Copper-catalyzed demethylative esterification of arylmethylketones: a new route for the synthesis of benzocaine

Article abstract of DOI:10.1007/s13738-019-01698-z

An efficient method for oxidative cleavage of C(CO)–C bonds of ketones has been developed. This procedure enables activated methyl ketones to react with tetraethyl orthosilicate for generation of the corresponding methyl esters using copper as the catalyst. Primary mechanistic studies revealed that in contrary to other related studies, the involvement of the aldehyde as the intermediate of this reaction is highly probable. Interestingly, this study provides an unprecedented positive effect of the elemental sulfur upon this oxidative esterification reaction. As an application of our method, we have reported a two-step procedure for the synthesis of benzocaine. The second step of this procedure involves a highly selective reduction in nitro group in the presence of ester functionality.

Full text of DOI:10.1007/s13738-019-01698-z

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01698-z
ORIGINAL PAPER
Copper‑catalyzed demethylative esterifcation of arylmethylketones:
a new route for the synthesis of benzocaine
Sajedeh Maddah Roodan1 · Arash Ghaderi1
Received: 21 January 2019 / Accepted: 20 May 2019
© Iranian Chemical Society 2019
Abstract
An efcient method for oxidative cleavage of C(CO)–C bonds of ketones has been developed. This procedure enables acti-
vated methyl ketones to react with tetraethyl orthosilicate for generation of the corresponding methyl esters using copper as
the catalyst. Primary mechanistic studies revealed that in contrary to other related studies, the involvement of the aldehyde
as the intermediate of this reaction is highly probable. Interestingly, this study provides an unprecedented positive efect of
the elemental sulfur upon this oxidative esterifcation reaction. As an application of our method, we have reported a two-step
procedure for the synthesis of benzocaine. The second step of this procedure involves a highly selective reduction in nitro
group in the presence of ester functionality.
Keywords Esterifcation · Copper · Ketone · Oxidation · Catalyst
Introduction
acids [9, 10], esters [11], amides [12–14], imides [15],
nitriles [16] and even aldehydes [17]. Herein, we report our
recent fndings for direct conversion of ketones to esters.
Ketones are usually known as relatively stable compounds
toward oxidation processes owing to the inertness of their
C–C bonds. These compounds could be oxidized only in
the presence of very strong oxidants under harsh reaction
conditions. Baeyer–Viliger oxidation is an example for doing
this function [1, 2]. One of the most susceptible ketones
for oxidation reactions involving C(C=O)–C bond cleavage
is methyl ketones. A classical oxidative test for identifca-
tion of methyl ketones is the iodoform test which converts
these compounds to carboxylates [3]. Some modifcations
upon this test have also been reported for the conversion of
ketones to carboxylates or amides [4–7]. Studies have shown
that pre-functionalization in α-position of methyl ketones
with electronegative groups facilitates the oxidation process
[8]. In recent years, a few mild and efcient approaches have
been reported for the conversion of ketones to carboxylic
Experimental section
General
All chemicals were purchased from Merck and Aldrich
chemical companies and used as received without further
purifcation. All experiments were run under air atmosphere
in an open fask unless stated otherwise. NMR data were
determined in Bruker instrument (300 MHz) using CDCl3
or DMSO-d6 as the solvent.
General procedure for the esterifcation of ketones
A mixture of ketone (0.5 mmol), tetraethyl orthosilicate
(2 mmol, 0.41 g) and KF (2 mmol, 0.116 g) was mixed in
the presence of CuI (0.4 mmol, 0.019 g) and 1,10-phenan-
throline (0.8 mmol, 0.036 g) in DMF (1.5 mL) under ambi-
ent atmosphere at 100 °C. The progress of the reaction was
monitored by TLC. After completion of the reaction, the
product was extracted with ethylacetate/water. The solvent
was evaporated, and the product was purifed by column
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-019-01698-z) contains
supplementary material, which is available to authorized users.
