Synthesis of enaminones via copper-catalyzed decarboxylative coupling reaction under redox-neutral conditions

Article abstract of DOI:10.1039/c7cc00260b

A novel copper-catalyzed C(sp³)-H oxidative functionalization of aromatic oxime acetates with α-oxocarboxylic acids was reported. This process involved N-O/C-C bond cleavages and C-C bond formations to furnish substituted enaminones under redox-neutral conditions. The oxime acetates served as both reactants and internal oxidants. Furthermore, this transformation also features good functional group tolerance and needs no ligands or additional bases.

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DOI: 10.1039/C7CC00260B
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Synthesis of Enaminones via Copper-Catalyzed Decarboxylative
Coupling Reaction under Redox-Neutral Conditions
Zhongzhi Zhu, Xiaodong Tang, Jianxiao Li, Xianwei Li, Wanqing Wu, Guohua Deng*, Huanfeng Jiang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A novel copper-catalyzed C(sp3)-H oxidative functionalization
of aromatic oxime acetates with α-oxocarboxylic acids was
reported. This process involved N-O/C-C bond cleavages and
C-C bond formations to furnish substituted enaminones
10 under redox-neutral conditions. The oxime acetates served as
both reactants and internal oxidants. Furthermore, this
transformation also features good functional group tolerance
and needs no ligands or additional bases.
crossꢀcoupling of benzoyl radical and αꢀcarbon radical can lead to
the formation of carbonꢀcarbon bond.
Oximes are fascinating and versatile building blocks in organic
synthesis due to their easy preparation, efficient reactivity and
50 harmless byꢀproducts. In addition, they were widely employed for
preparing the nitrogenꢀbearing heterocyclic compounds via SN2
reaction,15 radical cyclization.16 Recently, C−H functionalization
of oxime derivatives with transitionꢀmetal catalyzed have
attracted more and more chemists’ attentions 17, especially oxime
Enaminones are key core units found in biologically active
1a
15 molecules, natural products,1b materials science1c and synthetic 55 derivatives used as radical reagents for the transitionꢀmetalꢀ
pharmaceutical agents. They are also used in the synthesis of
heterocycles and pharmaceutical compounds. For example,
catalyzed aꢀC(sp3)ꢀH bond activation reactions14. Our group first
reported copperꢀcatalyzed radical crossꢀcoupling reaction of
oxime acetates with sodium sulfinates for the synthesis of sulfone
derivatives (Scheme 1a).14b Whereafter, Lei’s group developed
2b
2d
bioactive heterocycles (pyrroles,2a,
quinolines,2c,
2l
pyridaziniums,2e indoles2f, 2g, pyrazoles2h,
pyridines2m) and
20 drugs3(anticonvulsants3a, antibacterials 3b). Moreover, they were 60 copperꢀcatalyzed
αꢀphosphorylation of aryl ketone Oꢀ
also used as ligands for transition metalꢀcatalyzed reactions.4
Therefore, the development of efficient transformations for the
preparation of enaminones from readily available starting
materials has attracted considerable attention. Over the past
acetyloximes for the formation of ßꢀketophosphonates (Scheme
1b).14c Though the significant progress that has been achieved
along these lines, the obvious restrictive limitation is that the
product are usually hydrolysis to ketone because of the instability
25 decades, many methods have been developed for the synthesis of 65 of enamine group. Herein, we report the development of a
enaminones, such as the direct condensation reaction of 1,3ꢀ
dicarbonyl compounds with amines,5 the aldolꢀtype addition
reaction of carbonyl compounds with nitrile and the rings
opening reaction of heterocycles. There were also some good
30 work by polymerization approaches with alkynes, amines and
aldehydes9 or diaroyl chlorides.10 In spite of the successful
synthesis of enaminones, there are some limitations, including the
need of high functional substrates such 1,3ꢀdicarbonyls or the use
of excessive additives and/or ligands. Therefore, the preparation
35 of enaminones from readily accessible materials with mild
conditions is one continuing challenge.
copperꢀcatalyzed alkyl C(sp3 ꢀH bond activation and
decarboxylative with αꢀoxocarboxylic acids and directly afforded
enaminones as the stabilizing effect of intramolecular hydrogen
bonds.
