Iron-catalyzed aerobic oxidative cleavage of the C-C σ-bond using air as the oxidant: Chemoselective synthesis of carbon chain-shortened aldehydes, ketones and 1,2-dicarbonyl compounds

Article abstract of DOI:10.1039/c5cc07390a

A simple iron-catalyzed aerobic oxidative C-C σ-bond cleavage of ketones has been developed. Readily available and environmentally benign air is used as the oxidant. This reaction avoids the use of noble metal catalysts or specialized oxidants, chemoselectively yielding carbon chain-shortened aldehydes, ketones and 1,2-dicarbonyl compounds without overoxidation.

Full text of DOI:10.1039/c5cc07390a

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COMMUNICATION
IronꢀCatalyzed Aerobic Oxidative Cleavage of C–C σꢀ
Bond Using Air as Oxidant: Chemoselectively to
Carbon ChainꢀShortened Aldehydes, Ketones and 1,2ꢀ
Dicarbonyl Compounds
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Qi Xing,ab Hui Lv,a Chungu Xiaa and Fuwei Li*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
simple ironꢀcatalyzed aerobic oxidative C–C σꢀbond aldehydes with molecular oxygen as the oxidant (Scheme 1b).
cleavage of ketones has been developed. Readily available and Huang and coꢀworkers reported goldꢀcatalyzed oxidative C–C
environmently benign air is used as the oxidant. This reaction
prevents the use of noble metal catalysts or specialized
oxidants, chemoselectively yielding carbon chainꢀshortened
aldehydes, ketones and 1,2ꢀdicarbonyl compounds without
overoxidation.
In recent years, catalytic unstrained C–C bond cleavage, similar
to the emerging C–H bond functionalization, has attracted
much attention due to its fundamental scientific appeal and
potential application in organic synthesis.1 Up to now,
transitionꢀmetalꢀassisted approach to activate the inert C–C
bond has proved to be the most promising tool for this
purpose.2 Although the direct C–C bond cleavage has been
significantly developed in the past decades, transitionꢀmetal
involved oxidative C–C σꢀbond cleavage is still a challenging
task. In order to achieve this goal, noble metal catalysts and
stoichiometric oxidants, such as peroxides have traditionally
been required. Therefore, the development of milder and
greener process for oxidative C–C bond cleavage is highly
Scheme 1. Transition-metal-catalyzed oxidative C–C σ-bond cleavage
desirable.
Air is considered to be an ideal oxidant due to its easy
availability and environmentally benign character. Recently, a
aerobic conditions (Scheme 1c). Despite the progress achieved
few elegant examples of aerobic oxidative C–C σꢀbond
bond cleavage of aldehydes for synthesis of ynones under
in aerobic oxidative C–C bond cleavage, iron/air catalytic
cleavage have been developed for the synthesis of esters,
system promoted unstrained C–C single bond cleavages are
amides, ketones, aldehydes etc.3 For example, Jiao and coꢀ
workers developed a Mnꢀpromoted oxidative C–C bond
cleavage of aldehydes under oxygen atmosphere for formamide
synthesis (Scheme 1a). Later, the same group reported a
copperꢀcatalyzed aerobic oxidative C(CO)–C(alkyl) bond
cleavage of aryl alkyl ketones and C–N bond formation to
amides. The group of Bi and Liu reported a copper catalyzed
oxidative C(CO)–C(methyl) bond cleavage of ketones to
quite rare.4 As we know, iron, as an inexpensive, abundant and
nonꢀtoxic metal, offers a wide range of oxidation and spin
states.5 These features render it a potential catalyst for oxidative
C–C bond cleavage by means of single electron catalysis.
Herein, we reported an ironꢀcatalyzed aerobic oxidative
cleavage of C–C σꢀbond under air atmosphere. Various
phenylacetone derivatives with different alkyl chain, 3ꢀarylꢀ
substituted 2,4ꢀdicarbonyl compounds, 1,1ꢀdiphenylpropanꢀ2ꢀ
one and cyclic βꢀcarbonyl ketone were all suitable for this
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DOI: 10.1039/C5CC07390A
reaction, chemoselectively providing carbon chainꢀshortened products was obtained (entry 11). Prolonging the reaction time
aldehydes, ketones and 1,2ꢀdicarbonyl compounds in good to 20 h gave 2a in 90% yield (entry 12). As expected, the
yields (Scheme 1d). Particularly, this method terminated at reaction hardly proceeded in the absence of air (entry 13).
methyl ketone when 1ꢀmethylꢀ1ꢀarylꢀ2ꢀpropanone derivatives
R3
O
were used as the substrates. Whereas, in the work of Bi and Liu,
R2
FeCl3 (10 mol%)
R3
R1
methyl ketones were liable to undergo further C–C bond
cleavage to aldehydes under oxidative conditions (Scheme 1e).
