TBHP/I2-promoted oxidative coupling of acetophenones with amines at room temperature under metal-free and solvent-free conditions for the synthesis of α-ketoamides

Article abstract of DOI:10.1039/c2gc35489f

A novel and efficient TBHP/I₂-promoted oxidative coupling reaction of acetophenones with amines for the synthesis of α-ketoamides has been developed. The reactions proceeded smoothly at room temperature under metal-free and solvent-free conditions and generated the corresponding products in good yields.

Full text of DOI:10.1039/c2gc35489f

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COMMUNICATION
TBHP/I2-promoted oxidative coupling of acetophenones with amines at room
temperature under metal-free and solvent-free conditions for the synthesis of
α-ketoamides†
Xiaobin Zhanga and Lei Wang*a,b
Received 2nd April 2012, Accepted 25th May 2012
DOI: 10.1039/c2gc35489f
air-sensitive properties of metals or organometallics.11 A variety
A novel and efficient TBHP/I2-promoted oxidative coupling
of metal-free systems, such as TBHP (tert-butyl hydroperoxide)/
I2,12 TBPB (tert-butyl peroxybenzoate),13 TBHP,14 TBAF (tetra-
n-butylammonium fluoride),15 I2,16 I2/NBS (N-bromosuccini-
mide),17 and O2,18 have been reported for the organic transform-
ations. For example, Jiang et al. reported a TBHP/I2-mediated
domino oxidative cyclization for the one-pot synthesis of poly-
substituted oxazoles in 2010.12a Meanwhile, Wang and his
co-workers developed a series of TBHP/I2-promoted organic
reactions, such as a tandem oxidative cyclization of 2-amino-
1-phenylethanone with aromatic aldehydes for the preparation of
2,5-disubstituted oxazoles,12b a tandem reaction of 2-aminobenzo-
phenones and benzylic amines for the synthesis of 2-phenylqui-
nazolines,12c a decarboxylative cyclization from natural α-amino
acids to construct pyridine derivatives,12d an oxidative decarboxy-
lative coupling of primary α-amino acids with 2-aminobenzo-
ketones for the synthesis of quinazolines,12e and an oxidation of
benzylic methylenes to ketones and primary amines to nitriles.12f
Herein, we wish to report a TBHP/I2-promoted tandem direct
oxidative coupling of acetophenones with amine at room temp-
erature under metal-free and solvent-free conditions. The reac-
tions proceeded smoothly and generated the corresponding
α-ketoamide products in good yields under mild reaction con-
ditions (Scheme 1).
At the beginning of our investigation, the optimization of reac-
tion conditions was focused on a variety of reaction parameters
by using a model reaction of acetophenone (1a) with piperidine
(2a). As listed in Table 1, the oxidant plays an important role in
the reaction. When the model reaction was performed in
the presence of I2 (30 mol%) in toluene at room temperature
(20–25 °C), tert-butyl hydroperoxide (TBHP) exhibited the
highest reactivity to the reaction, and 78% yield of the desired
oxidative coupling product 3a was isolated (Table 1, entry 1).
Di-tert-butyl peroxide, H2O2, tert-butyl perbenzoate (TBPB),
reaction of acetophenones with amines for the synthesis of
α-ketoamides has been developed. The reactions proceeded
smoothly at room temperature under metal-free and solvent-
free conditions and generated the corresponding products in
good yields.
The α-ketoamides, as the key structural skeletal framework of
many natural products and pharmaceuticals, have attracted con-
siderable interest because of their important biologically active
properties.1 They are also the important synthetic intermediates
for the further transformation to Na-channel blocker GW
356194, 5-HT6 binding ligands, α-diones and β-lactams, etc.2
The traditional method for the synthesis of α-ketoamides is
based on the condensation of α-keto acids with amines in the
presence of dicyclohexylcarbodiimide or other activating agents,
however, the preparation of α-keto acids is impractical.3
Although a number of modifications on the synthesis of α-keto-
amides were developed,4 the drawbacks are the use of hazardous
reagents, harsh reaction conditions, multi-step processes, and
low yields in the most of the cases.
