Cobalt-catalyzed C(sp2)-H alkoxylation of aromatic and olefinic carboxamides

Article abstract of DOI:10.1002/anie.201409751

The cobalt-catalyzed alkoxylation of C(sp²)-H bonds in aromatic and olefinic carboxamides has been developed. The reaction proceeded under mild conditions in the presence of Co(OAc)₂?4H₂O as the catalyst and tolerates a wide range of both alcohols and benzamide substrates, including even olefinic carboxamides. In addition, this reaction is the first example of the direct alkoxylation of alkenes through C-H bond activation.

Full text of DOI:10.1002/anie.201409751

Angewandte
Chemie
DOI: 10.1002/anie.201409751
À
C H Activation
2
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Cobalt-Catalyzed C(sp ) H Alkoxylation of Aromatic and Olefinic
Carboxamides**
Lin-Bao Zhang, Xin-Qi Hao, Shou-Kun Zhang, Zhan-Jiang Liu, Xin-Xiang Zheng,
Jun-Fang Gong, Jun-Long Niu,* and Mao-Ping Song*
2
2
À
À
Abstract: The cobalt-catalyzed alkoxylation of C(sp ) H C(sp ) H bonds by using 8-aminoquinoline and picolinic
bonds in aromatic and olefinic carboxamides has been acid compounds.[14] Thereafter, they employed this catalyst
developed. The reaction proceeded under mild conditions in system for the coupling of alkynes and the carbonylation of
2
[15]
À
the presence of Co(OAc)2·4H2O as the catalyst and tolerates C(sp ) H bonds. However, to our knowledge, no applica-
a wide range of both alcohols and benzamide substrates, tion of a cobalt catalyst for C O bond formation has been
including even olefinic carboxamides. In addition, this reaction reported. We speculated that a cobalt catalyst might also
À
is the first example of the direct alkoxylation of alkenes promote the alkoxylation of 2-aminopyridine-1-oxide benz-
À
through C H bond activation.
amides on the basis of the following factors: a) cobalt salts
2
[16]
À
could activate C(sp ) H bonds, b) alkoxyl cobaltacycle
À
T
he direct C H bond functionalization method has emerged intermediates have been reported in enantioselective hydro-
as a practical strategy for the synthesis of natural products and acylation reactions,[17] c) 2-aminopyridine-1-oxide benzamide
medicinal compounds.[1] The synthesis of C O bonds is one of ligands could stabilize Co intermediates, as seen for 8-amino-
À
the fundamental reactions in organic chemistry, and ether- quinoline benzamides. As part of our interest in the field,[18]
À
containing compounds are widely employed in the synthesis we present a simple C H alkoxylation of aromatic and
of pharmaceuticals and functional materials.[2]. Comparing olefinic carboxamides using a Co(OAc)2·4H2O catalyst which
the continuous development of direct hydroxylation,[3] provides a new method for C O bond formation.
À
acetoxylation,[8a,4] and phenoxylation,[4d,5] reports on the
To probe the feasibility of this approach, we initiated our
À
more challenging C H bond alkoxylation are scarce because study with the reaction between ethanol (2a) and 2-benz-
of the fact that alkanols are easily transformed to the amidopyridine 1-oxide (1a). To our delight, the desired
corresponding aldehydes or ketones through cationic or ethoxylated product 3aa was obtained in 17% yield when
radical mechanisms.[6] Moreover, the alkoxyl–metal inter- a solution of benzamide (1a) in ethanol was treated with
mediates formed tend to undergo b-hydride elimination.[7]
a stoichiometric amount of Co(OAc)2·4H2O, NaOAc
À
Consequently, direct C H bond alkoxylation is largely (2 equiv), and PhI(OAc)2 (1 equiv) under an air atmosphere
restricted to using an established palladium-[8] or copper- at 808C (Table 1, entry 1). A decrease in the catalyst loading
catalyzed system.[9,10a]
only resulted in a slightly decreased yield (Table 1, entry 2).
