Iron-Catalyzed Cleavage Reaction of Keto Acids with Aliphatic Aldehydes for the Synthesis of Ketones and Ketone Esters

Article abstract of DOI:10.1002/chem.202000114

The radical–radical coupling reaction is an important synthetic strategy. In this study, the iron-catalyzed radical–radical cross-coupling reaction based on the decarboxylation of keto acids and decarbonylation of aliphatic aldehydes to obtain valuable aryl ketones is reported for the first time. Remarkably, when tertiary aldehydes were used as carbonyl sources, ketone esters were selectively obtained instead of ketones. The gram-scale preparation of aryl ketone through this strategy was easily achieved by using only 3 mol % of the iron catalyst. As a proof-of-concept, the bioactive molecule flurprimidol was synthesized in two steps by using this strategy.

Full text of DOI:10.1002/chem.202000114

A Journal of
Accepted Article
Title: Iron-catalyzed Cleavage Reaction of Keto Acids and Aliphatic
Aldehydes for the Synthesis of Ketone/Ketone Esters
Authors: Fangyuan Zhou, Lesong Li, Kao Lin, Feng Zhang, Guo-Jun
Deng, and Hang Gong
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To be cited as: Chem. Eur. J. 10.1002/chem.202000114
Link to VoR: http://dx.doi.org/10.1002/chem.202000114
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Iron-catalyzed Cleavage Reaction of Keto Acids and Aliphatic
Aldehydes for the Synthesis of Ketone/Ketone Esters
Fangyuan Zhou, Lesong Li, Kao Lin,Feng Zhang, Guo-Jun Deng, and Hang Gong*
Abstract: The radical-radical coupling reaction is an important
synthesis strategy. In this study, the iron-catalyzed radical-radical
cross-coupling reaction based on the decarboxylation of keto acids
and decarbonylation of aliphatic aldehydes to obtain valuable aryl
ketones is reported for the first time. Remarkably, when tertiary
aldehydes were used as carbonyl sources, ketone esters were
selectively obtained instead of ketones. The gram-scale preparation
of aryl ketone via this strategy was easily achieved using only 3 mol%
of the iron catalyst. As a proof-of-concept, the bioactive molecule
flurprimidol was synthesized in two steps by this strategy.
Radical reaction is one of the most basic and important synthetic
tools available in organic chemistry. It has been widely used as a
powerful strategy for the synthesis of pharmaceutical molecules,
natural products and functional materials[1]. As an important
component of radical reactions, the formation of a C-C bond
through a radical process has received increasing attention over
the past decade. The key step of these reactions is the formation
of the carbon radical. Aldehyde is a kind of safe, inexpensive and
Scheme 1. The strategies of decarboxylation vs. decarbonylation and our
readily available reagent, and could be served as a carbon radical
source via a decarbonylation reaction (Scheme 1, A)[2]. Various
C-C bonds, such as C(SP2)-C(SP2) [3], C(SP2)-C(SP3)[4],C(SP)-
C(SP3)[5] and C(SP3)-C(SP3)[6] etc. have been constructed based
on the decarbonylation reaction. In addition, decarboxylation of
carboxylic acids to obtain the carbon radical and then achieve the
chemo- and regioselective conversion has also been widely
investigated[7]. Among them, the decarboxylation of α-keto acid in
the presence of a radical initiator has been attractive for
preservation of the carbonyl skeleton (Scheme 1, B)[8].
There are only two reports about the construction of C–C via
tandem radical decarboxylation and decarbonylation (Scheme 1,
C & D)[9]. Despite the vigorous development of research on
decarboxylation and decarbonylation, the radical-radical coupling
reaction via decarboxylation and decarbonylation remains
challenging. In particular, the simultaneous cleavage of carboxylic
acid and aldehyde, and the competition coupling reactions of
multiple radicals (Scheme 1, E).
designed reaction.
reaction. However, besides having limited tolerance of functional
groups, such reaction consumes large amounts of Lewis acid, and
produces a large amount of by-products[10]
Aromatic ketones can also be prepared by the reaction of acid
chlorides or esters with organometallic species, such as
organolithium[11], Grignard[12], organoaluminum[13], organotin[14]
.
