Iron-catalyzed C[sbnd]C bond activation/C[sbnd]O bond formation: Direct conversion of ketones to esters

Article abstract of DOI:10.1016/j.tetlet.2017.10.068

The iron-catalyzed oxidative activation of the (O)C[sbnd]C bond in ketones has been developed. This method enables direct synthesis of esters by the reaction between ketones and alcohols via conversion of the (O)C[sbnd]C bond to the (O)C[sbnd]O bond. The

Full text of DOI:10.1016/j.tetlet.2017.10.068

Accepted Manuscript
Iron-catalyzed C-C bond activation / C-O bond formation: direct conversion of
ketones to esters
Ashot V. Arzumanyan
PII:
S0040-4039(17)31372-2
https://doi.org/10.1016/j.tetlet.2017.10.068
TETL 49430
DOI:
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
30 August 2017
20 October 2017
26 October 2017
Please cite this article as: Arzumanyan, A.V., Iron-catalyzed C-C bond activation / C-O bond formation: direct
conversion of ketones to esters, Tetrahedron Letters (2017), doi: https://doi.org/10.1016/j.tetlet.2017.10.068
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1
Tetrahedron Letters
journal homepage: www.els evier.com
Iron-catalyzed C-C bond activation / C-O bond formation: direct conversion of
ketones to esters
Ashot V. Arzumanyan*a,b
a A.N Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov str., Moscow 119991, Russian Federation
b Institute of Chemistry, Saint Petersburg State University, Universitetsky Pr. 26, 198504 Stary Petergof, Russian Federation
ARTICLE INFO
ABSTRACT
Article history:
Received
The iron-catalyzed oxidative activation of the (O)C-C bond in ketones has been developed. This
method enables direct synthesis of esters by the reaction between ketones and alcohols via
conversion of the (O)C-C bond to the (O)C-O bond. The reaction runs selectively: the (O)C-
CAlkyl bond is activated, while the (O)C-CAryl bond remains intact (i.e., iron-catalyzed
intermolecular anti-Baeyer-Villiger activation of the (O)C-C bond). The reaction conditions are
carefully optimized and allow the production of esters with yields of up to 95%. The method is
based on the inexpensive and commercially available catalyst (FeCl3), oxidant ((NH4)2S2O8),
and solvent (DCE) without using any ligands or additives.
Received in revised form
Accepted
Available online
Keywords:
Iron-catalyzed
C-C bond activation
C-O bond formation
Oxidation
2009 Elsevier Ltd. All rights reserved.
Ketone
Ester
Alcohol
group (Scheme 1, (2)),[7] and direct functionalization of (O)C-C
The C-C bond is fundamental to organic chemistry. The
processes involving C-C bond breaking are very common: they
take place in hydrocarbon metabolism, the petroleum industry,
etc. One of the current global problems is searching for effective
environmental waste disposal methods. These processes are
directly involved in the modification and refining of petroleum
products and plastics.[1] Furthermore, organic chemists have long
been focusing on synthesizing complex molecules from
hydrocarbons, their relatively simple precursors. Selective
functionalization of molecules with minimal pre-activation can
make synthesis simpler and broaden the possibilities for studying
the applicability of complex molecules in various fields, from the
pharmaceutical industry to materials science. This means that
elaboration of new methods for C-C bond activation is today a
high-demand and hot-button research area.[2]
bond (Scheme 1, (3) path. A).[8]
Reactions of transformation of carbonyl compounds hold a
central place in modern organic chemistry. Ketones are a
fundamental class of organic compounds used in such processes
as the Baeyer–Villiger oxidation (Scheme 1, (3) path. B),[3] the
Schmidt reaction,[4] the Haller–Bauer Reaction,[5] etc. However,
this class of compounds was recently also widely used in metal-
catalyzed C-C bond activation, and in particular in the processes
involving the new concept known as metal-organic cooperative
catalysis (MOCC, Scheme 1, (1)),[6] intramolecular activation of
the (O)C-C bond in ketone, which involve a targeting (chelating)
Scheme 1. Approaches to activation of the (O)C-C bond in
ketones.
