Cobalt-promoted dimerization of aminoquinoline benzamides

Article abstract of DOI:10.1021/acs.orglett.5b00155

A method for aminoquinoline-directed, cobalt-promoted dimerization of benzamides has been developed. Reactions proceed in ethanol solvent in the presence of Mn(OAc)₂ cocatalyst and Na₂CO₃ base and use oxygen as a terminal oxidant. Bromo, iodo, nitro, ether, and ester moieties are compatible with the reaction conditions. Cross-coupling of electronically dissimilar aminoquinoline benzamides proceeds with modest yields and selectivities.

Full text of DOI:10.1021/acs.orglett.5b00155

Letter
pubs.acs.org/OrgLett
Cobalt-Promoted Dimerization of Aminoquinoline Benzamides
Liene Grigorjeva and Olafs Daugulis*
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
S
* Supporting Information
ABSTRACT: A method for aminoquinoline-directed, cobalt-
promoted dimerization of benzamides has been developed.
Reactions proceed in ethanol solvent in the presence of
Mn(OAc)2 cocatalyst and Na2CO3 base and use oxygen as a
terminal oxidant. Bromo, iodo, nitro, ether, and ester moieties
are compatible with the reaction conditions. Cross-coupling of
electronically dissimilar aminoquinoline benzamides proceeds
with modest yields and selectivities.
a
Table 1. Optimization of Reaction Conditions
irect dehydrogenative arene coupling is the most efficient
1
Dmethod for the formation of biaryls. However, most of the
methods for arene homocoupling use polyfluorobenzene, five-
membered-ring heterocycles, or phenol starting materials.2
Cross-coupling of heterocycles and acidic arenes with each
other or simple benzene derivatives is also possible.3 In rare cases
when simple arenes such as toluene are coupled, isomer mixtures
of products are usually obtained.4 Furthermore, reactions often
require either palladium or rhodium catalysts or several equiva-
lents of silver salt additives. Few reports describe dimerization of
directing-group-containing arenes. In a notable exception, Yu has
described copper-promoted 2-arylpyridine dimerization.5 Meth-
ods for palladium- or ruthenium-catalyzed dimerization of
2-arylpyridines, phenylacetamides, and aryloxazolines have
been reported by Sanford, Greaney, and Oi.6 Miura has recently
reported a method for aminoquinoline-directed homocoupling
of thiophenes mediated by copper.7 Thus, directed arene
homocouplings are still rare. Furthermore, very few reports
describe dehydrogenative arene couplings that do not require
second-row transition metals as catalysts or additives.2f,3d,e,9g
We have reported that 8-aminoquinoline, picolinamide, and
2-pyridinylmethylamine auxiliaries can be used for palladium-,
copper-, and cobalt-catalyzed C−H bond functionalization.8,10a−c
Other research groups have extensively used these auxiliaries for
palladium-, ruthenium-, iron-, and copper-catalyzed reactions.9
The aminoquinoline auxiliary can direct cobalt-catalyzed C−H
activation/alkene, alkyne, and CO migratory insertion sequen-
ces, leading to ortho-functionalized benzoic acid derivatives.10a−c
We report here cobalt-promoted, aminoquinoline-directed
dimerization of arenes that affords biphenyldicarboxylic acid
derivatives, uses oxygen as a terminal oxidant, and does not
require second-row transition metal additives.
entry
solvent
oxidant
base
yield of 2, %
1
2
3
4
5
CF3CH2OH
EtOH
Mn(OAc)2/O2
Mn(OAc)2/O2
Mn(OAc)2/O2
Mn(OAc)2/air
Mn(OAc)3·2H2O/air
Mn(OAc)2/O2
Mn(OAc)2/O2
Mn(OAc)2/O2
Mn(OAc)2
Na2CO3
Na2CO3
NaOPiv
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
12
97
53
70
40
49
NR
8
EtOH
EtOH
EtOH
b
6
EtOH
c
7
EtOH
d
8
EtOH
e
9
EtOH
NR
a
Amide (0.1 mmol), oxidant (0.1 mmol), Co(acac)2 (0.05 mmol),
base (0.1 mmol), solvent (1 mL), Q = quinolin-8-yl. Yields determined
by NMR of crude reaction mixtures, with 1,1,2-trichloroethane as
b
c
d
internal standard. Co(acac)2 (10 mol %). No Co(acac)2. Reaction
e
temperature = 60 °C. Deoxygenated EtOH.
gives higher yield than the reaction under air (entries 2 and 4).
Manganese(III) acetate is an inferior co-oxidant compared with
Mn(OAc)2 (entry 5). Use of 10 mol % of catalyst affords lower
yield (entry 6), while the presence of cobalt is essential (entry 7).
Temperatures less than 100 °C afford lowered yields (entry 8).
Reaction scope is presented in Table 2. Aminoquinoline
benzamide (entry 1) affords dimerization product in 83% yield.
Amides possessing electron-withdrawing (entries 4−8, 10, and
11) as well as electron-releasing substituents (entries 2, 3, 9, and
12) are reactive. While amides substituted at 3- and 4-positions
afford products in high yields, substitution at the 2-position
results in lower yields (entries 3 and 7). Similar to other high-
valent cobalt-catalyzed C−H functionalization reactions,10a−c
On the basis of previous results,10a−c we decided to use
Co(acac)2 catalyst in an alcohol solvent. The reaction optimi-
zation was carried out with respect to solvent, oxidant, and base
(Table 1). In contrast with previous cobalt-catalyzed reactions,
trifluoroethanol is an inferior solvent compared to ethanol
(entries 1 and 2). Sodium carbonate base afforded higher yield
than sodium pivalate (entries 2 and 3). Reaction under oxygen
Received: January 16, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.5b00155
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Organic Letters
Letter
a
Table 2. Dimerization of Aminoquinoline Benzamides
a
Amide (1 mmol), Co(acac)2 (0.5 mmol), Mn(OAc)2 (1 mmol), Na2CO3 (1 mmol), EtOH (5 mL). Yields are isolated yields. Please see Supporting
b
c
Information for details. Reaction under air. Reaction was performed in MeOH (5 mL).
high functional group tolerance is observed. Iodo (entry 5), bromo
(entries 6 and 7), nitro (entry 8), and ester (entry 11) substituents
are tolerated. Heterocyclic substances, such as aminoquinoline
thiophenecarboxylic acid amides, are reactive and afford bithio-
phenedicarboxylic acid derivatives in high yields (entries 13
and 14). In all cases, only one isomer of product was obtained.
It would be useful to selectively cross-couple two different
aminoquinoline benzamides. A 1/1 mixture if amides 3 and 4 was
reacted in the presence of 1.5 equiv of Co(acac)2. The analysis of
the reaction mixture showed that a 0.1/1/0.6 ratio of three
possible coupling products 6, 7, and 8 was formed, with pre-
dominant formation of the cross-coupled product 7 (Scheme 1).
Compound 7 was isolated in an acceptable 52% yield. Similarly,
p-methoxybenzamide 3 was coupled with m-iodobenzamide
5 to afford a mixture of three coupling products 9, 10, and 8 in
0.2/1/0.5 ratio, with desired cross-coupling product 10 formed
predominately. Compound 10 was isolated in 46% yield
(Scheme 1). These experiments show that cross-coupling of
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DOI: 10.1021/acs.orglett.5b00155
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Cross-Coupling of Aminoquinoline Benzamides
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: olafs@uh.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Welch Foundation (Grant E-1571, Chair E-0044),
NIGMS (Grant No. R01GM077635), and University of
Houston for supporting this research.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterization data for
new compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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