Copper-catalyzed direct synthesis of iodoenamides from ketoximes

Article abstract of DOI:10.1002/chem.201301785

Iodide in copper's pathway: A new, efficient, and practical copper-catalyzed synthesis of Z-iodoenamides from readily available ketoximes has been developed (see scheme). The reaction was believed to proceed through a single-electron-transfer pathway. The corresponding Z-iodoenamides have been applied to the synthesis of substituted oxazoles, dienes, β-phenoxyl enamides, eneynes, β-acylenamides, and pyrroles (DCE=1,2-dichloroethane). Copyright

Full text of DOI:10.1002/chem.201301785

COMMUNICATION
Synthetic Methods
H. Liang, Z.-H. Ren, Y.-Y. Wang,
Z.-H. Guan* .................. &&&& — &&&&
Iodide in copperꢀs pathway: A new,
efficient, and practical copper-cata-
lyzed synthesis of Z-iodoenamides
from readily available ketoximes has
been developed (see scheme). The
reaction was believed to proceed
through a single-electron-transfer path-
way. The corresponding Z-iodo-
AHCTUNGTRENGNeUN namides have been applied to the
synthesis of substituted oxazoles,
dienes, b-phenoxyl enamides, eneynes,
bACTHNUTRGNEaUNG cylenamides, and pyrroles.
Copper-Catalyzed Direct Synthesis of
Iodoenamides from Ketoximes
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DOI: 10.1002/chem.201301785
Copper-Catalyzed Direct Synthesis of Iodoenamides from Ketoximes
Hao Liang, Zhi-Hui Ren, Yao-Yu Wang, and Zheng-Hui Guan*[a]
In modern organic chemistry, sp2 organic halides are ex-
tremely useful in transition-metal-catalyzed cross-coupling
reactions.[1] Accordingly, the development of novel methods
for the synthesis of sp2 organic halides is one of the most
promising research areas. Over the past few decades, a
number of protocols have emerged for the synthesis of aro-
matic halides.[2] However, apart from the conventional elec-
trophilic addition of HX to alkynes,[3] novel and efficient
methods for the synthesis of vinyl halides have rarely been
developed.
Vinyl iodides are versatile and powerful building blocks
in organic synthesis.[4] However, protocols for the synthesis
of vinyl iodides are still mainly limited to electrophilic addi-
tion of HI to the corresponding alkynes.[5] Novel and practi-
cal methods for the synthesis of vinyl iodides that are com-
Scheme 1. Copper-catalyzed synthesis of iodoenamides from ketoximes.
patible with various functional groups, use readily available
starting materials, and proceed under mild conditions
remain highly desirable.[6]
Oximes are valuable compounds for the preparation of
Table 1. Optimization of reaction condition.[a]
amides and nitriles.[7] In the past several years, ketoximes
have been applied in cross-coupling reactions to synthesize
heterocycles, such as indoles,[8] pyridines,[9] and isoquino-
lines.[10] The N O bond cleavage of ketoximes presents a
À
promising tool for organic transformations.[11] Inspired by
these works, we have developed a copper-catalyzed synthe-
Entry
Catalyst
MI
Solvent
Yield [%]
sis of Z-iodoenamides from ketoximes (Scheme 1). A mech-
anistic study shows that the reaction proceeded through
copper-catalyzed reductive acylation of the ketoxime fol-
lowed by oxidative iodination of in situ generated enamides.
We initially conducted our experiments by using palladi-
um as the catalyst. However, only acetophenone oxime ace-
tate was observed when a series of palladium catalysts, such
1
2
3
4
5
6
7
8
[Pd]
CuI
CuI
CuI
CuI
CuI
CuI
–
NaI
NaI
KI
Bu4NI
KI
KI
KI
KI
DCE
DCE
DCE
DCE
toluene
TCE
1,4-dioxane
DCE
0
45
72
14
39
53
28
0
as [Pd
ACHTUNGTRENNUNG(PPh3)4], [Pd2ACHTUNGTRENNUNG(dba)3] (dba=dibenzylideneacetone), or
[a] Reaction conditions: compound 1a (1.0 mmol), catalyst (10 mol%),
MI (1.2 equiv), Ac2O (2.5 equiv), solvent (10 mL), Ar, 1208C, 24 h
(DCE=1,2-dichloroethane, TCE=1,1,2,2-tetrachloroethane.