Transition-metal-catalyzed organic reactions for the construc-
tion of carbon–carbon and carbon–heteroatom bonds have
received much attention in modern organic synthesis.5 To syn-
thesize α-ketoamides, Pd-, and Cu-catalyzed double carbonyl-
ation of aryl iodides with amines,6 Pd-catalyzed aerobic
oxidative cleavage of preformed α-arylamino amides,7 and
Cu-catalyzed aerobic oxidative amidation–diketonization of
terminal alkynes,8 Cu-catalyzed oxidative coupling of aryl
acetaldehydes with anilines9 and Cu-catalyzed oxidative coup-
ling of aryl methyl ketones with amines10 have been developed
in recent years.
Most recently, organic reactions carried out under metal-free
conditions have also received much attention because it can
overcome the drawbacks of the expensive, poisonous, and
aDepartment of Chemistry, Huaibei Normal University, Huaibei, Anhui
235000, P.R. China. E-mail: leiwang@chnu.edu.cn; Fax: +86-561-309-
0518; Tel: +86-561-380-2069
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
P.R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35489f
Scheme 1 The direct oxidative coupling of acetophenones with
amines.
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Table 1 Optimization of the reaction conditionsa
reaction conditions for the model reaction were I2 (30 mol%),
TBHP (3.0 equiv.) at room temperature (20–25 °C) for 8 h
without additional solvent.
Under the optimized reaction conditions, the scope of substi-
tuted acetophenones and amines in the direct oxidative coupling
reactions was investigated. The results were shown in Table 2. A
variety of acetophenones bearing substituents on the benzene
rings were examined for the direct coupling with piperidine. The
results indicated that a variety of functional groups, including
electron-donating groups and electron-withdrawing ones were
tolerated, and the oxidative coupling reactions produced the cor-
responding products in good to excellent yields (Table 2, entries
1–12). Acetophenones with electron-withdrawing groups on the
para-positions of acetophenone, such as Cl, Br, I, and NO2, gen-
erated the corresponding oxidative coupling products in 88–94%
yields (Table 2, entries 3–6). On the other hand, acetophenones
with electron-donating groups, such as Me, MeO, and t-Bu
on the para-positions of their benzene rings, gave the desired
products in 85–89% yields (Table 2, entries 2, 7 and 8). It is
important to note that the reaction of 4-phenylacetophenone with
piperidine generated the corresponding product in 65% yield
(Table 2, entry 9). Meanwhile, the ortho-effect is not obvious for
methyl, and iodo groups on the ortho-positions of the aceto-
phenones in the reactions (Table 2, entries 10 and 11). Disubsti-
tuted acetophenone, such as 2,4-dichloroacetophenone, also
underwent the tandem reaction with piperidine under the present
reaction conditions and afforded the corresponding product in
92% yield (Table 2, entry 12). However, the reaction of 2-acetyl-
pyridine with piperidine only gave 68% yield of the product
(Table 2, entry 13). Under the recommended reaction conditions,
1-acetylnaphthalene also preceded the reaction smoothly to
generate the corresponding product in 85% yield (Table 2,
entry 14). The present reaction conditions were also suitable for
the reactions of acetophenone with other secondary amines, such
as diethylamine, dibenzylamine, 1-methylpiperazine and
morpholine, and the corresponding products were isolated in
58–87% yields (Table 2, entries 15–18). Finally, the reactions of
substituted acetophenones having p-Cl and p-Me groups, with
morpholine generated the desired products in 83 and 80% yields,
respectively (Table 2, entries 19 and 20).