,
organozinc[15], organoindium[16] reagents, etc[17]. However, the
stoichiometric consumption of metal reagents and the ready
generation of tertiary alcohols as side-products are unfavorable to
the organic synthesis [18]. Compared with these conventional
methods, our strategy does not consume large amounts of Lewis
acid, does not require pre-preparation of stoichiometric metal
reagents, is compatible with substrates containing active
hydrogen and strong electron-withdrawing group, and its
influence on the environment is greatly reduced. More importantly,
when tertiary aldehydes were used as carbonyl sources, ketone
esters were selectively obtained instead of ketones (Scheme 1, F,
b).
Our study began with the reaction of phenylglyoxylic acid with
isobutyraldehyde using Fe(OTf)3 (15 mol%) as the catalyst, di-
tert-butyl peroxide (DTBP) as the oxidant, and toluene as the
solvent, at 120 oC under an atmosphere of air (Table 1).
Gratifyingly, the yield of the desired radical coupling product
isobutyrophenone was 22% (entry 1). Motivated by this result,
various reaction conditions, including catalysts (entries 1–7, 22),
the ratio of reactants (entries 8–9), oxidants (entries 10-11, 23–
26), reaction temperature (entries 12–13), reaction time (entries
14–15) and solvents (entries 16–21) were optimized (More details
of the optimization reactions are given as supporting information).
The optimized reaction conditions were as follows: 30 mol%
Fe(OTf)3 as catalyst, 2 equiv DTBP and 1.5 equiv K2S2O8 as
In this paper, the iron-catalyzed decarboxylative and
decarbonylative radical cross-coupling reactions between keto
acids and aliphatic aldehydes are reported for the first time
(Scheme 1, F). This strategy is significant since the corresponding
aromatic ketones would be achieved when secondary aldehydes
are used as alkyl source (Scheme 1, F, a). Traditionally, aromatic
ketones are synthesized through the Friedel-Crafts acylation
[*] Mr. F. Zhou, Mr. L. Li, Mr. K. Lin, Prof. Dr. G.-J. Deng, Dr. H. Gong,
The Key Laboratory of Environmentally Friendly Chemistry and
Application of the Ministry of Education; The Key Laboratory for Green
Organic Synthesis and Application of Hunan Province; College of
Chemistry, Xiangtan University, Xiangtan 411105 (China)
E-mail: hgong@xtu.edu.cn
Ms. F. Zhang
College of Science, Hunan Agricultural University, Changsha 410128,
China
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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Table 1. Selected optimization results.a
Table 2. Scope of decarboxylation and decarbonylation radical coupling
reaction.a
Cat.
(mol%)
T
Time
(h)
3a
(%)
Entry
Oxidant
Solvent
(oC)
1
2
3
4
5
Fe(OTf)3
Fe(OTs)3
FeBr3
Fe(acac)2
Fe(acac)3
DPPF
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
DMSO
DMF
120
120
120
120
120
120
120
120
120
120
120
100
150
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
12
12
12
12
12
12
12
12
12
12
12
12
12
10
18
12
12
12
12
12
12
12
12
12
12
12
12
12
22
18
21
45
53
24
36
44
47
47
53
29
47
50
51
23
25
12
12
52
41
67
17
68
76
77
15
trace
6
7
DTBPF
8b
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
——
9c
10d
11e
12
13
14
15
16
17
18
19
20
21
22f
23f
24f,g
25f,h
26f,i
27i
28f
DTBP
DTBP
DTBP
DTBP
CHCl3
EtOAc
CH3CN
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
DTBP
K2S2O8
DTBP+K2S2O8
DTBP+K2S2O8
DTBP+K2S2O8
DTBP+K2S2O8
——
Fe(acac)3
[a] Unless otherwise noted, all reactions were conducted on a 0.2 mmol scale in
a sealed tube under the atmosphere of air; NMR yields were given using
CH3NO2 as internal standard; DPPF = 1,1'-Bis(diphenylphosphino)ferrocene;
DTBPF = 1,1'-Bis(di-tert-butylphosphino)ferrocene. For other results from the
optimization of the reaction conditions please see the Supporting Information;
[b] 2 equiv aldehyde was used; [c] 3 equiv aldehyde was used; [d] 1.5 equiv DTBP
was used; [e] 2.5 equiv DTBP was used; [f] Fe(acac)3 30 mol%; [g] 2 equiv DTBP
and 1 equiv K2S2O8 were used; [h] 2 equiv DTBP and 1.5 equiv K2S2O8 were
used; [i] 2 equiv DTBP and 2 equiv K2S2O8 were used.
