Aldehydes as Carbon Radical Acceptors: Silver Nitrate Catalyzed Cascade Decarboxylation and Oxidative Cyclization toward Dihydroflavonoid Derivatives

Article abstract of DOI:10.1002/adsc.201601407

Silver nitrate-catalyzed cascade decarboxylation and oxidative cyclization of α-oxocarboxylic acids, alkenes, and aldehydes is demonstrated for the first time. With ammonium persulfate as the oxidant, the cascade reactions afford dihydroflavonoid derivatives as products in moderate to good yields, exhibiting a broad substrate tolerance. Control experiments indicated that the mechanism includes a radical pathway with aldehydes as the carbon radical acceptors. (Figure presented.).

Full text of DOI:10.1002/adsc.201601407

Title: Aldehydes as Carbon Radical Acceptors: Silver Nitrate-Catalyzed
Cascade Decarboxylation and Oxidative Cyclization toward
Dihydroflavonoid Derivatives
Authors: Wen-Chao Yang, Peng Dai, Kai Luo, Yi-Gang Ji, and Lei Wu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201601407
Link to VoR: http://dx.doi.org/10.1002/adsc.201601407

10.1002/adsc.201601407
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Aldehydes as Carbon Radical Acceptors: Silver Nitrate Catalyzed
Cascade Decarboxylation and Oxidative Cyclization toward
Dihydroflavonoid Derivatives
Wen-Chao Yang,a,c Peng Dai,a Kai Luo,a,c Yi-Gang Ji, a,c and Lei Wu a,b,*
a
Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural
University, Nanjing 210095, P. R. China.
[Tel/Fax: +86-25-84396716; E-mail: rickywu@njau.edu.cn]
Beijing National Laboratory for Molecular Sciences and Institute of Chemistry, Chinese Academy of Sciences, Beijing
b
100190, P. R. China.
c
College of Plant Protection, Nanjing Agricultural University, Nanjing 210095, P. R. China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
hydroperoxide to obtain 2,3-dihydro-1H-inden-1-one
Abstract. Silver nitrate-catalyzed cascade decarboxyl-
ation and oxidative cyclization of α-oxocarboxylic acids,
alkenes, and aldehydes is demonstrated for the first time.
With ammonium persulfate as the oxidant, the cascade
reactions afford dihydroflavonoid derivatives as products
in moderate to good yields, exhibiting a broad substrate
tolerance. Control experiments indicated that the
mechanism includes a radical pathway with aldehydes as
the carbon radical acceptors.
derivatives (Scheme 1a).[5a] Lately, Li and co-workers
reported
a
TBHP-mediated
oxidative
difunctionalization of activated alkenes for the
synthesis of 3-(2-oxoethyl) indolin-2-ones using
aldehyde as the acylation source (Scheme 1b).[5c]
Notably, the aforementioned reactions relied on the
generation of acyl radicals from aldehydes, followed
by their addition to alkene functionalities. From
another point of view, aldehydes as acceptors
subjected to free carbon radicals offer an alternative
approach to ketone synthesis. However, owing to
their dissociation energy differentiation, the addition
intermediates (alkoxy radicals) preferably undergo C-
C β-scission reverting to the original aldehydes,
instead of undergoing C-H β-scission to form
ketones.[6] Consequently, this strategy is not only
challenging, but is also extremely underdeveloped. In
an elegant study, Zhu et al. reported the only example
of aldehydes as radical acceptors, in which a Cu-
catalyzed cascade annulation of enynals with alkenyl
or alkynyl α-bromocarbonyls was described for the
synthesis of various cyclohexenone-fused polycyclic
compounds (Scheme 1c).[6] Therefore, a milder, direct
and convenient approach such as the oxidative
cyclization of alkenes using aldehydes as radical
acceptors to achieve carbonyl-containing heterocyclic
compounds is still highly desirable.
Keywords: Ketones, Decarboxylation, Aldehydic C–H
bonds, Oxidative cyclization, Diacylation.