In addition, this method can be applied to the preparation of 2ꢀ
R1
O
air (1 atm), H2O (1 equiv)
solvent, 110 o
C
1
2
Producta
Substrate
Producta
Substrate
acetylaminoꢀbenzaldehydes from oꢀ(Nꢀacylamino)aryl ketones.
The latter could be synthesized efficiently from ortho
ꢀ
O
R2
CHO
n
iodoanline and 1,3ꢀdiones via C–C bond cleavage.6 As reported,
H
2a, 92%
2a, 24%
O
1m, n=1
1n, n=2
R2
R2
O
O
2ꢀacetylaminoꢀbenzaldehydes are reactive intermediates for
quinolinꢀ2(1 H
)ꢀone skeletons construction.7
1a, R2 = Me
1b, R2 = Et
1c, R2 = iPr
1d, R2 = Ph
2a, 90%
2a, 91%
2a, trace
O
R2
Table 1. Optimization of the reaction conditions.
O
R1
2a, 54%
R1
O
1o, R1 = Me, R2 = Me
2j, 91%b
2k, 66%b
2l, 81%b
1p, R1 = NO2, R2 = Me
1q, R1 = Me, R2 = Et
O
O
R1
R1
O
1e, R1 = H
1f, R1 = Me
2b, 72%
2c, 75%
OEt
OEt
Entry
1
Solvent
Catalyst
T (oC)
90
Yield (%)a
77
O
O
O
H
1r
O
2m, 73%b
DMSO
DMSO
FeCl3
ꢀꢀꢀꢀ
O
R1
O
R1
2
3b
90
n.r.
64
1g, R1 = Me
2d, 86%
2e, 75%
2f, 91%
2g, 84%
2h, 82%
90
DMSO
1,4ꢀdioxane
CH3CN
DMF
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl2
Fe(OTf)3
CuI
1h, R1 = OMe
1i, R1 = Cl
4
90
44
1j, R1 = NO2
1k, R1 = CN
1s
2n, 64%
5
90
29
O
6
90
54
O
OH
7
110
110
110
110
110
110
110
83
DMSO
DMSO
DMSO
DMSO
H
H
O
N
O
N
8
47
1t
O
1l
2i, 79%
2o, 85%
9
49
Scheme 2. C–C bond cleavage of different β-carbonyl compounds. Conditions: a1
(0.5 mmol), FeCl3 (10 mol%), DMSO (2 mL), H2O (0.5 mmol), air (1 atm), 110 oC,
20 h. bCH3CN (2 mL) was used as the solvent, 90 oC.
10
11c
12d
13e
78
Trace
90
DMSO
DMSO
HCl
FeCl3
DMSO
FeCl3
Trace
With the optimized conditions in hand, we further
investigate the substrate scope toward this oxidaitve C–C bond
cleavage (Scheme 2). Firstly, our efforts were directed to
aReaction conditions: 1a (0.5 mmol), H2O (0.5 mmol), 10.0 mol % of metal
catalyst, air (1 atm), solvent (2 mL), 110 oC, 12 h. Isolated yield. bO2 (1 atm)
was inflated instead of air. cHCl (30 mol %). d20 h. eThe reaction was propiophenone derivatives with different alkyl substituents.
conducted in the glove box and no air was inflated.
