Acid/Base-Co-catalyzed Formal Baeyer-Villiger Oxidation Reaction of Ketones: Using Molecular Oxygen as the Oxidant

Article abstract of DOI:10.1021/acs.orglett.8b02006

A novel acid/base-co-catalyzed formal Baeyer-Villiger oxidation of various ketones using O₂ as the sole oxidant under metal-free conditions has been developed for the first time. The reaction tolerates a wide range of ketones and anilines and provides a simple and efficient method for the construction of various amides and isoquinolin-1-ones from the reactions of ketones with anilines in a single step.

Full text of DOI:10.1021/acs.orglett.8b02006

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Acid/Base-Co-catalyzed Formal Baeyer−Villiger Oxidation Reaction
of Ketones: Using Molecular Oxygen as the Oxidant
Chun-Hua Liu,† Zhuo Wang,† Li-Yun Xiao, Mukadas, Dong-Sheng Zhu, and Yu-Long Zhao*
Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis, Faculty of Chemistry, Northeast Normal
University, Changchun 130024, China
S
* Supporting Information
ABSTRACT: A novel acid/base-co-catalyzed formal Baeyer−Villiger oxidation of various ketones using O2 as the sole oxidant
under metal-free conditions has been developed for the first time. The reaction tolerates a wide range of ketones and anilines
and provides a simple and efficient method for the construction of various amides and isoquinolin-1-ones from the reactions of
ketones with anilines in a single step.
he Baeyer−Villiger (BV) reaction of ketones with the
The amide bond is the key backbone of all natural peptides
in biological systems and is also a ubiquitous and favorable
functional group in all branches of organic chemistry.9
Numerous methods for the formation of amide bonds have
been developed. Traditionally, amides were generated by
reaction of carboxylic acids and their derivatives with amines,10
which suffers from harsh conditions, stoichiometric amounts of
activators or condensation reagents, and a large amount of
byproduct. Alternatively, oxidative or dehydrogenative ap-
proaches from alcohols or aldehydes and amines are desirable
methods from the viewpoint of green sustainable chem-
istry.11,12 However, the direct transformation of simple ketones
to amides using amines as a N-source has rarely been studied.13
Recently, several elegant methods for the direct amide
synthesis from simple ketones have been developed using
amines as a N-source via C−C bond cleavage; however,
transition-metal catalysts, superstoichiometric amounts of
iodine, and chemical oxidants were required.14 Obviously, a
convenient new method for the synthesis of amides from
simple ketones using amines as an N source under metal-free
and chemical oxidants conditions is highly desired. As part of
our research on C−N bond formation reaction,15 we recently
developed a acid/base-co-catalyzed direct intermolecular
oxidative α-amination of cyclic ketones using O2 as the sole
oxidant.16 As the result of our continuous research in this area,
we herein report the first acid/base-co-catalyzed type Baeyer−
Villiger oxidation of ketones using O2 as the sole oxidant in the
absence of transition-metal catalysts and chemical oxidants
(Scheme 1, c). The reaction works with a broad range of cyclic
T
formation of esters or lactones is an important trans-
formation in organic chemistry.1,2 This transformation has
been widely used in many different applications, including the
synthesis of antibiotics, steroids, pheromones, and monomers
for polymerization.2 The most commonly used oxidants in this
process are hydrogen peroxide,3 organic peracids,4 and other
chemical oxidants5 (Scheme 1, a). In addition, although
Scheme 1. Strategies for Baeyer−Villiger Oxidation of
Ketones
noncatalytic and catalytic BV reactions with molecular oxygen
as the oxidant have been reported, transition-metal catalysts,6
enzymes,7 or superstoichiometric amounts of aldehydes8 are
generally required (Scheme 1, b). Thus, the development of
new BV-type reactions catalyzed by simple organic molecules
with molecular oxygen as the oxidant in the absence of
transition metals, chemical oxidants, enzymes, and aldehydes
remains a challenge.
Received: June 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02006
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and acyclic ketones and provides a simple and efficient method
for the construction of various amides and isoquinolin-1-ones
in a single step.