As a chemical skeleton for a range of materials, drugs,
and naturally occurring bioactive molecules,[1]
ketones are of great importance and have stimulated
numerous scientists toward research. Various
efficient synthetic approaches have been developed
for ketones over the past decades. Recently,
transition-metal-catalyzed
C-H
acylation
of
aldehydes has become a powerful strategy for
preparing ketones via the aldehydic C(sp2)-H bond
cleavage.[2,3] Within this regime, the addition of
carbonyl radical, which is generated from aldehydes
either by transition-metal catalysis or under metal-
free conditions, to alkenes or alkynes represents a
significant choice for producing ketones.[4] Recent
progress on the oxidative tandem coupling of alkenes
with aldehydic C–H bonds and other species for the
formation of heterocyclic compounds have gained
extensive attention owing to its prominent advantage
of step-and-atom efficiency.[5] For example, Li’s
group disclosed a novel iron-catalyzed carbonylation-
peroxidation reaction of alkenes with aldehydes and
Decarboxylative cross-coupling reactions for the
construction of C-C and C-X bonds have currently
attracted a great deal of interest due to their potential
advantage, such as using readily available, stable, and
low-cost carboxylic acids, extruding nontoxic CO2
gas as the byproduct, and high selectivity.[7] The
catalytic decarboxylative coupling reactions were
achieved with transition-metal complexes, including
1
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10.1002/adsc.201601407
Advanced Synthesis & Catalysis
those of palladium,[8] rhodium,[9] nickel,[10] copper,[11]
silver,[12] and other metals,[13] or by photoredox
catalysis.[14] Of these, the high efficiency of
decarboxylative functionalization of alkenes with
tandem cyclization makes this most appealing to
synthetic scientists.[15] For instance, Mai et al.
2-oxo-2-phenylacetic acid (1a) and 2-(allyloxy)-
benzaldehyde (2a) in DMF was treated with AgNO3/
K2S2O8 at 90 oC under air for 12 h. To our delight, the
reaction gave the desired cyclization product 3a,
albeit with only 10% isolated yield (entry 1, Table 1).
Subsequently, solvent effects were investigated, and
we found that the conversion efficiency strongly
depended on the solvents used. No product was
detected in dichloroethane, H2O, or acetonitrile
(entries 2-4). Nevertheless, the reaction proceeded
described
a
AgNO3/K2S2O8
promoted
decarboxylative tandem alkyl-arylation of N-aryl-
cinnamamides with aliphatic carboxylic acids to
access
a variety of 3,4-disubstituted dihydro-
quinolin-2(1H)-ones in aqueous solution.[15c] Later on, quite smoothly in aqueous solution, providing the
Wang’s group developed a direct decarboxylative
carbonyl-arylation of alkenes with α-oxocarboxylic
acids. The reaction was catalyzed by mild
hypervalent iodine reagents (HIRs) under visible-
light irradiation to give carbonyl-containing
oxindoles.[15e] Mechanistically, these reactions were
initiated by acyl radicals through decarboxylation. We
thus envisioned that, with intramolecular alkenes and
aldehydes as successive acceptors, the radical
propagation might undergo cyclization to afford
dihydroflavonoid skeletons.[16] However, to the best
of our knowledge, apart from the pioneering works
cyclization product in acceptable yields ranging from
27% to 47% (entries 5-8). Further investigations
revealed that acetone/H2O (1:1) is the most
appropriate medium, which afforded 3a in 52% yield
(entry 9). Various oxidants were used to improve the
efficiency, among which oxone, TBHP, and DTBP
gave no conversion at all (entries 10-12). Utilizing
ammonium persulfate as the oxidant slightly
improved the yield of 3a to 57% (entry 13). Other
metal precursors such as silver oxide, silver carbonate,
cuprous chloride, and copper acetate proved to be less
effective (entries 14-17). The cascade reaction
exhibited higher efficiency when the loading of silver
nitrate was increased to 10 mol% and 20 mol%,
giving comparably higher yields of isolated 3a (entry
18-19). Delightfully, when the ratio of 1a/2a was
changed to 2:1, the adduct formed in 73% yield
(entry 20). It was noticed that further increasing the
amount of (NH4)2S2O8 to 4.0 equiv gave only a
moderate yield of 45% (entry 21). Eventually, no
diacylation product was obtained in the control
experiments in the absence of silver catalysts or
oxidants (entries 22-23). These investigations
revealed that the amount of catalyst/oxidant, the ratio
of the substrates, and solvents are essential to the
feasibility and efficiency of the reaction.