* Arash Ghaderi
aghaderi@hormozgan.ac.ir
1
Department of Chemistry, College of Sciences, University
of Hormozgan, Bandar ‘Abbas 71961, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Table 1 Optimization of the reaction conditions
O
O
O
conditions
Si(OEt)4
NO2
NO2
Entry
Copper catalyst
Ligand
Solvent (mL)
Time (h)
Yield (%)
1a,b
2a,b
3c
Cu(OAc)2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
–
CuI
CuI
CuI
CuI
CuI
CuI
Py
Py
Py
Py
Phen
–
Phen
Phen
Phen
Phen
Phen
Phen
Phen
Phen
bipy
Phen
DMF
DMF
DMF
PhCl
DMF
DMF
DMF
DMF
18
24
4
4
4
4
4
4
4
4
4
4
4
4
4
4
Trace
33
52
–
80
Trace
–
–
20
10
80
–
–
Trace
Trace
–
4c
5
6
7d
8e
9
–
10
11
12
13
14
15
16f
DMF
DMSO
Toluene
1,4-Dioxane
PEG400
DMF
DMF
Reaction conditions 3-Nitroacetophenone (0.5 mmol), tetraethyl orthosilicate (2 mmol), catalyst (20 mol%), phenanthroline (0.2 mmol), KF
(2 mmol), solvent (1.5 mL), at 100 °C under air
aPyridine (1.5 mmol) was used as the ligand
bThe reaction was conducted at 60 °C
cPyridine (0.2 mmol)
dThe reaction was conducted in the absence of KF
eThe reaction was run under N2 atmosphere
fNaOEt (2 mmol) was used instead of tetraethyl orthosilicate
chromatography over silica gel using n-hexane/ethylacetate
H), 8.28–8.36 (m, 2 H), 8.78 (s, 1 H); 13C NMR (75 MHz,
CDCl3) δ; 22.51, 25.20, 37.31, 64.65, 124.55, 127.31,
129.61, 132.28, 135.27, 148.32, 164.56.
(8:2) as the eluent.
Ethyl 3‑nitrobenzoate 1H NMR (500 MHz, CDCl3) δ;
1.45 (t, J=7.1 Hz, 3 H), 4.46 (q, J=7.1 Hz, 2 H), 7.67 (t,
Typical procedure for the synthesis of benzocaine
J=7.9 Hz, 1 H), 8.41 (m, 2 H), 8.88 (t, J=1.9 Hz, 1H), 13
C
NMR (125 MHz, CDCl3) δ; 14.27, 61.95, 124.55, 127.30,
In a reaction vessel, CaH2 (0.2 mmol, 0.008 g) was added
to a solution of Pd(OAc)2 (0.025 mmol, 0.005 g) in DMSO
(3 mL) and stirred at 100 °C. In 10 min, the brown color
of the mixture turned to black. Then, ethyl 4-nitrobenzo-
ate (1 mmol, 0.195 g), prepared using the above procedure,
was added to this mixture followed by Cu(OAc)2 (0.2 mmol,
0.036 g), KF (2.4 mmol, 0.139 g) and HCO2Na (2 mmol,
0.136 g). After completion of the reaction, the product was
extracted with ethylacetate/water. The solvent was evapo-
rated, and the product was purifed by column chromatogra-
phy over silica gel using n-hexane/ethylacetate as the eluent.
129.58, 132.21, 135.28, 148.25, 164.47.
Ethyl 4‑nitrobenzoate 1H NMR (300 MHz, DMSO-d6) δ;
1.29 (t, J=7.1 Hz, 3 H), 4.30 (q, J=7.1 Hz, 2 H), 8.05–8.10
(m, 2 H), 8.21–8.26 (m, 2 H), 13C NMR (75 MHz, DMSO-
d6) δ; 14.33, 62.09, 124.12, 130.91, 135.58, 150.44, 164.63.