Reactions with benzyl methyl ketone 1a and benzyl ethyl
ketone 1b proceeded smoothly, giving benzaldehyde 2a in
excellent yields. However, for benzyl isopropyl ketone 1c, only
trace amount of 2a was obtained and most of 1c was preserved,
possibly due to the larger steric hindrance. To our delight, 1,2ꢀ
We commenced our study with Feꢀcatalyzed aerobic
oxidation of 1ꢀphenylpropanꢀ2ꢀone 1a (Table 1). Initially, the
aimed product benzaldehyde 2a was obtained in 77% yield with
FeCl3 as the catalyst (entry 1). The reaction did not work in the
diphenylethanone 1d was also suitable for the C–C bond
absence of Fe catalyst (entry 2). What’s more, when 1 atm O2
cleavage reaction with a modest yield. Fortunately, substrates
was used instead of air, a lower yield of 2a was obtained along
1e and 1f bearing methyl group on the benzyl carbon atom were
with benzoic acid as the main byproduct, which probably
also compatible with the standard reaction conditions,
resulted from further oxidation of 2a under O2 (entry 3). Then
providing acetophenone 2b and 2c in 72% and 75% yield,
we screened other solvents (1,4ꢀdioxane, CH3CN, DMF), but
respectively. Subsequently, we investigated substrates with
no better results were obtained (entries 4ꢀ6). Increasing the
different substituents on the aryl ring. The results showed that
temperature to 110 oC led to an improved yield (83%) (entry 7).
electronꢀdonating (ꢀMe, ꢀOMe), electronꢀwithdrawing (ꢀNO2, ꢀ
In addition, other Fe catalyst such as FeCl2 and Fe(OTf)3 could
CN) and halogen groups were all well tolerated under the
also catalyze this reaction, but the efficiency was much lower
standard conditions, giving the corresponding aldehyde in 75%ꢀ
than FeCl3 (entries 8 and 9). As reported, CuI is widely used to
91% yields. Moreover, heteroaryl ketone such as 1l also
catalyze oxidative C–C bond cleavage. Under the same reaction
underwent the reaction successfully to furnish the
conditions, CuI is as efficient as FeCl3 (entry 10). The Brønsted
corresponding product in good yield. Interestingly, the standard
acid HCl was also tested, however, only trace of the desired
conditions were also compatible with benzenacetaldehyde 1m
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with benzaldehyde as the final product in 92% yield. However, antiplatelet, antiviral agents, and various types of receptor
in contrast to 1m
1n showed a much lower reactivity providing antagonists.8 Our method provides a useful alternative pathway
)ꢀones.
,
2a in 24% yield and 66% of 1n was recovered. It’s probably for the synthesis of quinolinꢀ2(1
due to a stepwise C–C bond cleavage reaction, in which
PhCH2CH2CHO was firstly converted into PhCH2CHO
followed by a second C–C cleavage from PhCH2CHO to
PhCHO.9 In this process, the conversion from PhCH2CH2CHO
to PhCH2CHO is probably the rateꢀdetermining step in view of
the facile transformation from 1m to 2a. Notably, 3ꢀarylꢀ
substituted 2,4ꢀdicarbonyl compounds 1o to 1r could also be
used in this reaction, giving the corresponding 1,2ꢀdicarbonyl
compounds 2j to 2m in good yields. In addition, 1,1ꢀ
diphenylpropanꢀ2ꢀone 1s was also a suitable substrate to
provide benzophenone 2n in 64% yield. Fortunately, cyclic βꢀ
carbonyl ketone 1t could also undergo this reaction smoothly,
giving ringꢀopening product 3ꢀ(2ꢀformylphenyl)propanoic acid
2o in 85% yield. It’s worth to note that the aromatic ring of
substrates is required for this reaction and only trace amount of
an alternative carboxylic acid and formaldehyde were observed.
Taking the reaction of 1a for example, benzaldehyde was the
main product while only trace amount of benzoic acid was
H
detected by GCꢀMS. As shown in Scheme 5, enolate
I formed
on the side which is close to the aromatic ring is more stable
Scheme 4. Control C–C bond cleavage of 1a
.
due to conjugation action. Subsequent reaction of
provides benzaldehyde as the major product.
I with O2
We performed some control experiments to explore the
reaction mechanism (Scheme 4). In the absence of H2O, 49% of
1a was recovered and only trace amount of 2a was observed.
Meanwhile, 43% yield of 1ꢀphenylpropaneꢀ1,2ꢀdione 4a was
obtained (Eq. 1). This result confirmed the essential role of H2O
in this C–C bond cleavage reaction. Then we placed 4a under
the optimized conditions for C–C cleavage, but no reaction
occured, which suggested that 4a was not the intermediate for
this reaction (SEq.
6 in the Electronic Supplementary
Information). It’s noteworthy that experiment of Eq. 1 was
carried out under O2 in order to avoid the interference of H2O in
air. When the reaction was conducted under Ar, only trace
amount of the desired product was detected (Eq. 2). So the
presence of air (or O2) is also essential for the present reaction.