Although the exact mechanism of this reaction is not clear up
till now, a possible cascade pathway was proposed and shown in
Scheme 2. The reaction occurs involving an iodination of aceto-
phenone, subsequently intermolecular nucleophilic substitution
with amine, and finally oxidation of methylene unit. When the
reaction was carried out in the presence of a radical scavenger,
such as TEMPO (2,2,6,6-tetramethyl-piperidyl-1-oxyl) and
ascorbic acid up to 2.0 equiv., the oxidative coupling reaction
did not occur and the starting material was recovered.19 On the
other hand, when the reaction of 1a without 2a was performed
under the present reaction conditions, it also did not occur and
the starting material was recovered, suggesting that 2a was
essential for the formation of 2-iodo-1-phenylethanone. Initially,
acetophenone reacted with piperidine (a secondary amine) to
afford enamine I.20 Then obtained enamine I reacted with I+,
which was generated from the reaction of I2 with an oxidant
TBHP,17 via a iodocyclization of the enamine CvC double
bond, forming a three-membered iodonium intermediate II,
followed by an intramolecular ring opening to give III, followed
Yield of 3ab
(%)
Entry Solvent
Oxidant
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
tert-Butyl hydroperoxide (TBHP) 78
Di-tert-butyl peroxide
H2O2
tert-Butyl perbenzoate (TBPB)
PhI(OAc)2
Cumyl hydroperoxide
O2
(C6H5COO)2
59
43
32
24
20
18
14
9
tert-Butyl hydroperoxide (TBHP) 0c
tert-Butyl hydroperoxide (TBHP) 30
tert-Butyl hydroperoxide (TBHP) 43
tert-Butyl hydroperoxide (TBHP) 51
tert-Butyl hydroperoxide (TBHP) 62
10
11
12
13
14
15
16
17
18
19
20
21
22
23
CH3CN
THF
Dioxane
Dichloromethane tert-Butyl hydroperoxide (TBHP) 65
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
tert-Butyl hydroperoxide (TBHP) 91
tert-Butyl hydroperoxide (TBHP) 76d
tert-Butyl hydroperoxide (TBHP) 50e
tert-Butyl hydroperoxide (TBHP) 92f
tert-Butyl hydroperoxide (TBHP) 45g
tert-Butyl hydroperoxide (TBHP) 76h
tert-Butyl hydroperoxide (TBHP) 92i
—
—
Trace
Tracej
a Reaction conditions: 1a (1.0 mmol), 2a (0.60 mL, 6.07 mmol),
I2 (0.30 mmol), oxidant (3.0 mmol), solvent (2.0 mL) if needed at room
temperature (20–25 °C) for 8 h. b Isolated yields. c In the absence of I2.
d TBHP (2.0 mmol). e TBHP (1.0 mmol). f TBHP (4.0 mmol).
h I2 (0.10 mmol). h I2 (0.20 mmol). i I2 (0.40 mmol). j I2 (1.0 mmol).
PhI(OAc)2, cumyl hydroperoxide, O2, and (C6H5COO)2 were
subsequently inferior, and only 14–59% yields of 3a were
obtained (Table 1, entries 2–8). However, no desired product 3a
was observed when the model reaction was carried out in the
presence of tert-butyl hydroperoxide (TBHP) without I2 in
toluene (Table 1, entry 9). Subsequently, the effect of solvent on
the model reaction was also examined and a significant solvent
effect was observed. When the model reaction was performed in
toluene, 78% yield of 3a was obtained, and 30–65% yields of 3a
was obtained when the reaction was performed in DMF,
CH3CN, THF, dioxane, or dichloromethane (Table 1, entries
10–14). To our delight, 91% yield of 3a was isolated, represent-
ing the best results, when the model reaction was carried out in
the presence of I2 and tert-butyl hydroperoxide (TBHP) at room
temperature without additional solvent (Table 1, entry 15). For
the optimization of the amount of I2 and tert-butyl hydroperox-
ide (TBHP) used in the model reaction, less than 30 mol% of I2
and 3 equiv. of TBHP led to the incompletion of the reaction
(Table 1, entries 16, 17, 19 and 20). Meanwhile, up to 40 mol%
of I2 and 4 equiv. of TBHP did not increase the yield of 3a sig-
nificantly (Table 1, entries 18 and 21). However, when the
model reaction was carried out in the presence of 0.30 equiv. or
1.0 equiv. of I2 in the absence of TBHP, only trace amounts of
3a was observed (Table 1, entries 22 and 23). The optimized
2142 | Green Chem., 2012, 14, 2141–2145
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Table 2 The direct oxidative coupling of acetophenones and aminesa
Entry
1
Ketone
Amine
Product
3
Yieldb (%)
91
3a
2
3
3b
3c
89
94
4
5
3d
3e
92
90
6
7
8
3f
88
85
89
3g
3h
9
3i
65
10
11
12
3j
3k
3l
87
82
92
13
14
3m
3n
68
85
15
3o
58
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Table 2 (Contd.)