Scheme 2. Acid/Base-Co-catalyzed Tandem Reaction of 2-
Methylcyclohexanone 1a with Arylamines 2a−n
a,b
Initially, the reaction of 2-methylcyclohexanone 1a with 4-
methoxyaniline 2a was investigated using molecular oxygen as
the sole oxidant (Table 1). In the presence of 2,2,6,6-
a
Table 1. Optimization of Reaction Conditions
b
temp
time
(h)
yield
(%)
entry catalyst (mol %)
(°C)
solvent
m-xylene
m-xylene
m-xylene
1
2
3
TfOH (5)
V (50)
V (50)/TfOH
(5)
130
130
130
8
6
6
c
55
c
80
4
5
V (30)/TfOH
130
120
m-xylene
m-xylene
6
8
70
45
(5)
V (50)/TfOH
(5)
6
7
I (50)/TfOH (5)
II (50)/TfOH
(5)
III (50)/TfOH
(5)
IV (50)/TfOH
(5)
130
130
m-xylene
m-xylene
6
6
47
23
a
Reaction conditions: 1a (0.4 mmol), 2 (0.2 mmol), TfOH (0.01
mmol), TMP (0.1 mmol), m-xylene (2.0 mL), O2 (1 atm), at 130 °C
b
8
9
130
130
m-xylene
m-xylene
6
6
16
39
for 6−10 h in a sealed tube. Isolated yield.
substitutes and heteroaromatic amine, could react with 2-
methylcyclohexanone 1a to give the corresponding amides
3aa−n in good yields. It is noteworthy that the amination
reaction also tolerates ortho-monosubstituted arylamine,
indicating that the transformation exhibits good tolerance of
steric hindrance. In contrast, substrates 2e and 2m with
sterically hindered groups as well as 2j−l with electron-
deficient aromatic rings required longer reaction times for full
conversion and gave slightly lower yields. However, the
reactions of 1a with aliphatic amines, such as benzylamine
and butan-1-amine, are messy; presumably, their higher
nucleophilicity renders them able to disrupt multiple substrate
interactions and interact with intermediates and prone to
oxidation under the reaction conditions. In addition, it should
be noted that this tandem reaction is easy to scale up. A gram-
scale reaction of 1.34 g of 1a and 0.74 g of 2a was carried out
under the optimal conditions, furnishing 1.06 g of the desired
product 3aa in 71% isolated yield.
Next, we turned to extend the scope of the reaction with
respect to the ketone component. As shown in Scheme 3,
various substituted cyclohexanone-derived substrates 1b−f can
react readily with 4-methoxyaniline 2a to give the correspond-
ing amides 3b−fa in good to high yields. The incorporation of
various alky and phenyl substituents at position 2 and 5 of the
cyclohexanone ring is well-tolerated. Similarly, 2-alkycyclopen-
tanones 1g and 1h were also suitable partners for this tandem
reaction. Notably, in the case of fused cyclic ketone 1i, the
tandem reaction also worked well, providing the desired
product 3ia in 57% yield (Scheme 3).
10
11
12
V (50)/HCl (5)
V (50)/TFA (5)
V (50)/TfOH
(5)
V (50)/TfOH
(5)
V (50)/TfOH
(5)
130
130
130
m-xylene
m-xylene
chlorobenzene
8
8
8
12
38
52
13
14
130
130
toluene
DMF
8
31
10
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), I−V (0.06−0.1
mmol), HX (0.01 mmol), solvent (2.0 mL), O2 (1 atm), at 120−130
b
1
°C for 6−10 h in a sealed tube. Estimated by H NM spectroscopy
c
using dimethyl phthalate as an internal standard. Isolated yield.
tetramethylpiperidine (TMP, 50 mol %), the reaction of 1a
(0.4 mmol) with 2a (0.2 mmol) gave amide 3aa in 55% yield
in m-xylene (2.0 mL) at 130 °C for 6 h (entry 2). The addition
of trifluoromethanessulfonic acid (TfOH, 5 mol %) improved
the yield of 3aa to 80% (entry 3). However, no reaction was
observed when TfOH alone (5 mol %) was used as the catalyst
(entry 1). Decreasing the amount of TMP and reducing
temperature led to comparatively lower yields (entries 4 and
5). Other amines and acids, such as I−IV, hydrochloric acid,
and trifluoroacetic acid (TFA), were less effective (entries 6−
11). Among the solvents tested, m-xylene seemed to be the
best choice. Other solvents, such as chlorobenzene and
toluene, gave lower product yields (entries 12 and 13). No
desired product was observed when the reaction was carried
out in N,N-dimethylformamide (entry 14).
With the optimized conditions in hand (Table 1, entry 3),
the scope and generality of substituted amines were examined.