The latter approach is preferred, since no chelating groups,
which are often expensive and not commercially available, need
to be introduced to the molecule being activated. Furthermore,
this approach can also be carried out using inexpensive
———
* Corresponding author. Tel.: +7-499-135-9347; fax: +7-499-135-5085; e-mail: aav@ineos.ac.ru

2
Tetrahedron
catalysts.[9] For example, Ning Jiao et al. have conducted
organochlorine solvents, such as chlorobenzene (17%),
tetrachlormethane (28%), and dichloroethane (30%) (Table 1 and
Supporting Information). Subsequent optimization was carried
out in dichloroethane in the presence of iron (III) chloride as a
catalyst.
copper-catalyzed oxidation of acetophenone derivatives to the
corresponding benzoic acid esters.[10]
In this study, the iron-catalyzed activation of the (O)C-CAlkyl
bond in ketones performed with an inexpensive and
commercially available catalyst, oxidant, and solvent without
using any ligands or additives has been described. This approach
allows one to directly cleave the (O)C-CAlkyl bond and form the
(O)C-O bond.
The study involved two stages: the reaction conditions were
thoroughly optimized at the first stage and the applicability of the
reaction to a broad range of substrates was determined at the
second stage.
At the next stage, optimization was carried out based on the
dependence of the yield of product 3b,b on reaction temperature.
The initial reagents were converted into the reaction product 3b,b
0
at a temperature as low as 80 C (5%, S. I.); the best yield (35%)
0
was achieved at 120 C (No. 5, Table 1). Further increase in
temperature did not raise the yield of the product 3b,b due to the
side reactions (Supporting Information).
The yield of 3b,b slightly rose to 41% (No. 6, Table 1) when
the molar ratio of FeCl3/1b was increased from 0.2 to 0.3. The
decrease in the amount of the catalyst (FeCl3) to 0.02 eq. abruptly
reduced the yield of 3b,b to trace amounts due to incomplete
conversion of the initial reagents and the side reactions of
oxidation of 1b (Supporting Information). Further increase in
FeCl3 content to 0.5 and 1 eq. also caused incomplete conversion
of 1b and, as a result, reduced the yield of the reaction product
3b,b. This presumably occurred due to the increased contribution
of the side process, viz., intermolecular dehydration of alcohol 2b
In order to optimize the reaction conditions, an easy-to-
analyse model system based on inexpensive and commercially
available initial reagents was selected, namely, 4-
methylacetophenone 1b and hexanol 2b (Table 1).
At the first optimization stage, we needed to determine the
optimal catalyst/solvent combination ensuring oxidative cleavage
of the (O)C-CAlkyl bond and formation of the (O)C-O bond. Salts
of inexpensive and commercially available metals (Fe, Cu, Ni,
Mn, Co, Cr) as catalysts, solvents of different nature, (NH4)2S2O8
and etc. as oxidant were used (Table 1 and Supporting
Information).
1
to give dihexyl ether that we observed when analyzing the H
NMR spectrum.
Next, the oxidant nature was varied. The main focus was
placed on simple and inexpensive reagents: tert-
Table 1. Examination of reaction conditions.a
butylhydroperoxide,
di-(tert-butyl)peroxide,
phenyl
iodosoacetate, oxygen, ammonium and potassium persulfate,
dichlorodicyanoquinone (DDQ), as well as sodium molybdate
and tungstate (Supporting Information). The best results were
achieved in the presence of DDQ (28%), potassium persulfate
(41%), and ammonium persulfate (55%). The higher solubility of
ammonium persulfate in organic solvents compared to potassium
persulfate was one of the possible reasons why the highest yield
was achieved with ammonium persulfate. Only trace amounts of
the product 3b,b (less than 5%) were observed in the presence of
other oxidants (Table 1 and Supporting Information).
No.
1
x[M]
y[O]
Sol
T (oC)
100
100
100
100
120
120
120
120
Yield) (%)d
0.2FeCl3
0.2FeCl3
0.2FeCl3
0.2СuCl
0.2FeCl3
0.3FeCl3
0.3FeCl3
0.3FeCl3
0.3FeCl3
0.3FeCl3
2K2S2O8
2K2S2O8
2K2S2O8
2K2S2O8
2K2S2O8
2K2S2O8
DCE
Tol
30
10
5
2
3
MeNO2
DCE
DCE
DCE
Further optimization of the reagent ratio has shown the best
1b/2b/FeCl3/(NH4)2S2O8 ratio to be 1/1.5/0.3/2. An abrupt
decrease in the yield of product 3b,b was observed when the
molar ratio of the substrates 1b/2b was changed. This effect was
particularly pronounced when an excess of ketone 1b with
respect to alcohol 2b (2/1) was taken. No product formation was
observed when the reaction was carried out in alcohol
(Supporting Information). Increased and reduced molar ratios of
(NH4)2S2O8/1b differing from (2/1) decreased the yield of
product 3b,b (Supporting Information). In addition, the reaction
under study is highly sensitive to changes in the amount of
solvent: dilution of the reaction mass resulted in a decrease in the
reaction rate, while concentration of the reaction mass caused
occurrence of oxidative side reactions involving 1b (Supporting
Information).