[a] Unless otherwise noted, all reactions were carried out with phenylglyoxylic
acid (0.2 mmol), isobutyraldehyde (1.0 mmol), Fe(acac)3 (30 mol%), DTBP (2
equiv) and K2S2O8 (1.5 equiv) in a sealed tube in 1 mL EtOAc at 120 oC for 12
hrs under the atmosphere of air; Isolated yields were given; [b] Without K2S2O8.
oxidants, 1 mL ethyl acetate (EtOAc) as solvent, reaction
temperature of 120 oC and reaction time of 12 h (entry 24). Control
experiments were also conducted and the results revealed that
both the iron catalyst and oxidant play an important role for this
reaction (entries 27–28).
yields (3x, y). In particular, when tertiary aldehydes were used as
carbonyl sources in the reaction with phenylglyoxylic acid, ketone
esters, instead of ketones, were selectively obtained in good
yields (3z–zb). This is probably caused by the rapid generation of
tertiary carbon radicals, which were then coupled with carboxyl
radicals rather than carbonyl radicals after decarboxylation. When
1-adamantane carboxaldehyde (1h) was used, a bridgehead
carbon radical with less stability would be formed, thus ketone 3h
is achieved instead of ketone ester. In addition, in the case of
using 1z-1zb as reactants, K2S2O8 were not benifite for this
reaction. However, this reaction is sensitive to steric hindrance
and when the substituent is present in the ortho-position almost
no desired products could be found (3zc, zd). Primary aldehydes
and alkenyl substituted aldehydes were also incompatible with
this reaction due to the poor stability of the radical after
decarbonylation (3ze, zf).
With the optimized reaction conditions in hand, the generality of
this strategy was investigated (Table 2). First, various secondary
aldehydes for use as alkyl source were scanned. When aliphatic
aldehydes with short or long chains were used, moderate to good
yields were obtained (3a–d). Gratifyingly, this reaction is also
compatible with cyclo-aliphatic aldehydes, and the corresponding
aromatic ketones were obtained in good yields (3e–g).
Additionally, the sterically hindered adamantane moiety, which is
frequently found in pharmaceutical, nanomaterials and
supramolecular assemblies[19], was successfully introduced into
the aryl ketone (3h). The vinyl bond in the aliphatic ring could be
preserved after this reaction, and a good yield could also be
obtained (3i). Subsequently, the substitutes on the aromatic ring
of the keto acid were explored. It was determined that some of the
substituents were electron-donating groups (3j–p) while others
were electron-withdrawing groups (3q–w), and good yields were
obtained. Importantly, this reaction is compatible with substrates
containing active hydrogen such as hydroxyl group (3p). α-
Naphthyl and α-heteroaryl keto acids also followed this rule well,
and the corresponding ketones were obtained with acceptable
To investigate the reaction mechanism, a radical inhibition
experiment was conducted using 2 equiv of TEMPO as inhibitor,
and almost no desired aromatic ketone 3a was detected (Scheme
2, A). Remarkably, when the reaction was performed in the
presence of 1,1-diphenylethylene, besides the desired ketone 3a,
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Scheme 4. Gram-scale reaction and the synthesis of flurprimidol.
To prove the feasibility of this method, a gram-scale synthesis
of aryl ketone 3a was conducted and a good yield also obtained
using only 3 mol% of the iron catalyst with a longer reaction time
(72 h) (Scheme 4, A). The convenient decarboxylation and
decarbonylation radical coupling reaction can be readily applied
in the synthesis of bioactive molecules. As a proof-of-concept, the
plant-growth regulator flurprimidol was effectively synthesized
from 1o and isobutyraldehyde in two steps (Scheme 4, B). The
key intermediate 3o could be obtained by a one-step reaction with
a good yield of 64%.