As described in Scheme 2, various amines, such as arylamines
with electron-rich, electron-neutral, or electron-deficient
Interestingly, under the identical conditions as above, the
reaction of 2-methyl-2,3-dihydro-1H-inden-1-one 1j with 2a
afforded isoquinolin-1-one 4a in 35% yield (Scheme 4). To our
B
DOI: 10.1021/acs.orglett.8b02006
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Scheme 3. Acid/Base-Co-catalyzed Tandem Reaction of
Cyclic Ketones 1b−i with 2a
Scheme 5. Acid/Base-Co-catalyzed Tandem Reaction of
Acyclic Ketones 1p−s with 2a
a,b
To further probe the reaction mechanisms, some control
experiments were designed and investigated (Scheme 6). We
Scheme 6. Control Experiments for Mechanistic Studies
a
Reaction conditions: 1b−i (0.4 mmol), 2a (0.2 mmol), TfOH (0.01
mmol), TMP (0.1 mmol), m-xylene (2.0 mL), O2 (1 atm), at 130 °C
b
for 6−8 h in a sealed tube. Isolated yield.
Scheme 4. Base-Catalyzed Reaction of Fused Cyclic Ketones
1j−o with 2a
found that lactone 5j was obtained in 50% yield when 1j was
treated under the optimal reaction conditions for 5 h (Scheme
6, a), implying that the Baeyer−Villiger type oxidation of
ketone is feasible to afford the ester as the key intermediate in
the reaction. In addition, no desired product was observed
when the reaction of 1a with 2a was carried out under nitrogen
atmosphere either in the presence or absence of TEMPO
(Scheme 6, b and c). The yield of product 3aa decreased
greatly from 80 to 10% under the standard conditions when
2.0 equiv of BHT (2,6-di-tert-butyl-4-methylphenol) was used
as a radical inhibitor (Scheme 6, d). These results indicate that
molecular oxygen as oxidant is required for the tandem
reaction, and a radical mechanism may be involved in this
process. In addition, no product 5j was obtained when 6j was
treated under the optimal reaction conditions for 5 h (Scheme
6, e). Similarly, no product 4a was observed when 6j was
treated with 2a in the presence of TMP (10 mol %) in m-
xylene at 130 °C for 8 h (Scheme 6, f). The result indicates
that intermediate I (Scheme 7) was not produced in the
tandem reaction and the reaction route from intermediate I to
3 was not involved.18 In addition, further GC−MS studies of
the reaction of 1a with p-tolylboronic acid under the optimal
reaction conditions identified that p-cresol was generated,
which provides further evidence for the generation of
superoxide anion from SET from A to oxygen (Scheme 7).
On the basis of the above experimental results and related
reports,3,6−8,16,19 a possible mechanism for the formation of 3
and 4 is proposed (Scheme 7, with cyclic ketone as example).
Initially, TfOH promotes the condensation of 1 with TMP to
form the enamine intermediate A, which is oxidized by O2 via a
single-electron transfer (SET) process to give the cationic
radical intermediate B/B′. Subsequently, the radical inter-
delight, the yield of 4a increased to 51% when TMP (10 mol
%) was used alone without TfOH. Similarly, isoquinolin-1-
ones 4b−f were obtained in moderate to good yields from the
reactions of 2-methyl-2,3-dihydro-indenones 1k−o with 2a
(Scheme 4). Thus, the reaction provides a new strategy for
direct and highly efficient synthesis of isoquinolin-1-one
derivatives.17
To further explore the generality of this reaction for ketones,
the tandem reaction of acyclic ketones 1p−s with 2a was
investigated. Under essentially the identical conditions, when
pentan-3-one 1p was used as the substrate, no desired product
3pa was obtained. Fortunately, when 3-methylbutan-2-one 1q
was used as the substrate, the desired amide 3qa was produced
in 60% yield. Similarly, acyclic ketones 1r and 1s bearing aryl
unit at the α-position were suitable for the reaction, affording
the desired amides 3qa and 3sa in 71% and 60% yields
(Scheme 5), respectively. In addition, GC−MS studies of the
reaction of 1s with 2a identified that 2-fluorobenzaldehyde was
generated in this reaction.
C
DOI: 10.1021/acs.orglett.8b02006
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Letter
Scheme 7. Proposed Mechanisms for Formation of 3 and 4
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhaoyl351@nenu.edu.cn.
ORCID
Yu-Long Zhao: 0000-0001-6577-1074
Author Contributions
†C.-H.L. and Z.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support of this research by the National Natural
Sciences Foundation of China (21472017) is acknowledged.
■
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02006.
Experimental procedures, full spectroscopic data and 1H
and 13C NMR spectra of all compounds (PDF)
Accession Codes
Dierks, A.; Muller, T.; Christoffers, J. Chem. - Eur. J. 2017, 23, 7245.
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Accession Codes CCDC 1832565 contains the supplementary
crystallographicdata for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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