
[*] J. Y. Kim, S. H. Cho, Dr. J. Joseph, Prof. Dr. S. Chang
Department of Chemistry and Molecular-Level Interface Research
Center, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 305-701 (Republic of Korea)
Fax: (+82)42-350-2810
E-mail: sbchang@kaist.ac.kr
[**] This research was supported by the Korea Research Foundation
Grant (KRF-2008-C00024, Star Faculty Program), and MIRC (NRF-
2010-0001957).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005922.
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Table 1: Optimization of reaction conditions.[a]
Entry Catalyst (mol%) Oxidant
Additive T [8C] Yield [%][b]
1
2
3
4
5
6
7
8
Ag2CO3 (120)
Ag2CO3 (10)
Ag2CO3 (10)
Co(OAc)2 (2)
Co(OAc)2 (2)
Co(OAc)2 (2)
–
Co(acac)3 (10)
Co(OAc)2 (2)
Co(OAc)2 (2)
Co(OAc)2 (2)
Co(OAc)2 (2)
Co(OAc)2 (2)
Mn(OAc)2 (10)
Fe(OAc)2 (10)
–
–
PhCO2H 60
PhCO2H 60
92
6
8
85
4
19
12
44
70
10
13
68
84
43
47
T-HYDRO[d] PhCO2H 60
T-HYDRO
–
PhCO2H 60
PhCO2H 60
T-HYDRO
T-HYDRO
T-HYDRO
tBuOOH
BzOOtBu
H2O2
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
–
60
PhCO2H 60
PhCO2H 60
PhCO2H 60
PhCO2H 60
PhCO2H 60
PhCO2H 25
9
10
11
12
13[c]
14
15
Scheme 2. Direct amination of azoles. Reaction conditions: 1
(0.5 mmol), 2a (1.2 equiv), Co(OAc)2 (2 mol%), T-HYDRO (1.2 equiv),
AcOH (1.2 equiv) in CH3CN (1 mL) at 258C for 12 h under air (yield of
isolated products). [a] Used 5 mol% of Co(OAc)2 and 2.0 equiv of
AcOH relative to 1. [b] 1 (2 equiv), 2a (0.5 mmol), Co(OAc)2
(10 mol%), BzOOtBu (1.1 equiv), Zn(OAc)2 (5 mol%) in CH3CN
(1 mL) at 708C for 12 h under O2. Bz=benzoyl.
AcOH
25
PhCO2H 25
PhCO2H 25
[a] Reaction conditions: 1a (1.2 equiv), 2a (0.5 mmol), acid (1.2 equiv),
peroxide (1.2 equiv), catalyst in CH3CN (1 mL) for 12 h. [b] Yield based
on NMR spectroscopy. [c] The reaction was carried out with 0.5 mmol of
1a and 1.2 equiv of 2a. [d] T-HYDRO is the trademark name for 70 wt%
tBuOOH in H2O. acac=acetylacetonate.
Not only 2-aminobenzoxazoles bearing substituents at the
5-position but also one substituted at the 4-position was
readily obtained (3g). In the case of benzothiazole, the
desired aminated product 3h was obtained in moderate yield
when tert-butyl peroxybenzoate was employed instead of
T-HYDRO in the presence of catalytic amounts of zinc(II)
acetate as an additive at higher temperatures under an O2
atmosphere.[19] Likewise, a reaction of 6-methylbenzothiazole
afforded the 2-aminated product 3i in a slightly higher yield
under the same reaction conditions.
We next examined the scope of the amine reactant in the
cobalt-catalyzed amination of benzoxazoles (Scheme 3).
Cyclic amines were readily employed for this reaction. For
instance, benzoxazoles bearing cyclic amino groups such as
morpholinyl (Scheme 2) and piperidinyl (4a), as well as
piperazinyl (4b) could be isolated in good yields. It should be
mentioned that important functional groups such as N-Boc
(4b) were completely tolerated under the present reaction
conditions. Acyclic secondary amines such as diallyl amine or
benzylmethyl amine were smoothly reacted to give the
corresponding products in high yields (4c and 4d, respec-
tively). Notably, the amination reaction can be easily carried
out in a gram scale without difficulty, thereby delivering 4d in
excellent yield. Interestingly, an amine bearing a propargylic
moiety was tolerated under the present reaction conditions
(4e). Dialkylamine was readily employed in the coupling
reaction with 5-chlorobenzoxazole albeit at a slightly higher
temperature (4 f).
line; Table 1, entry 4). In fact, it was previously known that
some cobalt, manganese, and iron complexes readily react
with alkyl peroxides to generate organic radical species which
exhibit high activity as an oxidizing agent.[17,18]
Importantly, in the absence of either of the reagents, metal
species, oxidant, or Brønsted acid, the reaction efficiency was
significantly decreased (Table 1, entries 5–7). Cobalt species
other than Co(OAc)2 exhibited reduced catalytic activity
(Table 1, entry 8). The choice of oxidants and acid additives
turned out to be crucial for the reaction efficiency, and
T-HYDRO was the most effective among various oxidants
investigated (Table 1, entries 9–11). The amination reaction
could even be carried out at room temperature (Table 1,
entry 12), thus affording an excellent product yield especially
when acetic acid was employed as an additive (Table 1,
entry 13). Although manganese or iron species can also
facilitate this transformation, the reaction efficiency was
slightly lower compared to that of cobalt catalyst (Table 1,
entries 14 and 15).