mentioned
above,
cascade
oxidative
difunctionalization of alkenes with aldehydic C(sp2)-
H bonds and decarboxylation has never been reported
so far. Inspired by these works and our ongoing
interest in transition-metal catalysis and radical
chemistry,[17] we herein report the first silver nitrate-
catalyzed
cascade
decarboxylative
radical
addition/cyclization of alkenes and α-oxocarboxylic
acids with aldehydes as radical acceptors to furnish
dihydroflavonoid derivatives (Scheme 1).
Table 1. Reaction conditions optimization.[a]
yield
entry
catalyst
oxidant
solvent
(3a,%)[b]
1
2
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
Ag2O
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Oxone
DMF
10
N.R.
N.R.
N.R.
43
dichloroethane
H2O
3
4
CH3CN
5
DMF/H2O (1:1)
THF/H2O (1:1)
dioxane/H2O (1:1)
CH3CN/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
6
31
7
27
8
47
9
52
10
11
12
13
14
N.R.
N.R.
N.R.
57
Scheme 1. Representative studies of aldehydes as carbon
radical donors in difunctionalization of alkenes (a, b),
aldehydes as carbon radical acceptor (c), and this work.
TBHP
DTBP
(NH4)2S2O8
(NH4)2S2O8
To investigate the AgNO3-catalyzed oxidative
tandem diacylation of alkenes, initially, a solution of
48
2
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10.1002/adsc.201601407
Advanced Synthesis & Catalysis
yields. In general, the reactions exhibited remarkable
sensitivity to the electron density of the substituents.
For instance, the para-methoxy substituent, which is
a representative strong electron-donating group, led
to an apparent drop in efficiency with 54% yield of
isolated 3e. Likewise, strong electron-withdrawing
groups such as trifluoromethyl, nitro, and nitrile
substantially impaired the cascade reactions (3i, 3j,
and 3k) resulting in sluggish conversions.
15
16
Ag2CO3
CuCl
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
51
N.R.
N.R.
61
17
Cu(OAc)2
AgNO3
AgNO3
AgNO3
AgNO3
-
18[c]
19[d]
20[e]
21[f]
22
60
(NH4)2S2O8 acetone/H2O (1:1)
73
(NH4)2S2O8
(NH4)2S2O8
-
acetone/H2O (1:1)
acetone/H2O (1:1)
acetone/H2O (1:1)
45
0
23
AgNO3
0
Delightfully,
substrates
with
para-halogen-
substituents were applicable with moderate yields
ranging from 56-74% (3f-3h). In addition, α-
oxocarboxylic acids with meta-Br-, Cl-, and methyl-
substituents (3l-3n) underwent smoothly to afford
comparable or higher yields to para-substituted ones.
When it came to ortho-substitution, substrate 2o gave
the diacylation product 3o in 59% yield, which
indicated that the cascade reaction was relatively
insensitive to steric effect. Moreover, heteroaromatic
thiophene substituted α-oxocarboxylic acid proved a
reliable substrate, furnishing 3p in 51% yield. The
reactions were also extended to other potential acyl
radical precursors, including methyl 2-oxo-2-
phenylacetate and 1,3-dioxoisoindolin-2-yl benzoate,
in which no target adducts were isolated (see SI for
details).