Isoamyl 3‑nitrobenzoate 1H NMR (300 MHz, CDCl3) δ;
0.91 (d, J=6.4 Hz, 6 H), 1.63 (q, J=6.7 Hz, 2 H), 1.69–1.82
(m, 1 H), 4.35 (t, J = 6.8 Hz, 2 H), 7.58 (t, J = 7.9 Hz, 1
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Journal of the Iranian Chemical Society
1
Ethyl 4‑aminobenzoate H NMR (300 MHz, CDCl3) δ; 1.26
showing the important role of molecular oxygen in this reac-
tion (Table 1, entry 8). The product was obtained in low
yield when the reaction was run in the absence of either
DMF or CuI (Table 1, entries 9, 10). Changing the solvent
to DMSO gave the same result as DMF, forming the desired
product in 80% yield (Table 1, entry 11). Other solvents
such as toluene, 1,4-dioxane and PEG400 were inefcient
for this reaction (Table 1, entries 12–14). The reaction was
failed using bipyridine as the ligand (Table 1, entry 15).
No desired product was obtained using NaOEt instead of
Si(OEt)4 (Table 1, entry 16). Thus, the conditions mentioned
in entry 5 were chosen as the optimum conditions.
(t, J=7.1 Hz, 3 H), 4.05 (s, 2 H), 4.22 (q, J=7.1 Hz, 2 H),
6.53 (d, J=8.6 Hz, 2 H), 7.76 (d, J=8.6 Hz, 2 H), 13C NMR
(75 MHz, CDCl3) δ; 14.42, 60.34, 113.77, 119.88, 131.56,
150.97, 166.82.
Results and discussion
In order to check the efect of diferent parameters upon the
reaction, we chose the reaction of 3-nitroacetophenone with
tetraethyl orthosilicate as the model reaction in the presence
of copper salts as the catalysts (Table 1). When the reaction
was conducted in the presence of Cu(OAc)2 as the catalyst
and pyridine as the ligand in DMF, only a trace amount of
the desired product was detected after 18 h reaction time
at 60 °C (Table 1, entry 1). Changing the catalyst to CuI
under the same conditions gave the product in 33% yield
after 24 h (Table 1, entry 2). Increasing the temperature to
100 °C resulted in the formation of the corresponding ester
in 52% yield (Table 1, entry 3). The reaction failed employ-
ing the conditions similar to the available procedure [11],
using CuI/Py in PhCl as the solvent (Table 1, entry 4). By
switching the ligand to 1,10-phenanthroline, the yield of the
desired product was increased drastically up to 80% within
4 h reaction time in DMF (Table 1, entry 5). The reaction
in the absence of this ligand was totally inefcient (Table 1,
entry 6). The reaction also failed in the absence of KF
(Table 1, entry 7). We have also run the reaction under N2
atmosphere. No product was obtained under these conditions
Under the optimized conditions, some other methyl
ketones were subjected to the esterifcation reactions using
tetraethyl orthosilicate (Scheme 1). m- and p-nitroacetophe-
none gave the desired ethyl esters in 80 and 83% isolated
yields, respectively. Unfortunately, our eforts failed for the
synthesis of ethyl p-aminoacetobenzoate (3c). To our sur-
prise, acetophenone was unreactive under the optimum con-
ditions and only a trace amount of ethyl benzoate (3d) was
detected by GC–MS. In light of these results, we interested in
examining the competitive reaction between 3-nitroacetophe-
none and acetophenone. For this aim, we mixed 3-nitroace-
tophenone (1 eq.), acetophenone (1 eq.) and tetraethyl ortho-
silicate (2 eq.) and performed the reaction under the standard
conditions. Analysis of the reaction mixture after 4 h revealed
that 3-nitroacetophenone selectively converted to the desired
product leaving acetophenone intact.