After determining the optimal catalyst, oxidant, temperature,
and the molar ratio between the reaction components, the effect
of acid, base, free radical scavengers and other additives on the
yield of the reaction product 3b,b was studied (Supporting
Information). The presence of both basic and acidic additives in
the reaction mass was found to drastically reduce the yield of the
product 3b,b, in most cases down to 0% (Supporting
Information). Such free radical scavengers as γ-terpinene and
hydroquinone reduced the yield of the product 3b,b to 5 and
10%, respectively (Supporting Information).
4
0
5
35
41
55
64b
71c
34
16
6
7
2(NH4)2S2O8 DCE
2(NH4)2S2O8 DCE
8
9
2(NH4)2S2O8 DCE/CCl4 120
10
11
1(NH4)2S2O8 DCE
0.3Fe(ClO4)3 2(NH4)2S2O8 DCE
120
120
a
Reaction conditions: 1b (0.299 mmol, 1 eq.), 2b (0.598 mmol, 2 eq.),
catalyst (0.0598 – 0.0897 mmol, 0.2 – 0.3 eq.), oxidant (0.299 – 0.598 mmol,
1 – 2 eq.), and solvent (1.00 mL) were stirred in Schott culture tubes (H ×
diam. 160 mm × 16 mm) at 100 – 120 °C for 24 h.
b Reaction conditions: 1b (0.299 mmol, 1 eq.), 2b (0.224 mmol, 0.75 eq.),
FeCl3 (0.0897 mmol, 0.3 eq.), (NH4)2S2O8 (0.598 mmol, 2 eq.), and DCE
(0.75 mL) were stirred in Schott culture tubes (H × diam. 160 mm × 16 mm)
at 120 °C for 3 h. Further, after adding a solution of 2b (0.0748 mmol, 0.25
eq.) and DCE (0.08 mL), the reaction mixture was stirred at 120 °C for 3 h (3
times at 3-hour intervals). Next it was stirred at 120 °C for 12 h.
c Solvent - DCE/CCl4 = 1/1 (1.00 mL).
The yield of 3b,b was determined by 1H NMR (CDCl3) after the
d
filtration of the reaction mass through a thin pad (0.4 – 0.5 cm) of silica gel
(0.015 – 0.040 mm) using 20 mL of DCM.
As mentioned previously, the reaction under study is highly
sensitive to additives of various nature; hence, it was interesting
and important to determine the effect of the atmosphere on the
yield of the reaction product 3b,b. The yield of the product 3b,b
Only certain iron salts, such as FeCl3, FeCl2, Fe(ClO4)3, and
Fe(NO3)3 were found to catalyze oxidative coupling in

3
3d,c: R=iBu
3d,d: R=iAmyl
68
73
for the reaction in the atmosphere of oxygen (37%) and argon
(30%) was found to be lower than that for the reaction carried out
in air (55%) (Supporting Information). Atmospheric oxygen
presumably acts as an oxidant, along with ammonium persulfate.
The use of pure oxygen (~99%) resulted in oxidation of the
product 3b,b under the reaction conditions as pure oxygen is
more reactive compared to atmospheric oxygen.
Studying the effect of the sequence of adding the reagents on
the yield of the reaction product was the next key stage in
optimizing the reaction (Supporting Information). When 4-
methylacetophenone 1b, ammonium persulfate, and the 4-
methylacetophenone (1b)/1.5*hexanol (2b) mixture were added
in four steps, the yield of the product 3b,b decreased to 8, 19, and
5%, respectively. The fact that the yield of the product 3b,b was
increased to 64% by adding hexanol 2b to the solutions in four
steps was an exception (No. 8 ,Table 1, and Supporting
Information). Stepwise addition of alcohol 2b inhibited the
intermolecular dehydration of 2b, which would otherwise give
rise to dihexyl ether under these reaction conditions.