)
(a)
Our Previous work
O
Cu(OAc)2,100 o
C
OAc
N
SO2R3 C-S formation
Enamine hydrolysis
R3SO2Na
+
R1
Ar
Ar
ref 16a
Ar
R1
(b)
(c)
Lei 's work
OAc
O
O
O
R
CuCl, 130 o
C
N
R'
C-P bond formation
Enamine hydrolysis
+
P
H
P
R'
R
R'
Ar
R'
ref 16b
This work
Carbonꢀcarbon bond formation plays a central role in organic
synthesis.11 As an important strategy, the selective radical/radical
crossꢀcoupling12 between a persistent radical and a transient
40 radical have represented a powerful approach for achieving this
goal. Benzoyl radical may be considered as a persistent radical
which is easily produced from αꢀoxocarboxylic acids.13 Based on
our and other’s continuous studies on oxime acetates14, it is
believed that enamide intermediate radicals or αꢀcarbon radical
45 species could be formed by the oxime esters. Thus, the selective
OAc
N
O
NH2
R
O
CuI, 90 o
C
C-C bond formation
Enamine retain
OH
+
R
Ar'
Ar
Ar'
Ar
O
70
Scheme 1. The aꢀC(sp3)ꢀH bond activation of oxime acetates.
In our initial studies, we took acetophenone oxime acetate (1a),
2ꢀoxoꢀ2ꢀphenylacetic acid (2a) as the model substrates to
75 investigate different reaction conditions, and the results were
summarized in Table 1. To our delight, the desired product
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enaminone 3aa was detected in 55% GC yield when using CuI as
oxime acetates such as thiophene, furan, pyridine were
demonstrated compatible under our standard conditions,
the catalyst in CH3CN at 90 oC in a sealed tube for 6 h (entry 1).
Further copper salts screening suggested that copper(I) such as 40 rendering 3an, 3ao and 3ap in 73%, 61% and 52% yields,
CuCl, CuBr also catalyzed this reaction, but none of them were
found to be more efficient than CuI (entries 2−3). In contrast, the
copper(II) salts such as CuBr2, Cu(OAc)2, and Cu(OTf)2 did not
catalyze this conversion (entries 4ꢀ6). Subsequently, different
solvents such as DMSO, DMA, DMF, toluene and DCE were
also tested. DMF was revealed best to the reaction and gave 3aa
respectively. 1ꢀAcetylnaphthalene oxime acetate was a suitable
substrate for this reaction in 48% yield (3aq), In addition,
tetralone oxime acetate also afforded the desired product 3ar in
good yield.
5
45 Table 2. Substrate scope of various oxime acetates.a
10 in 72% GC yield (entries 7ꢀ11). However, no significant effects
were observed when the reactions were performed under O2 and
N2 atmosphere (entries 12ꢀ13). Moreover, considering the effect
of water on the reaction, molecular sieves (4Å) were added as an
additive increased the yield to 92% (entry 14). Thereafter,
15 investigation of the temperature showed that 90 oC exhibited best
for this transformation (entries 15ꢀ16). Finally, no product was
obtained in the absence of copper salt (entry 17).
Table 1. Screening for optimal reaction conditions.a
Entry
[Cu]
Additive
Solvent
T (oC)
Yield (%) b
1
2
CuI
CuCl
ꢀ
CH3CN
CH3CN
90
90
55
9
ꢀ
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
DMF
90
90
90
90
90
90
90
90
90
90
90
90
80
3
CuBr
ꢀ
11
4
CuBr2
Cu(OAc)2
ꢀ
n.d.
n.d.
n.d.
n.d.
72
5
ꢀ
6
Cu(OTf)2
CuI
ꢀ
a
7
ꢀ
Reaction conditions: unless otherwise noted all reactions were
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
ꢀ
performed with 1 (0.3 mmol), 2a (0.36 mmol), CuI (10 mol %), 4 Å M.S.
50 mg and dry DMF (2.0 mL) in a sealed tube for 6 h.
8
ꢀ
toluene
DCE
9
ꢀ
n.d.
n.d.
54
50 Table 3. Substrate scope of various αꢀoxocarboxylic acids.a
10
11
12 c
13 d
14
15
16
17
a
ꢀ
ꢀ
DMA
DMF
NOAc
O
NH2 O
ꢀ
70
CuI, M.S.(4A)
O
H
+
DMF
DMF
DMF
DMF
DMF
R
DMF, 90 o
C
ꢀ
72
92 (84) f
O
e M.S.
e M.S.
e M.S.
e M.S.
R
3
1a
2
66
NH2
O
100
90
55
NH2
O
NH2
O
n.d.
F
20
Reaction conditions: unless otherwise noted, all reactions were
performed with 1a (0.3 mmol), 2a (0.36 mmol), [Cu] (10 mol %),
additive (0.3 mmol) and solvent (2.0 mL) in a sealed tube for 6 h.
3as, 85%
NH2
3at, 84%
NH2
3au, 67%
NH2
b
O
O
O
Yields analyzed by GC/MS based on 1a with dodecane as internal
c
d
standard. n.d. = not detected. Under N2 atmosphere. Under O2
f
f
Cl
O
Br
25 atmosphere. M.S. = molecular sieves (4 Å), 50 mg was used. Isolated
yield.