To shed light on the mechanism of the reaction, a
series of experiments were conducted. We assumed that
Scheme 1 Esterifcation
reactions under the optimized
conditions
O
O
CuI (20 mol%), 1,10-Phen (40 mol%)
OEt
Si(OEt)4
R
R
KF, DMF, 100 oC
Under air
1
2
3
O
O
O
O
OEt
OEt
OEt
OEt
O2N
H2N
3c (24 h, trace)
NO2
3a (4 h, 80%)
3b (4 h, 83%)
3d (24 h, trace)
Scheme 2 Reaction of possible
intermediates with tetraethyl
orthosilicate under the opti-
mized conditions
O
R
O
CuI (20 mol%), 1,10-Phen (40 mol%)
Si(OEt)4
+
KF, DMF, 100 oC, 4 h
Under air
NO2
NO2
Isolated Yield (R=CHO): 34%
(R=COOH):
-
(R=CH2OH): trace
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Journal of the Iranian Chemical Society
O
O
In order to check if the reaction involves radical
intermediates, we performed the TEMPO test upon the
reaction. Interestingly, the reaction was not affected
by TEMPO indicating that the process goes through a
non-radical pathway. This result is the opposite of that
observed by Jiang et al. for oxidative esterifcation of
ketones under CuBr/Py catalysis [11]. The same group
also reported that only pyridine was an efective ligand
for the reaction and CuBr/1,10-phenanthroline was totally
inactive [11]. However, in our case, 1,10-phenanthroline
plays the key role increasing the yield of the product
drastically while pyridine did not put a signifcant efect.
Therefore, we decided to discover the active catalyst for
our reaction. Looking through the literature, we found a
report for the synthesis of iodobis(1,10-phenanthroline)
copper (II) iodide octasulfur complex, [Cu(1,10-phen)2I]
I.S8, which had been characterized by X-ray difraction
data [18]. We supposed that for our reaction, the same
complex is being produced in situ and catalyzes the reac-
tion. Hence, we prepared the complex and used it as the
catalyst for the reaction. The reaction proceeded well giv-
ing the desired product in 92% yield under the air atmos-
phere (Scheme 3). However, when the same reaction was
conducted under N2 atmosphere, no product was obtained.
This result shows that in our standard conditions (Table 1,
O
[Cu(1,10-Phen)2I]I.S8
Si(OEt)4
KF (4 mmol),
DMF (1.5 mL)/ 100 °C
18 h, Under air or N2
NO2
NO2
Under air: 92%
Under N2: -
Scheme 3 Oxidative esterifcation of 3-nitroacetophenone catalyzed
by [Cu(1,10-phen)2I]I.S8
3-nitrobenzaldehyde, 3-nitrobenzoic acid and 3-nitroben-
zyl alcohol could be potential intermediates in this reac-
tion. Therefore, we conducted three reactions using these
compounds as the starting materials under the optimized
conditions. When 3-nitrobenzaldehyde was used as the
substrate, the desired ester was isolated in 34% yield
showing the possibility of aldehyde to be the actual inter-
mediate (Scheme 2). However, the second experiment
including 3-nitrobenzoic acid gave no yield of the prod-
uct (Scheme 2). Only a trace amount of the product was
obtained in our third experiment utilizing 3-nitrobenzyl
alcohol (Scheme 2). These results excluded the possibil-
ity of 3-nitrobenzoic acid and 3-nitrobenzyl alcohol as the
intermediates for this reaction.
Si(OEt)4 + KF
O
SiF(OEt)4
O
OEt
Ar
Ar
SiF(OEt)4
Ar
H
I
EtOH
CuI + O2
Phen
VII
O
OSiF(OEt)3
+
HCOOH
Ar
H
VI
CuII + O2
II
SiF(OEt)3
CO2 + H2
O
H
I
C
u
Ar
OEt
O
VIII
(EtO)3FSiO
HSiF(OEt)3
O
H
OH
O
H
Cu(II)
Ar
Ar
OH
III
V
SiF(OEt)3
transmetallation
O-O bond
cleavage
O
O
Ar
H2O
OSiF(OEt)3
Cu(II)
O
IV
β-elimination
Cu(II)H
Ar
OEt
H
O
IX
Ar
OEt
X
Scheme 4 Plausible mechanism for copper-catalyzed oxidative esterifcation of ketones
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Journal of the Iranian Chemical Society
entry 5), oxygen of atmosphere not only cause the conver-
sion of Cu(I) to Cu(II) [19, 20] but also take part in the
oxidation of ketone.