1e
1f
3e,b
76
3f,a: R=Bu
3f,b: R=Hex
92
95
1g
1h
3f,b
64
Since it has been found (Table 1 and Supporting Information)
that the reaction runs best in organochlorine solvents, it was
optimized by combining various chlorine-containing solvents
(Supporting Information). The optimal solvent combination was
DCE/CCl4 (1/1); the yield of the product increased by 7% (up to
71%) (No. 9, Table 1, and Supporting Information).
62 (1/1)
3f,b / 3a,b
a General method of synthesis of 3a-f,a-d: 1a-h (0.0299 – 0.0532 g, 0.299
mmol, 1 eq.), 2a-d (0.0166 – 0.228 g, 0.224 mmol, 0.75 eq.), FeCl3 (0.0145 g,
0.0897 mmol, 0.3 eq.), (NH4)2S2O8 (0.1363 g, 0.598 mmol, 2 eq.), and
DCE/CCl4 (0.75 mL, 1/1) were stirred in Schott culture tubes (H × diam. 160
mm × 16 mm) at 120 °C for 3 h. Further, after adding a solution of 2a-d
(0.055 – 0.0763, 0.0748 mmol, 0.25 eq.) and DCE/CCl4 (0.08 mL, 1 / 1), the
reaction mixture was stirred at 120 °C for 3 h (3 times at 3-hour intervals).
Next it was stirred at 120 °C for 12 h. The yields of 3a-f,a-d were determined
by 1H NMR (CDCl3) after the filtration of the reaction mass through a short
pad (0.4 – 0.5 cm) of silica gel (0.015 – 0.040 mm) using 20 mL of DCM and
evaporation of solvent in vacuum.
Complete conversion of the initial reagents under the
optimized conditions takes 24 h (Supporting Information).
Hence, although the reaction conditions were very simple, the
reagents were inexpensive and commercially available, the
system under study turned out to be very sensitive to various
additives, to changes in concentrations of the initial components,
and to other parameters.
At the second stage, we tested the applicability of the
synthetic method of iron-catalyzed C-C bond activation to
reactions of acetophenone derivatives having various substituent
moieties on the aromatic ring, 1,3-diketons, and other carbonyl
compounds with various alcohols (Table 2).
It turned out that the proposed synthesis approach can be used
for a broad range of acetophenones1а-е (Table 2), mostly those
having electron-donating substituents in the aromatic ring, as
well as for dicarbonyl compounds 1f-h (Table 2). Acetophenone
derivatives 1а-е (Ar-C(O)-Ме) were selectively oxidized to the
corresponding benzoic acid esters 3а-е (Ar-C(O)-OR), while the
Ar-C(O) bond remained intact.
Table 2. Substrate scope of esters.a
Benzophenone, dibenzylidenacetone, benzaldehyde, and
derivatives of methyl vinyl ketone were found to be inert in this
reaction. Furthermore, secondary, tertiary and aromatic alcohols,
amines, thiols, selenols and diorganophosphine oxides did not
react with 4-methylacetophenone, which attests to the high
selectivity of the reaction under study.
Yeild of
3a-f,a-d
(%)
Structure of 1a-h
Structure of 3a-f,a-d
Scheme 2 shows the reaction mechanism in general simplified
form. At the first stage of the catalytic cycle, FeCl3 acts as a
Lewis acid and binds to the oxygen in carbonyl group of ketone
1, giving rise to complex A. This process increases
electrophilicity of the carbon atom of carbonyl group, thus
activating ketone 1 for the nucleophilic attack of alcohol 2.
Further interaction between the alcohol and complex A gives rise
to complex B; further oxidation of the complex B results in
cleavage of the C-R2 bond and formation of the reaction product
3 and FeCl2, which subsequently interacts with the oxidant to be
converted back to FeCl3, thus closing the catalytic cycle.
1a
3a,b
53
1b
1c
1d
3b,a: R=Bu
3b,b: R=Hex
70
71
3c,a: R=Bu
3c,b: R=Hex
75
78
The catalyst (FeCl3) acts in this reaction as a Lewis acid and
an oxidant in combination with (NH4)2S2O8 and O2. Probably, the
[Fe] oxidation state under the oxidizing reaction conditions can
vary not only in the range of +2 and +3, but also +4, +5, +6 and
+7, which, presumably, explains the higher catalytic activity of
[Fe] under these reaction conditions in comparison with other
catalysts (Supporting Information).