3ax, 61%
NH2
3av, 76%
NH2
3aw, 71%
NH2
O
O
O
Based on the optimization study, we first investigated the
scope of oxime acetates and the results are summarized in Table
2. Generally, this reaction proceeded smoothly and gave the
30 desired products 3 in moderate to good yields. A series of paraꢀ
substituted acetophenone oxime acetate, including some with
electronꢀdonating groups (Me, Et, OMe) and some with electronꢀ
withdrawing groups (F, Cl, Br, CF3) were converted into the
corresponding enaminones in good yields (3aa–3ah).
35 Furthermore, ortho, metaꢀsubstituted and polyꢀsubstituted
acetophenone oxime atceates were able to give the desired
products in moderate to good yields (3aiꢀ3am). The heteroarene
O
S
NO2
NH2
3az, 65%
3ba, 61%
3ay, 64%
O
NH2
O
3bb, 54%
3bc, 0%
a
Reaction conditions: unless otherwise noted all reactions were
performed with 1a (0.3 mmol), 2 (0.36 mmol), CuI (10 mol %), 4 A M.S.
(50 mg) and dry DMF (2.0 mL) in a sealed tube for 6 h. Isolated yields.
55
Next, the scope of αꢀoxocarboxylic acids was examined in this
reaction (Table 3). In general, a variety of αꢀoxocarboxylic acids
2
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with different substituents on the aromatic ring were found to be
tolerable in this process. For example, αꢀoxocarboxylic acids with
electronꢀdonating substituents (Me, MeO) afforded the desired
abstracted a single electron from carboxylate to produce the
carboxyl radical, and the latter was quickly decarboxylated to
produce acyl radical III. Subsequently, selective crossꢀcoupling
enaminones in good yields (3as, 3at, 3ax). While αꢀ 45 of the radical species II and III constructed a new CꢀC bond to
5
oxocarboxylic acid derivatives bearing electronꢀwithdrawing
substituents (F, Cl, Br, NO2) provided the desired products in
moderate yields ranging from 64% to 76% (3auꢀ3aw, 3ay).
Satisfactorily, furoylformic acid and 2ꢀthienylglyoxylic acid
offered the desired products in 65% and 61% yields, respectively.
afford imine IV, which rapidly isomerized to the desired product
3.
10 Gratifyingly, naphthyl oxocarboxylic acid also efficiently reacted
with 2a, giving the product 3bb in 54% yield. Unfortunately,
aliphatic oxocarboxylic acid failed to undergo desired reaction
(3bc).
O
O
Scheme 4. Possible reaction mechanism.
4
99%
50
In summary, we have developed an novel copperꢀcatalyzed
C(sp3)ꢀH oxidative functionalization of oxime acetates with αꢀ
oxocarboxylic acids. Furthermore, the reaction was accomplished
through NꢀO/CꢀC bond cleavages and CꢀC bond formations to
afford substituted enaminones. Moreover, the use of simple
H
R
NH2
O
H
N
N
N
NOAc
R
NH2
2a, 1.8g
6
standard condition
82%
3aa, 1.83g
6a, R = H, 84%
6b, R = Ph, 90%
1a, 1.77g
NH2OH
55 starting materials, no need of bases or ligands, no toxic
byproducts make this method practical and particularly attractive.
N
O
We are grateful to the National Natural Science Foundation of
China (21172076 and 21420102003) and the Fundamental
60 Research Funds for the Central Universities (2015ZY001).
5, 90%
15
Scheme 2. Largeꢀscale synthesis of 3aa and its application
To demonstrate the practical utility of this novel method, the
reaction of 1a and 2a was performed at the 10 mmol scale. The
Notes and references
Key Laboratory of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640, China. Fax: +86 20ꢀ
65 87112906; Tel: +86 20ꢀ87112906; Eꢀmail: jianghf@scut.edu.cn;
ghdeng@scut.edu.cn.
† Electronic Supplementary Information (ESI) available: Experimental
section, characterization of all compounds, copies of 1H and 13C NMR
spectra for selected compounds. See DOI: 10.1039/b000000x/
70
20 desired 3aa was formed in 82% yield (Scheme 2). Hydrolysis of
3aa with dilute sulphuric acid is smoothly proceeded, almost
quantitatively providing the desired product 4. The
hydroxylamine cyclization of 3aa with hydroxylamine
hydrochloride also gave isoxazole 5 with high yield. Moreover,
25 3aa reacted smoothly with hydrazine hydrate or phenylhydrazine,
and gave the pyrrole products 6a and 6b with good yields.
To gain more insight into the mechanism of the process, we
conducted two control experiments illustrated in Scheme 3. When
2.0 equiv of TEMPO or butylated hydroxytoluene (BHT) was
30 added, the reaction was completely shut off. This result indicated
that a radical process may involve in this transformation.
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Scheme 3. Investigation into the reaction mechanism.
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