Pd(OAc)2, was used as the catalyst (Table 1, entry 1). To our
ACHTUNGTRENNUNG
delight, when CuI was used as the catalyst, iodoenamide 2a
was obtained in moderate isolated yield (Table 1, entry 2)
and only the Z-iodoenamide isomer was observed in the re-
action. The structure of 2a was also confirmed by X-ray dif-
fraction. Encouraged by these results, we have screened dif-
ferent iodide reagents in the presence of CuI as the catalyst.
When KI was used in the reaction, the yield of 2a was in-
creased to 72% (Table 1, entry 3). Furthermore, screening
of various solvents, such as toluene, TCE, and 1,4-dioxane,
revealed that DCE provided the best performance (Table 1,
entries 5–7). It should be noted that no reaction was ob-
served in the absence of the CuI catalyst (Table 1, entry 8).
Finally, the optimum reaction conditions were ketoxime 1
(1.0 equiv), KI (1.2 equiv), Ac2O (2.5 equiv), and CuI
(10 mol%) in DCE at 1208C under argon.
[a] H. Liang, Z.-H. Ren, Prof. Y.-Y. Wang, Prof. Dr. Z.-H. Guan
Key Laboratory of Synthetic and
Natural Functional Molecule Chemistry of Ministry of Education
Department of Chemistry & Materials Science
Northwest University, Xiꢁan 710069 (P.R. China)
E-mail: guanzhh@nwu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301785.
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Synthesis of Iodoenamides from Ketoximes
COMMUNICATION
Table 2. CuI-catalyzed synthesis of iodoenamides 2 from ketoximes 1.[a]
ly to give the corresponding Z-iodoenamides in good to
high yields (Table 2, entries 1–12). Ketoximes bearing an
electron-donating substituent, such as methyl, methoxyl, or
acetamino groups, on aromatic rings, showed good reactivity
and gave the Z-iodoenamides 2a–2d and 2i–2l in high
yields (Table 2, entries 2–4 and 9–12). Ketoximes bearing an
electron-withdrawing substituent, such as fluoro, chloro,
bromo, or nitro groups, on aromatic rings, proceeded slowly
and gave the corresponding Z-iodoenamides 2e–2h in mod-
erate to good yields (Table 2, entries 5–8).
Entry
Ketoxime 1 (R)
Product 2, yield [%][b]
As well as aryl methyl ketoximes, 2-naphthyl methyl ke-
toxime 1m and benzylideneacetone oxime 1n also produced
Z-iodoenamides 2m and 2n in good yields (Table 2, en-
tries 13 and 14). Phenyl ethyl ketoxime 1o gave a low yield
of the corresponding product 2o (Table 2, entry 15). We an-
ticipated that the reaction was being inhibited by the pres-
ence of the flexible ethyl group. To test this hypothesis and
further extend the reaction scope, cyclic ketoximes derived
from a-tetralones (1p–1q) and indanones (1r–s) were inves-
tigated under the standard conditions. Satisfactorily, the cor-
responding iodoenamides 2p–2s were obtained in 70–82%
yields (Table 2, entry 16–19). Additionally, N-propyl iodoe-
namide 2t was obtained in 67% yield in the presence of
propionic anhydride (Table 2, entry 20).
1
2
3
4
5
6
7
8
1a (H)
1b (4-Me)
1c (4-OMe)
1d (4-NHAc)
1e (4-F)
2a, 72
2b, 69
2c, 64
2d, 75
2e, 66
2 f, 59
2g, 62
2h, 50
2i, 67
2j, 65
2k, 57
2l, 76
1 f (4-Cl)
1g (4-Br)
1h (4-NO2)
1i (3,4-di-CH3)
1j (3,4-(CH2CH2)2)
1k (2,5-di-CH3)
1l (3-OMe)
9
10
11
12
13
To demonstrate the efficiency and practicality of this reac-
tion, a scaled up reaction was performed with low catalyst
loading [Eq. (1)]. Gram-scale synthesis of Z-iodoenamide
2a was achieved in 68% yield after isolation by crystalliza-
tion.