À
Recently, the chelation-assisted C H bond functionaliza- The catalyst loading was decreased to 20 mol% of
tion strategy has attracted widespread attention as a result of Co(OAc)2·4H2O and various oxidants were investigated,
the pioneering work of Daugulis and co-workers[10] on the use with Ag2O proving to be most effective (Table 1, entries 3–
of 8-aminoquinoline, picolinamide, and 2-pyridinylmethyl- 11). This is in agreement with findings by Gooßen and co-
amine compounds. Other groups have extensively applied this workers.[9] Alternative cobalt catalysts (Table 1, entries 12–
strategy to various Pd-, Ru-, Fe-, Ni-, and Cu-catalyzed 14) were found to be inferior compared to Co(OAc)2·4H2O.
reactions.[11] On the other hand, the use of a cobalt catalyst in The replacement of NaOAc with other bases and the addition
À
C C bond-forming reactions has attracted significant atten- of various cosolvents did not promote the efficiency of the
tion[12] since the chelation-assisted direct metalation of reaction (see the Supporting Information). However, low-
azobenzene was reported by Murahashi and Horiie in ering the reaction temperature substantially influenced the
1956.[13] Furthermore, Daugulis and Grigorjeva recently yields (Table 1, entry 15). We speculated that a decrease of
reported Co/Mn-catalyzed coupling of alkynes with temperature could slow down the decomposition of the
starting material and diminish the formation of by-products (a
trace amount of the homocoupled product of 1a was detected
by HRMS analysis under the reaction conditions at 808C). A
decrease in the reaction temperature to room temperature
and a longer reaction time failed to afford an ideal result as
a large portion of the starting materials were unreacted. By
screening the reaction temperature, we found that 608C was
the optimal temperature and afforded the ethoxylated
product in 82% yield. Control experiments revealed that
the cobalt catalyst is essential, as no reaction takes place in
the absence of the Co(OAc)2·4H2O catalyst. Notably, using
other bidentate coordinating groups (Figure 1, A–C), the
[*] L.-B. Zhang, Dr. X.-Q. Hao, S.-K. Zhang, Z.-J. Liu, X.-X. Zheng,
Dr. J.-F. Gong, Dr. J.-L. Niu, Prof. M.-P. Song
The College of Chemistry and Molecular Engineering
Henan Key Laboratory of Chemical Biology and Organic Chemistry
Zhengzhou University, Kexue Avenue 100
Zhengzhou, Henan 450001 (P. R. China)
E-mail: niujunlong@zzu.edu.cn
mpsong@zzu.edu.cn
[**] We are grateful to the National Natural Science Foundation of China
(Nos. 21072177 and 21272217) for financial support of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409751.
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Table 1: Optimization of cobalt-catalyzed alkoxylation reaction between
substrates 1a and 2a.[a]
Table 2: Cobalt-catalyzed ethoxylation of benzamide derivatives 1 with
ethanol (2a).[a]
Entry
Catalyst
T [8C]
Oxidant
Yield [%]
1[b]
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18[c]
19
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(acac)2
80
80
80
80
80
80
80
80
80
80
80
80
80
80
50
60
70
RT
60
PhI(OAc)2
PhI(OAc)2
K2S2O8
Mn(OAc)3·2H2O
NaIO4
NMO
BQ
O2
Ag2CO3
AgOTf
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
17
15
18
45
26
n.r.
n.r.
n.r.
18
27
67
42
25
29
75
82
71
26
n.r.
Co(acac)3
CoSO4·6H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
none
[a] reaction conditions: 1a (0.2 mmol), 2a (1.5 mL), [Co] (20 mol%),
NaOAc (2.0 equiv), in air. Yield indicates yield of isolated product.
[b] Co(OAc)2·4H2O (0.2 mmol). [c] 24 h. NMO=N-methylmorpholine
oxide, BQ=1,4-benzoquinone, acac=acetylacetonate. OTf=trifluoro-
methanesulfonate. n.r.=no reaction.