To explore the substrate scope, we examined a range of
azole derivatives in the coupling reaction with morpholine
(2a) under the optimized reaction conditions (Scheme 2). It
was observed that electronic variation of the substituents at
the 5-position of benzoxazole did not significantly affect the
reaction efficiency. In fact, 2-aminated products of benzox-
azoles substituted with a 5-methyl (3a), 5-phenyl (3b), and
5-methoxy (3c) group were obtained in satisfactory yields at
room temperature. Unsubstituted benzoxazole smoothly
reacted with morpholine to provide the desired product 3d.
Benzoxazoles bearing electron-withdrawing groups such as
chloride or acetyl could also be employed as facile substrates
that provided the corresponding products (3e and 3 f,
respectively) in acceptable yields at ambient temperature.
In sharp contrast to the above results with secondary
amines, no desired products were obtained when primary
amines or ammonia were employed under the cobalt-
catalyzed reaction conditions (e.g., 4g or 4h). Thus, we
turned our attention to other catalytic systems especially
those based on manganese or iron species as they were also
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amine was directly introduced at the 2-position of
benzoxazoles without racemization (4k). Notably,
aqueous ammonia can undergo the amination to
afford the 2-aminobenzoxazole derivative 4h albeit
in low yield at the present stage.
In addition, the amination reaction took place
smoothly with oxadiazoles under the manganese
catalyst system using tert-butyl peroxybenzoate
[Eq. (1)].[20] This result is significant in that the
substrate scope can be extended to not only fused
heteroarenes but also to a monomeric heterocycle to
give the corresponding aminated product 5a, which
is an important pharmacophore exhibiting a broad
spectrum of biological activity.[21]
During the course of the present study, it was
observed that the ring-opened amidine species 9a
was also generated along with the desired amination
product 4a when the reaction was carried out in the
absence of an acid additive [Eq. (2) versus Eq. (3)].
Under the standard reaction conditions applied for
the optimized amination, no trace of the amidine 9a
Scheme 3. Amination of benzoxazoles with various amines. Reaction conditions: 1
(0.5 mmol), 2 (1.2 equiv), Co(OAc)2 (2 mol%), T-HYDRO (1.2 equiv), AcOH
(1.2 equiv) in CH3CN (1 mL) at 258C for 12 h (isolated yields). [a] Used 5 mol% of
Co(OAc)2. [b] Carried out at 408C.
effective in the preliminary screening albeit with lower
efficiency (Table 1, entries 14–15) when compared to the
cobalt catalyst system.
To our delight, we found that the amination efficiency in
the reaction with primary amines was significantly improved
upon the replacement of cobalt(II) acetate catalyst with
manganese(II) acetate (Scheme 4). Although a higher reac-
tion temperature (708C) was required to achieve satisfactory
product yields, the manganese catalyst system was efficient
enough to include various types of amine reactants. For
instance, 2-aminobenzoxazole derivatives substituted with
N-isobutyl (4g), n-pentyl (4i), and cyclohexyl (4j) groups
were easily obtained. Notably, an optically active primary
was detected in the crude reaction mixture, and only 4a was
obtained in high yield [Eq. (2)]. This result suggests the
possibility that the ring-opening process might be involved
during the course of the amination reaction, thus giving a
“nonproductive” intermediate amidine species.
In fact, when the ring-opened amidine 9a was subjected to
our standard amination conditions, a full conversion was
attained within 1 hour at room temperature to afford a similar
mixture of benzoxazole 1a and its 2-aminated product 4a
[Eq. (4)]. Interestingly, when the same transformation was
tried in the absence of cobalt catalyst, lower conversion was
observed leading to only 38% yield of the benzoxazole 1a,
but no aminated compound 4a was produced [Eq. (5)]. In
addition, the absence of an external oxidant (T-HYDRO)
resulted in no formation of 4a [Eq. (6)].
Scheme 4. Manganese-catalyzed amination of benzoxazoles with pri-
mary amines. Reaction conditions: 1 (1.2 equiv), 2 (0.5 mmol), Mn-
(OAc)2 (10 mol%), T-HYDRO (1.2 equiv), AcOH (2.0 equiv) in CH3CN
(1 mL) at 708C for 12 h (yield of isolated products). [a] Used 2 mol%
of Mn(OAc)2. [b] The enantiomeric excess (ee) of both the amine and
product were determined to be 96%. [c] Aqueous ammonia solution
(28%, 0.5 mmol) was used in the presence of anisic acid (1.2 equiv)
instead of AcOH.