14
15
Oxazole is a prevalent structural motif in many bioactive
natural products, including 8-deshydroxyajudazol A, diazo-
namide A, oxaprozin, bengazole A, berninamycins, and gri-
seoviridin.[12,13] The intramolecular cyclization of b-iodo-
16
17
18
19
1p (n=2, R=H)
1q (n=2, R=4-OMe)
1r (n=1, R=H)
2p, 70
2q, 82
2r, 80
2s, 72
AHCTUNGTREGeNNNU namides would give an efficient and practical route for the
1s (n=1, R=4-Cl)
synthesis of substituted oxazoles.[14] After a brief survey of
reaction conditions, we found that a 94% yield of oxazole
3a was obtained in the presence of CuI (5 mol%) and
K2CO3 (1.5 equiv) in DMF at 1008C (Table 3, entry 1).[15]
Therefore, Z-iodoenamides were easily transformed into
their corresponding oxazoles under the aforementioned con-
ditions (Table 3). A series of 4-aryl-substituted oxazoles 3a–
3l and 3t were synthesized in good to excellent yields. 4-
Naphthyl- and 4-styryl-substituted oxazoles 3m–3n were ob-
tained in 83 and 96% yields, respectively. Notably, 2,4,5-tri-
substituted oxazoles 3o–3q were formed in 93–99% yields
by intramolecular cyclization of tetrasubstituted iodoena-
mides 2o–2q.
20[c]
[a] Reaction conditions: compound 1 (1.0 mmol), CuI (10 mol%), KI
(1.2 equiv), Ac2O (2.5 equiv), DCE (10 mL), Ar, 1208C. [b] Yields of the
isolated products. [c] Propionic anhydride (2.5 equiv) was used.
Under the optimized reaction conditions, the scope of the
reaction was investigated (Table 2). This redox-neutral acy-
lation/iodination reaction displayed high functional group
tolerance and proved to be an efficient method. Again, only
the Z-iodoenamide isomer was obtained in all cases. The re-
actions of various aryl methyl ketoximes proceeded smooth-
Encouraged by these results, further extension of the re-
action scope of the Z-iodoenamides was attempted
(Scheme 2). Palladium-catalyzed Heck cross-coupling of the
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Z.-H. Guan et al.
Table 3. Cu-catalyzed intramolecular cyclization of iodo-enamides 2 for
the synthesis of oxazoles 3.[a]
methoxyphenylacetylene, has been explored. The desired
eneyne product 9 was obtained in 95% yield under mild re-
action conditions.[15,19] Moreover, eneyne product 9 can be
further transformed into b-acylenamide 10 and pyrrole 11
through simple operations.[20,21] It should be noted that the
Z geometry of the double bond in the iodoenamides was
preserved and only 1 mol% of the palladium catalyst was
used in all of aforementioned cross-coupling reactions.
Moreover, palladium-catalyzed deiodination of the iodoena-
mides can also be performed efficiently in the presence of
Et3N in DMF at 608C.
The tentative mechanisms are proposed in Scheme 3. One
proposed mechanism is reductive acylation of the acetophe-
none oxime acetate intermediate A by Cu+ in a two-step
single-electron-transfer reaction to afford enamide inter-
mediate C.[22] Next, oxidative iodination of enamide C
occurs through a two-step single-electron-transfer oxidation
by Cu2+ (path a).[23] The selectivity for cis stereochemistry is
assumed to occur because of the steric effects of enamide in-
termediate C. Alternatively, electrophilic iodination of en-
amide intermediate C by I+ would give iodoenamide 2
(path b).[24] Another possible mechanism is based on oxida-
Entry
Product 3
Yield [%]
1
2
3
4
5
6
7
8
3a, R=H
3b, R=4-Me
3c, R=4-OMe
3d, R=4-NHAc
3e, R=4-F
94
92
83
81
85
93
95
81
94
94
92
91
3 f, R=4-Cl
3g, R=4-Br
3h, R=4-NO2
3i, R=3,4-di-CH3
3j, R=3,4-(CH2CH2)2
3k, R=2,5-di-CH3
3l, R=3-OMe
9
10
11
12
À
tive addition of the N O bond of acetophenone oxime ace-
13
83
tate intermediate A to CuI to generate CuIII intermediate
F.[25] After tautomerization and acylation by anhydride, vinyl
III
À
C H activation by Cu occurs to give intermediate H. Fi-
nally, reductive elimination of CuIII intermediate H produces
iodoenamide 2 (path c).[26]
14
15
16
96
93
95
To gain more insight into the mechanism of the reaction,
methoxyl-substituted enamide 2cc (1.0 equiv) was directly
added to the reaction of acetophenone oxime 1a (1.0 equiv)
under the aforementioned conditions [Eq. (2)]. The enamide
8 was obtained in 58% yield along with iodoenamide 2c in
57% yield. These results indicate that the Z-iodoenamides
should be generated from the corresponding enamides.