[a] Reaction conditions: 1 (0.2 mmol), ethanol 2a (1.5 mL), Co(OAc)2·
4H2O (20 mol%), Ag2O (0.2 mmol), NaOAc (2.0 equiv), under air, 608C,
12 h. [b] Ag2O (0.3 mmol).
group also underwent reacted under these conditions to form
3 fa in 69% yield. The ethoxylation of ortho-methoxylbenz-
amide took place sluggishly with a yield of 19% (data not
shown). For meta-substituted substrates (1g–1n), the reac-
tions proceeded with high regioselectivity, with the reaction in
each case occurring exclusively at the less-hindered position
of the arenes. Notably, the fluoro, chloro, bromo, and even
iodo halogen functional groups on the arene derivatives
remained intact under these reaction conditions. Moreover,
increasing the amount of the oxidant Ag2O was beneficial in
these transformations (3ia–3la). Disubstituted substrates (1o,
1q, and 1r) bearing methyl and methoxy groups provided the
ethoxylated products (3oa, 3qa, and 3ra) in good yields. The
trisubstituted benzamide 1p also underwent reaction to form
the corresponding product 3pa in 77% yield. To our delight,
the heterocyclic substrate 1s underwent ethoxylation to
produce product 3sa in 75% yield.
Next, we explored various alkanol substrates in the
cobalt-catalyzed alkoxylation and found that the procedure
can be widely applied (Table 3). A variety of linear and
branched alcohols were efficiently coupled with benzamide
1 to give products 3aa–3aj. Simple primary alkyl alcohols,
such as methanol and butanol, afforded good yields (3ab–
3ac). The successful formation of products 3ad and 3ae
showed that ether- and phenyl-containing alcohol substrates
are well tolerated in the reaction. Trifluoroethanol (2 f)
Figure 1. Screening of directing groups for the alkoxylation reaction.
Dec.=decomposed.
reaction was inefficient under the same optimized reaction
conditions. Notably, 8-aminoquinolinebenzamide (C) did not
give a satisfactory yield in our system. No reaction was
detected in the case of the structurally similar but mono-
dentate directing groups (D–G), highlighting the unique
property of the N,O-bidentate coordinating group (Figure 1).
With these optimized reaction conditions in hand, we
probed the substrate scope in the ethoxylation of benzamides
1. As shown in Table 2, a wide variety of functionalized
benzamides were tolerated by the cobalt catalyst to give rise
to the desired products 3. The reactions proceeded success-
fully for both electron-rich and electron-poor amides. In all
cases studied, bisethoxylated products were not obtained. The
benzamides which have electron-rich functional groups in the
para position afforded coupling products in good yields (3ba–
3ea). The electron-poor amide 1 f bearing a fluoro functional
2
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Table 3: Cobalt-catalyzed alkoxylation of benzamide 1a with various
alcohol substrates 2a–2j.[a]
Table 4: Cobalt-catalyzed alkoxylation of olefinic carboxamides 1.[a]
[a] Reaction conditions: 1 (0.2 mmol), alkanols 2 (1.5 mL), Co(OAc)2·
4H2O (20 mol%), Ag2O (0.2 mmol), NaOPiv·H2O (2.0 equiv), under Ar,
408C, 12 h. Piv=pivalate.
reaction system showed the existence of the free radical (g =
2.00905, see the Supporting Information). These results
indicate that a radical pathway is involved. A 1:1 mixture of
1a and [D5]-1a was then treated with ethanol. No kinetic
isotope effect (KIE; kH/kD ꢀ 1.0) was obtained (Scheme 1),
[a] Reaction conditions: 1a (0.2 mmol), alkanols 2 (1.5 mL), Co(OAc)2·
4H2O (20 mol%), Ag2O (0.2 mmol), NaOAc (2.0 equiv), under air, 608C,
12 h.
underwent reaction with 1a to form the desired product 3af
which was isolated in 67% yield. The sterically hindered
alcohol 2g was employed to provide corresponding ether
product 3ag, albeit in a lower yield. Benzyl alcohols 2h and 2i
were also transformed into the corresponding benzyl ether
derivatives 3ah and 3ai in synthetically useful yields, despite
the fact that they are sensitive to oxidation.[6a] The hetero-
cyclic compound tetrahydrofurfuryl alcohol (2j) underwent
alkoxylation upon reaction with 1a to yield 3aj in 68% yield.