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primary amines or ammonia is more prone to open leading to
its amidines, thus resulting in lower product yields compared
to the amination reaction with secondary amines.
It is postulated that a metal catalysis is operative in the
rearomatization step of the putative 2-aminobenzoxazolidine
intermediate 7. Indeed, it is known that two radical species,
alkoxy ROC and alkylperoxy ROOC, are generated, in addition
to water, from two molecules of alkylhydroperoxides upon
the action of cobalt or manganese species.[17,18] As a result, it is
reasonable to propose that the in situ formed alkoxy and
alkylperoxy radicals abstract two hydrogen atoms, one each,
from the putative intermediate 7 to produce 2-aminobenzox-
azole 8.[23,24]
In summary, we have developed a new catalytic system for
the direct amination of azoles with amines by using cobalt or
manganese catalyst in the presence of peroxide and acid
additive. The reaction is highly attractive from the synthetic
point of view in that the catalyst loadings are low, optimal
reaction conditions are mild, and substrate scope is broad. In
addition, a mechanistic proposal is made on the basis of the
kinetic isotope effects and isolation of amidine compounds.
The present reaction, therefore, is anticipated to be a
powerful tool for the synthesis of 2-aminoazoles which are
an important pharmacophore of high biological activity.
These experiments suggest that 2-aminoazole products
are generated most efficiently by the combined action of the
cobalt catalyst, oxidant, and acid additive. In addition, poor
product yields obtained from the reaction of certain azoles
and amines can be attributed to the formation of stable ring-
opened amidine compounds.
A competition experiment between 1a and its deuterated
derivative at the 2-position ([D]-1a) was next performed to
identify any kinetic isotope effects [Eq. (7)].[22] It was
observed that the amination reaction does not exhibit kinetic
À
isotope effects (kH/kD = 1.0), which may imply that the C H
bond cleavage at the C2 position of benzoxazoles is not
involved in the rate-determining step in the overall catalytic
processes.
Experimental Section
Representative procedure: A test tube equipped with a magnetic stir
bar was charged with 5-methylbenzoxazole (0.5 mmol, 1.0 equiv),
Co(OAc)2 (0.01 mmol, 2 mol%), T-HYDRO (0.6 mmol, 1.2 equiv),
acetic acid (0.6 mmol, 1.2 equiv), and morpholine (0.6 mmol,
1.2 equiv) in acetonitrile (1.0 mL) under air. The test tube was
sealed with a rubber septum and stirred for 12 h at 258C under air.
The crude mixture was filtered through a plug of celite and washed
with dichloromethane (15 mL). The filtrate was washed with a
saturated solution of NaHCO3 (3 ꢀ 15 mL) and the aqueous layer was
extracted again with dichloromethane (3 ꢀ 15 mL). The combined
organic layers were dried over MgSO4 and concentrated under
reduced pressure. The resulting residue was purified by column
On the basis of the above mechanistic
clues and data, a plausible pathway of the
cobalt-catalyzed amination reaction is pro-
posed in Scheme 5. It is believed that an
acid additive initially protonates the het-
eroarene 1 to provide the salt 6 which is
more electrophilic than 1, thus enabling
the subsequent nucleophilic attack of
amine to be more facile. In contrast,
direct amine addition to the neutral ben-
zoxazole 1 rather than its protonated
species 6 can also be possible albeit at a
slower rate. It is now presumed that the
adduct 7 is in equilibrium with its ring-
opened amidine species 9, and the extent
of which is dependent upon the type of
heteroarenes and amine reactants. In this
context, it is assumed that 2-aminobenzox-
azolidine 7 derived from the addition of Scheme 5. A proposed pathways of the cobalt-catalyzed amination of azoles.
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chromatography on silica gel (n-hexane/EtOAc = 1:1) to afford
2-(4-morpholinyl)-5-methylbenzoxazole (3a, 84%)
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Received: September 21, 2010
Published online: November 23, 2010
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[19] A similar result was obtained in the Ag-mediated amination
reaction of benzothiazoles (Ref [15]). Additional investigations
on the effects of Lewis acid additives are now underway.
[20] a) H. L. Yale, K. Losee, J. Med. Chem. 1966, 9, 478; b) S. A. F.
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[21] The direct amination with 1-methylbenzimidazole or 4,5-dime-
thylthiazole was unsuccessful under Co or Mn catalysis con-
ditions, and the starting materials were recovered unchanged.
[22] See the Supporting Information for details.
Keywords: amination · cobalt · heterocycles · manganese ·
peroxides
.
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