In conclusion, in this study for the first time the iron-catalyzed
decarboxylative and decarbonylative radical-radical cross-
coupling reactions between keto acids and aliphatic aldehydes
were shown to produce valuable aryl ketones. Compared with
traditional methods of preparing aryl ketones, our strategy does
not consume large amounts of Lewis acid, does not require pre-
preparation of stoichiometric metal reagents, is compatible with
substrates containing active hydrogen and strong electron-
withdrawing group, and its influence on the environment is greatly
reduced. Additionally, the gram-scale reaction was easily
achieved with a much lower iron catalyst loading (3 mol%). As an
example, the rapid synthesis of bioactive molecules, such as
flurprimidol was successfully performed by this strategy.
Scheme 2. Radical inhibition/capture experiments.
the radical-capturing product 4a was obtained with a yield of 67%
(Scheme 2, B). In addition, when phenylglyoxylic acid (1a) was
treated with 1,1-diphenylethylene under the standard condition,
the radical-capturing product 5a was detected (Scheme 2, C).
These results revealed that this reaction is a radical process, and
the alkyl radical and acyl radical were formed through
decarbonylation and decarboxylation reactions, respectively.
Based on these control experiments, a plausible mechanism is
depicted in Scheme 3. For payway A, the aryl ketonic acid 1a
loses a hydrogen atom, which in the presence of the composite
initiator and is transferred to carboxyl radical (I). Subsequently,
the decarboxylation process occurs and the corresponding
carbonyl radical (II) is generated. In addition, aldehyde is
converted to the carbonyl radical, which is followed by
decarbonylation to form the alkyl radical. When the secondary
aldehyde is used, the derived alkyl radical couples with the
carbonyl radical (II) to generate the desired product aryl ketone
3a. Instead, when tertiary aldehydes are used, the derived alkyl
radical couples with carboxyl radical (I) and the ketone esters 3z
are obtained. This is probably caused by the rapid generation of
tertiary carbon radicals, which leads to the occurrence of the
radical-radical coupling reaction before the decarboxylation of the
carboxyl radical (I). In pathway B, Fe(III) species interact with the Acknowledgements
α-keto acid, and followed by decarboxylation (for tertiary
aldehydes, this process was skipped) and radical addition and
reduction elimination to give the corresponding products.
We are grateful to the Key Project Program of the Educational
Department of Hunan Province (No. 18A069), Scientific Research
Foundation of Hunan Provincial Education Department (No.
15B232), National Natural Science Foundation of China (No.
21402168) and Hunan 2011 Collaborative Innovation Center of
Chemical Engineering
&
Technology with Environmental
Benignity and Effective Resource Utilization for their support of
our research.
Experimental Section
A typical experimental procedure: A solution of aryl keto acid (0.2
mmol), Fe(acac)3 (21.2 mg, 0.06 mmol), aldehyde (1.0 mmol),
DTBP (79.5 μL, 0.4 mmol) and K2S2O8 (81.1 mg, 0.3 mmol) in
EtOAc (1 mL) is prepared by stirring in a sealed tube under an
atmosphere of air at 120 oC for 12 h. Subsequently, the reaction
mixture is cooled to room temperature, and the solid residue is
filtered through a short silica gel column, and washed with 10 mL
EtOAc. Afterwards, the solvent is evaporated in vacuo. The
residue is then purified by preparative thin-layer chromatography
Scheme 3. The proposed mechanism.
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10.1002/chem.202000114
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(TLC) on silica gel with petroleum ether/EtOAc to obtain the pure
product.
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Keywords: Radical-radical cross-coupling • decarboxylation •
decarbonylation • iron-catalyzed reaction • synthetic methods
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10.1002/chem.202000114
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Fangyuan Zhou, Lesong Li, Kao
Lin,Feng Zhang, Guo-Jun Deng, and
Hang Gong*
Page No. – Page No.
Iron-catalyzed Cleavage Reaction of
Keto Acids and Aliphatic Aldehydes
for the Synthesis of Ketone/Ketone
Esters
The iron-catalyzed radical-radical cross-coupling reaction based on the
decarboxylation/dehydrogenation of keto acids and decarbonylation of aliphatic
aldehydes to obtain the valuable aryl ketones/ketone esters were reported.
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