3d,a: R=Bu
3d,b: R=Hex
84
88

4
Tetrahedron
the Centre for Magnetic Resonance of Saint Petersburg State
University for recording of 1H and 13C NMR spectra. High-
resolution mass spectra were recorded in the Department of
Structural Studies of Zelinsky Institute of Organic Chemistry,
Moscow. This work is supported by a grant of the Russian
Science Foundation (RSF grant 14-50-00126).
O
FeCl3
R1
R2
[O]
1
FeCl3
A
[O] = (NH4)2S2O8, O2
O
FeCl2
References and notes
R1
R2
O
1.
2.
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[O]
R
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Scheme 2. The proposed mechanism.
The proposed method has reversed selectivity in C-C bond
oxidation, as opposed to the known Baeyer-Villiger reaction,[3]
where intramolecular oxidation of the (O)C-CAr bond to the
(O)C-OCAr bond occurs in the first place. The substituent
moieties can be arranged in order of decreasing ease of oxidation
of the C-C(O) bond in the Baeyer-Villiger reaction[3] as follows:
4-Me-C6H4-C(O) >C6H5-C(O) > 4-Cl-C6H4-C(O) >Me-C(O) in
CAr-C(O)-CAlkyl. Furthermore, oxidation of the C-C(O) bond in
this reaction is usually non-selective and gives rise to a mixture
of products of oxidation of the (O)C-CAlkyl and (O)C-CAr bonds.
In addition, this reaction is intramolecular. The approach
proposed by us involves selective oxidation of (O)C-Me to (O)C-
OR with the (O)C-CAr bond remaining intact (Table 2) (i.e., iron-
catalyzed intermolecular anti-Baeyer-Villiger activation of the
(O)C-C bond), which allows one to selectively synthesize
derivatives of esters of benzoic and other carboxylic acids.
Thus, we proposed iron-catalyzed activation of the (O)C-C
bond in ketones. This method enables a direct synthesis of esters
by reactions between ketones and alcohols by converting the
(O)C-C bond to the (O)C-O bond. Thorough optimization of the
reaction conditions has been carried out. The method is based on
the inexpensive and commercially available catalyst (FeCl3),
oxidant ((NH4)2S2O8), and solvents (DCE, CCl4) without using
any ligands or additives. The reaction runs selectively and
involves activation of the (O)C-CAlkyl bond only, while the (O)C-
CAr bond remains intact.
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Experimental Section
General method of synthesis of 3a-f,a-d: 1a-h (0.0299 – 0.0532 g, 0.299
mmol, 1 eq.), 2a-d (0.0166 – 0.228 g, 0.224 mmol, 0.75 eq.), FeCl3 (0.0145 g,
0.0897 mmol, 0.3 eq.), (NH4)2S2O8 (0.1363 g, 0.598 mmol, 2 eq.), and
DCE/CCl4 (0.75 mL, 1/1) were stirred in Schottculture tubes (H × diam.
160 mm × 16 mm) at 120 °C for 3 h. Further, after adding a solution of 2a-d
(0.055 – 0.0763, 0.0748 mmol, 0.25 eq.) and DCE/CCl4 (0.08 mL, 1 / 1), the
reaction mixture was stirred at 120 °C for 3 h (3 times at 3-hour intervals).
Next it was stirred at 120 °C for 12 h. The yields of 3a-f,a-d weredetermined
by 1H NMR (CDCl3) after the filtration of the reaction mass through a short
pad (0.4 – 0.5 cm) of silica gel (0.015 – 0.040 mm) using 20 mL of DCM and
evaporation of solvent in vacuum. Products 3c,b and 3d,a-d were isolated by
gradient flash chromatography (eluent: petroleum ether / ethyl acetate).
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Acknowledgments.
The author is very grateful to Prof. Valentine P. Ananikov for
helpful discussions. The author also expresses their gratitude to

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6
Tetrahedron
- The Fe-catalyzed activation of the (O)C-
C bond in ketones has been developed.
- This method enables direct synthesis of
esters.
- The reaction runs selectively: (O)C-CAlkyl
is activated / (O)C-CAryl remains intact.
- The method is based on the inexpensive and
commercially available oxidizing system.