[a] Reaction conditions: 2-oxo-2-phenylacetic acid (1a, 0.30
mmol), 2-(allyloxy)benzaldehyde (2a, 0.60 mmol), catalyst
(5 mol%), oxidant (2.0 equiv.), solvent (2.0 mL) at 90 C
o
for 12 h.
[b] Isolated yields, N.R.=no reaction.
[c] 10% mmol AgNO3.
[d] 20% mmol AgNO3.
[e] 10% mmol AgNO3 was used, 1a : 2a (2:1).
[f] 4.0 equiv oxidant.
Table 2. Substrate scopes of α-oxocarboxylic acids.[a]
Table 3. Reactivity of 2-(allyloxy)benzaldehyde
derivatives. [a]
[a]
Reaction conditions: 2-(allyloxy)benzaldehyde (0.30
mmol), 2-oxo-2-phenylacetic acid (0.60 mmol), AgNO3
(10 mol%), (NH4)2S2O8 (2.0 equiv.), acetone/H2O (1:1, 2.0
mL) at 90 oC for 12 h. Isolated yields.
[a]
Reaction conditions: 2-(allyloxy)benzaldehyde (0.30
mmol), 2-oxo-2-phenylacetic acid (0.60 mmol), AgNO3
(10 mol%), (NH4)2S2O8 (2.0 equiv.), acetone/H2O (1:1, 2.0
mL) at 90 oC for 12 h. Isolated yields.
Encouraged by the optimal results, we further
evaluated the generality and scope of the substrates
for this cascade reaction. Initially, a variety of
substituted α-oxocarboxylic acids, with both electron-
donating and electron-withdrawing substituents on
the phenyl moiety, was investigated. As summarized
in Table 2, it was found that most of the substrates
exhibited good group tolerance, producing the
corresponding adducts in moderate to good yields.
Electron-donating substituents such as methyl-, t-
butyl-, and phenyl performed well in the system
affording the desired products (3b-3d) with 68-75%
Next, the scope of 2-(allyloxy)benzaldehyde
derivatives was investigated. Quite interestingly, the
electron density effect of the substituents in this case
was similar to that of the aforementioned results. As
depicted in Table 3, the substrates containing weak
electron effect groups, whether withdrawing or
donating, reacted quite well with the α-oxocarboxylic
acid derivatives. Even for the dichloro-substituted
substrate,
6,8-dichloro-3-(2-oxo-2-phenylethyl)-
chromen-4-one (3q) was obtained in a moderate yield
3
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10.1002/adsc.201601407
Advanced Synthesis & Catalysis
of 57%. In addition, the efficiency remained
unchanged when different α-oxo-carboxylic acid
derivatives were used, producing the target adducts
(3r-3x) with medium yields. It was observed that 4-
methoxy substitution retarded the reaction with a
comparatively lower yield of 3y. Moreover, nitro
substitution on 2-(allyloxy)-benzaldehyde inhibited
the reaction, just as 2j and 2k did, as shown in Table
2. Considering that the reaction might follow a
radical pathway, we reasoned that the distinct
electron effects shown in Table 2 and Table 3 could
be attributed to two aspects: 1) strong electron-
donating groups reduce the susceptibility of aldehyde
to radical attack; and 2) strong electron-withdrawing
groups, especially the nitro group, are notorious for
inhibiting radical attack. These observations
prompted us to verify the validity of aldehydes as
radical acceptors.
the experimental conditions (Scheme 2, eq. c).
Overall, the control experiments collectively
demonstrated that aldehydes serve as carbon radical
acceptors.