In addition, the reaction proceeded well in the presence of
radical scavengers such as 1,1ꢀdiphenylethylene, butylated
hydroxytoluene (BHT) and 1,1,5,5ꢀtetramethylpentamethylene
nitroxide (TEMPO), providing the desired product 2a in 78% to
87% yields (Eq. 3). These reactions indicate that a radical
process might not be involved in the present transformation.
Scheme 3. C–C bond cleavage of o-(N-acylamino)aryl ketones and further
transformation to quinolin-2(1 H)-one. Condition
a1 (0.25 mmol), FeCl3 (10
mol%), CH3CN (1 mL), H2O (0.25 mmol), air (1 atm), 90 oC, 20 h, isolated yield.
b110 oC. Condition (0.125 mmol), Cs2CO3 (0.625 mmol), DMF (1 mL), 60 oC,
12 h, isolated yield based on
A
:
B: 2
2.
In our previous work, oꢀ(Nꢀacylamino)aryl ketones could be
synthesized efficiently from orthoꢀiodoanline and 1,3ꢀdiones
via C–C bond cleavage.6 To our delight, through iron catalyzed
aerobic oxidative C–C bond cleavage, these products could also
be converted to the corresponding aldehydes in good yields.
Taking
Nꢀ(2ꢀ(3,3ꢀdimethylꢀ2ꢀoxobutyl)phenyl)acetamide and
its derivatives (1u to 1x) for example, they could be converted
into 2ꢀacetylaminoꢀbenzaldehydes (2p to 2s) efficiently via
ironꢀcatalyzed C–C bond cleavage with the acetyl amino group
remained. Further more, in the presence of base, 2ꢀacetylaminoꢀ
benzaldehydes underwent further cyclization to form quinolinꢀ
2(1
Quinolinꢀ2(1
H
)ꢀone and its derivatives (3a to 3d) (Scheme 3).7
)ꢀone skeleton is frequently found in many
H
pharmacologically useful compounds, such as antitumor,
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DOI: 10.1039/C5CC07390A
Scheme 5. Plausible mechanism for C–C bond cleavage.
(h) Ziadi, A.; Correa, A.; Martin, R. Chem. Commun. 2013
(i) Subramanian, P.; Indu, S.; Kaliappan, K. P. Org. Lett. 2014
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49, 4286.
,
16
,
,
5
Based on the results above, a proposed mechanism for this
oxidative C–C bond cleavage reaction is drawn in Scheme 5.
First, with the catalysis of Fe(III), propiophenone is converted
,
, 5
,
,
into the iron enolate I, which is attacked by molecular oxygen
to yield a peroxide (II or III) coordinated by iron.9 Then this
peroxide suffers nucleophilic attack of H2O to form
intermediate IV. Subsequently, C–C bond cleavage delivers the
benzaldehyde along with one equivalent of acetic acid.
,
,
,
,
Jin, H. M.; Liu, X.; Cheng, Y. X.; Zhu, C. J. Chem. Commun.
, 2013,
In conclusion, we have developed an ironꢀcatalyzed aerobic
oxidative C–C bond cleavage of ketones under air, which
chemoselectively provides carbon chainꢀshortened aldehydes,
ketones and 1,2ꢀdicarbonyl compounds as the final products
without overoxidation. In this transformation, environmently
benign air and naturally abundant iron salt were used as the
oxidant and catalyst, respectively. In addition, this method
could be applied to the synthesis of 2ꢀacetylaminoꢀ
benzaldehyde and its derivatives, which are facile synthetic
49, 2933.
3
For selected examples of transitionꢀmetal involved oxidative C–C σꢀ
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For selected examples of ironꢀcatalyzed oxidative C–C bond cleavage,
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,
,
Acknowledgements
,
This work was supported by the Chinese Academy of Sciences
and the National Natural Science Foundation of China
(21133011, 21373246 and 21522309).
,
,
,
Notes and references
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000 (China).
b University of Chinese Academy of Sciences, Beijing, 100049, China.
,
,
†
Electronic Supplementary Information (ESI) available: Detailed
experimental procedures and spectral data for all compounds, including
scanned images of and NMR spectra. See
1H 13C
DOI: 10.1039/b000000x/
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