Entry
16
Ketone
Amine
Product
3
Yieldb (%)
87
3p
17
18
19
3q
3r
3s
66
81
83
20
3t
80
a Reaction conditions: ketone (1, 1.0 mmol), amine (2, 0.60 mL), TBHP (3.0 mmol), I2 (0.30 mmol) at room temperature (20–25 °C) without
additional solvent for 8 h. b Isolated yields.
Scheme 3 The transformation of 2-iodo-1-phenylethanone and 2-
cyclohexyl-1-phenylethanone in the presence of TBHP.
reaction, the prepared 2-iodo-1-phenylethanone was reacted with
piperidine in the presence of TBHP (3 equiv.) to generate the
desired product 1-cyclohexyl-2-phenylethane-1,2-dione in 94%
yield (Scheme 3). On the other hand, the reaction of 2-iodo-1-
phenylethanone with piperidine generated 2-cyclohexyl-1-
Scheme 2 Possible reaction mechanism.
phenylethanone in 97% yield. The obtained 2-cyclohexyl-
1-phenylethanone was oxidized in the presence of TBHP
by hydrolysis to give 2-iodo-1-phenylethanone.16a The generated
(2 equiv.) to give the final product 1-cyclohexyl-2-phenylethane-
1,2-dione in 96% yield.
2-iodo-1-phenylethanone then underwent a nucleophilic substi-
tution with piperidine to form the key intermediate 2-cyclohexyl-
1-phenylethanone. The obtained 2-cyclohexyl-1-phenylethanone
proceeded free radical substitution of its methylene with a tert-
Conclusion
butylperoxy free radical generated from the reactions of eqn (1)
and (2)21 in Scheme 2 via homolytic cleavage of tert-butyl
In conclusion, a highly efficient synthetic method for the prep-
hydroperoxide to generate intermediate IV,12f which underwent
aration of α-ketoamides has been developed. In the presence of
further oxidation by TBHP to afford the final product 1-cyclo-
hexyl-2-phenylethane-1,2-dione.
To verify the formation of the key intermediates 2-iodo-1-
phenylethanone and 2-cyclohexyl-1-phenylethanone in the
I2 and tert-butyl hydroperoxide (TBHP), the direct oxidative
coupling reactions of acetophenones and amines proceeded
smoothly at room temperature under metal-free and solvent-free
conditions to generate the corresponding products in good
2144 | Green Chem., 2012, 14, 2141–2145
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Experimental section
All the direct oxidative coupling reactions of acetophenones with
1
amines were carried out under an air atmosphere. H and 13C
NMR spectra were measured on a Bruker Avance 400 MHz
NMR spectrometer with CDCl3 as solvent and recorded in ppm
relative to an internal tetramethylsilane standard. High resolution
mass spectroscopy data of the product were collected on a
Waters Micromass GCT or a Bruker Apex IV FTMS instrument.
General chemicals were purchased from commercial suppliers
and used without further purification.
Typical procedure for the metal-free and solvent-free reaction
A sealable reaction tube equipped with a magnetic stirrer bar
was charged with acetophenone (1.0 mmol), piperidine
(0.60 mL), I2 (0.30 mmol), tert-butyl hydroperoxide (TBHP,
3.0 mmol). The reaction vessel was carried out at room tempera-
ture (20–25 °C). After stirring the mixture for 8 h, it was diluted
with ethyl acetate, washed with water and brine, dried with
Mg2SO4. After the solvent was removed under reduced pressure,
the residue was purified by column chromatography on silica gel
(eluant: hexane–ethyl acetate 5 : 1) to afford the corresponding
product.
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Acknowledgements
This work was financially supported by the National Science
Foundation of China (nos. 21172092 and 20102039).
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