17
18
99
84
On the basis of these results, iodination of enamide 8 was
examined by using a stoichiometric amount of CuBr2 as the
oxidant [Eq. (3)]. The iodoenamide 2a was obtained in 7%
yield along with bromoenamide 12 in 56% yield [Eq. (3)].
When the reaction was performed without KI, the bromoe-
namide was obtained in 78% yield [Z/E=3:1; Eq. (4)]. It is
impossible to oxidaize BrÀ to give Br+ with Cu2+. There-
fore, path b can be ruled out as the mechanism. Further-
more, path c seems less likely because the CuIII species does
not exist in these systems.
[a] Reaction conditions: Compound 2 (0.3 mmol), CuI (5 mol%), K2CO3
(1.5 equiv), DMF (3 mL), Ar, 1008C.
Z-iodoenamide 2q with tert-butyl acrylate or acrylonitrile
proceeded smoothly to give the corresponding diene prod-
ucts 4 and 5 in 78 and 71% yields, respectively.[15,16] Notably,
styrene, which is generally less reactive than electron-poor
alkenes in Heck reactions, produced the corresponding
diene 6 in high yield.[15,17] Surprisingly, Suzuki cross-coupling
of Z-iodoenamides with phenylboronic acid gave unantici-
pated b-phenoxyl enamide products 7a–7c in high yields
under argon.[15,18] Furthermore, Sonogashira cross-coupling
of the Z-iodoenamide 2a with a terminal alkyne, such as 4-
To further confirm that the mechanism follows path a, a
radical-trapping reagent, (2,2,6,6-tetramethylpiperidin-1-
yl)oxidanyl (TEMPO), was added to the reaction. Forma-
tion of iodoenamide 2a was completely suppressed
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Synthesis of Iodoenamides from Ketoximes
COMMUNICATION
proceeds through the single-
electron-transfer pathway (path
a).
In summary, a novel and effi-
cient copper-catalyzed regiose-
lective synthesis of Z-iodoena-
mides from readily available
ketoximes has been developed.
The reaction tolerates a wide
range of functional groups and
allows gram-scale synthesis of
iodoenamide products. Prelimi-
nary mechanistic studies indi-
cate that the reaction is likely
to proceed through a single-
electron-transfer pathway. Fur-
thermore, the corresponding Z-
iodoenamide products have
been applied in the synthesis of
substituted oxazoles, dienes, b-
phenoxyl enamides, eneynes, b-
acylenamides, and pyrroles.
Further investigation of the re-
action scope and mechanistic
studies are underway in our
Scheme 2. Transformations of the Z-iodoenamides (TFA=trifluoroacetic acid; PMP=para-methoxyphenyl).
laboratory and will be reported
in due course.
Scheme 3. Tentative mechanisms.
[Eq. (5)]. Similarly, when TEMPO was added to the oxida-
tive halogenation reaction of enamide 8, neither iodoen-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Z.-H. Guan et al.
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Experimental Section
Typical procedure for the synthesis of Z-iodoenamides 2: In a round-bot-
tomed flask (25 mL), ketoximes 1 (1 mmol), acetic anhydride (2.5 mmol,
255.0 mg), KI (1.2 mmol, 199.2 mg), and CuI (10 mol%, 19.1 mg) were
stirred in 1,2-dichloroethane (10 mL) at 1208C under an argon atmos-
phere. After completion of the reaction (detected by TLC), the reaction
mixture was cooled to room temperature, extracted with ethyl acetate
(30 mL), and washed with brine (30 mL). The organic layer was dried
over anhydrous Na2SO4 and evaporated in vacuo. The desired Z-iodo-
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Acknowledgements
We thank the National Natural Science Foundation of China (NSFC-
21272183, 21002077), and the Rising Scientific Stars Foundation of
Shanxi Province (2012KJXX-26) for financial support.
Keywords: asymmetric catalysis · copper · iodoenamides ·
ketoximes · synthetic methods
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Synthesis of Iodoenamides from Ketoximes
COMMUNICATION
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Received: May 9, 2013
Published online: && &&, 0000
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These are not the final page numbers!