On the basis of the experimental results and some
[12d,15d]
previous reports,
a plausible mechanism is
proposed in Scheme 3. Initially, Ag(I) is oxidized by
the persulfate anion to generate the Ag(II)
intermediate, which then abstracts a single electron
from the carboxylate to produce the carboxyl
radical.[12g] A quick decarboxylation of the carboxyl
radical yields the corresponding acyl radical A.15b,d
The nucleophilic acyl radical then attacks the carbon-
carbon double bond of 2a, leading to the formation of
a radical intermediate B.15b,d Sequential radical attack
on the aldehyde moiety affords an alkoxyl radical C,
followed by a formal 1,2-H shift to deliver the benzyl
radical D.6 Finally, the sulfate radical anion
abstracted a hydrogen from D, giving product 3a.15b
Scheme 2. Control Experiments
Scheme 3. Plausible mechanism
In conclusion, we have developed, for the first time,
a silver nitrate-catalyzed cascade decarboxylation and
oxidative cyclization reaction of α-oxocarboxylic
acids, alkenes, and aldehydes. With ammonium
persulfate as the oxidant, the cascade reactions
afforded dihydroflavonoid derivatives with moderate
to good yields, exhibiting a broad substrate tolerance.
The reaction mechanism was exemplified via a
radical pathway with aldehydes as the carbon radical
acceptors. Various carbonyl-incorporated dihydro-
flavonoid derivatives were accessed by this new
methodology featuring simple operation and mild
conditions.
To gain further insight into the reaction mechanism,
several control experiments were designed and
conducted. Firstly, 2 equiv of TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy), a well-known radical
scavenger, was added to the reaction of 1a with 2a
under the standard conditions, which completely
inhibited the cascade reaction. Notably, only the acyl
radical generated from 2-oxo-2-phenylacetic acid (2a)
was trapped successfully by TEMPO, affording the
corresponding adduct (4) in 39% yields (Scheme 2,
eq. a). The results implied that the reactions might
have occurred through a radical pathway and ruled
out radical initiation from aldehyde. As a comparison,
when 2-(allyloxy)benzonitrile (5) was employed
instead of 2-(allyloxy)benzaldehyde (1a), the cascade
reaction proceeded to give cyclized product 3a in
48% yield (Scheme 2, eq. b). This observation
indicated that the generation of acyl radical from α-
oxo-carboxylic acids occurred first, followed by
hydrolysis to yield ketones,18 since the cyano group
could serve as an ideal carbon radical acceptor.
Finally, benzaldehyde (6) was reacted with 1a, which
did not yield the desired product 3a; this further
revealed that the carbonyl radical could not be
generated from either of the aldehydic groups under
Experimental Section
Typical Procedures for the Silver Nitrate
Catalyzed
Cascade
Decarboxylation
and
Oxidative Cyclization of α-Oxocarboxylic Acid,
Alkene and Aldehyde:
A reaction tube equipped with a magnetic stir bar was
charged with 2-(allyloxy)benzaldehyde (1a, 48.6 mg,
0.30 mmol), 2-oxo-2-phenylacetic acid (2a, 90.0 mg,
0.60 mmol), AgNO3 (5.1 mg, 0.03mmol), (NH4)2S2O8
(136.8 mg, 0.60 mmol), and acetone/H2O (1:1, 2.0
mL), respectively. The mixtures were allowed to
o
react at 90 C for 12 h. When the reaction was
4
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10.1002/adsc.201601407
Advanced Synthesis & Catalysis
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complete, the solution was concentrated under
reduced pressure before further purification by flash
chromatography on silica gel (eluent: petroleum
ether/ethyl acetate = 10:1 to 5:1, v/v) to furnish the
desired product 3a (58.3 mg, 73%).
Acknowledgements
This project is supported by the Foundation Research Project of
Jiangsu Province (The Natural Science Found No. BK20141359),
the Fundamental Research Funds for the Central Universities
(Grant No. KYTZ201604), and “333 High Level Talent Project”
of JiangSu Province.
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Advanced Synthesis & Catalysis
COMMUNICATION
Aldehydes as Carbon Radical Acceptors:
Silver
Nitrate
Catalyzed
Cascade
Decarboxylation and Oxidative Cyclization
toward Dihydroflavonoid Derivatives
Adv. Synth. Catal. 2017, Volume, Page –
Page
Wen-Chao Yang, Peng Dai, Kai Luo, Yi-
Gang Ji and Lei Wu*
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