Unfortunately, the use of secondary alcohols such as isopro-
panol in the reaction was unsuccessful under these reaction
conditions.
Scheme 1. Determination of the kinetic isotope effect.
À
suggesting that C H bond cleavage of arenes is not the rate-
limiting step. On the basis of the seminal studies by Kochi
et al. on the oxidation of aromatic compounds by CoIII[19] and
the chelation-directed chlorination reactions of 2-phenyl-
pyridine reported by Yu and co-workers[4d] in which a low
KIE (= 1.0) was measured, we speculate that an oxidative
substitution of arenes mediated by single-electron transfer
(SET) is very possible in the cobalt-catalyzed alkoxylation.
However, the exact oxidation state of the catalytic Co species
(CoII or CoIII) is for now unclear.
Furthermore, the reactions were not restricted to aromatic
amides. 2-aminopyridine-1-oxide olefinic carboxamides also
À
served as valuable substrates for the cobalt-catalyzed C H
bond alkoxylation (Table 4). In these reactions, a lower
temperature was employed. As part of our investigations, we
found that NaOPiv·H2O (OPiv = pivalate) was a more effec-
tive base compared to NaOAc. The reaction afforded a higher
yield under an atmosphere of argon (see the Supporting
Information). These necessary changes indicate that olefinic
carboxamides in this reaction are more sensitive to the
reaction conditions compared with aromatic amides. For
example, a,b-unsaturated amide 1t was treated with metha-
nol (2b) and the methoxylation product 3tb was isolated in
75% yield.
Finally, the 2-aminopyridine-1-oxide (PyO) directing
group can be easily removed. As shown in Scheme 2, the
ethoxylated product 3aa was treated with NaOH in EtOH at
808C to obtain 2-ethoxybenzoic acid (4) in a high yield.
In conclusion, we have developed the first cobalt-
2
À
catalyzed alkoxylation of C(sp ) H bonds using 2-amino-
pyridine-1-oxide as an N,O-bidentate directing group. The
The mechanistic details of the cobalt-catalyzed alkoxyla-
À
tion of C H bonds are unclear. Our control experiments
revealed that the addition of 2,2,6,6-tetramethylpiperidine-N-
oxyl (TEMPO, 1.5 equiv) as a radical quencher inhibited the
reaction. Moreover, the experimental EPR spectrum of the
Scheme 2. Removal of the 2-aminopyridine-1-oxide (PyO) directing
group.
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À
alkoxylation of alkenes through C H bond activation. This
method provides a unique reaction procedure among the
otherwise broadly applicable palladium- or copper-catalyzed
À
C H alkoxylation reactions, thus extending the scope of
À
cobalt-catalyzed C H functionalization reactions. Moreover,
the N,O-bidentate directing group can be easily removed.
Further exploration of the detailed mechanism is currently
ongoing in our laboratory.
Received: October 3, 2014
Published online: && &&, &&&&
À
Keywords: alkoxylation · C H activation · cobalt ·
directing groups · transition-metal catalysis
.
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C H Activation
L.-B. Zhang, X.-Q. Hao, S.-K. Zhang,
Z.-J. Liu, X.-X. Zheng, J.-F. Gong,
J.-L. Niu,* M.-P. Song*
&&&& — &&&&
2
À
Cobalt-Catalyzed C(sp ) H Alkoxylation
of Aromatic and Olefinic Carboxamides
Alcohols in action: A wide range of
alcohols and benzamide substrates
functionalized with electron-rich orelec-
tron-poor substituents are tolerated in
the title reaction. This practical reaction
occurs under mild conditions.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!