intermediate IV. Going through the species V, the interme-
diate IV undergoes degradation to form the corresponding
aldehyde VI plus formic acid. This aldehyde reacts with the
activated silicon I to form intermediate VII. This hyperva-
lent organosilicon has increased tendency to transfer an eth-
oxide group to the electrophilic carbon [22]. After ethoxide
transfer, transmetallation followed by β-hydride elimination
results in the desired ester X. The hydride is abstracted by
the silane specious [24] regenerating the active Cu(II). In
this fnal step, molecular oxygen plays no role since we have
observed that under N2 atmosphere, 3-nitrobenzaldehyde
gives the corresponding ester smoothly.
Since this complex involves S8 which acts as the two-
electron ligand, we were interested in checking the efect
of this ligand for our reaction. For this aim, the standard
reaction (Table 1, entry 5) was conducted in the presence of
S8 as the additive. The result showed that S8 does not afect
the efciency of the reaction. To understand the fate of the
methyl group during the reaction, we connected our reaction
vessel to the saturated solution of lime. During the course of
the reaction, we observed that the clear lime solution became
opaque showing that CO2 has been evolved under these con-
ditions (Scheme S1).
As mentioned above, the model reaction in the absence of
the catalyst gives the desired product in 10% isolated yield
(Table 1, entry 10 and Scheme S2) indicating that another
pathway might be simultaneously involved during the reac-
tion. Interestingly, when the reaction was conducted under
copper-free conditions, the lime test showed that CO2 was
not produced in the reaction. Thus, we propose a diferent
mechanism for these conditions (Scheme S3).
The probable reaction pathway has been depicted in
Scheme 4. The reaction of Si(OEt)4 with fuoride anion
which is highly silaphilic reagent generates the pentavalent
silicate I. This ate complex has reduced electron density on
the silicon regardless of its formal charge and coordinates
to the oxygen of ketone to form pentavalent species II [21].
In this step, extracoordination to hexavalent silicon cannot
also be excluded [16]. This oxophilic intermediate promotes
enolization of the ketone. Activation of molecular oxygen
by copper (I) [19, 20, 22] resulted in the incorporation of
dioxygen to form intermediate III [23] which converts to
We have also used our procedure for the carbon–carbon
bond cleavage of α-bromo-3-nitroacetophenone as an acti-
vated ketone. Utilizing this starting material, the correspond-
ing ethyl ester was obtained in 83% isolated yield within 4 h
reaction time (Scheme 5).
Scheme 5 Preparation of ethyl
3-nitrobenzoate from a bromo-
activated methyl ketone
O
O
O
Br
CuI (20 mol%), Phen (40 mol%)
Si(OEt)4
+
KF (2 mmol), DMF (1.5 mL)
100 °C, 4 h, Under air
NO2
NO2
83 %
Pd(OAc)2 + CaH2
Scheme 6 Synthesis of benzo-
caine
DMSO
100 oC
O
O
O
O
CuI, 1,10-phen
Pd(0), Cu(OAc)2
O
H2N
KF, DMF
100 oC, 4 h
Under air
83%
KF, HCOONa,
DMSO, 5 h
80%
O2N
O2N
overall yield: 66%
Scheme 7 Efect of sulfur
ligand in the esterifcation
reaction
O
O
O
CuI, Phen
OH
KF, DMF, 100 oC
Under air
NO2
NO2
with or without S8
No S8: 24%
With S8: 51%
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Journal of the Iranian Chemical Society
Scheme 8 Efect of sulfur
ligand in the esterifcation
reaction
O
O
O
[Cu(1,10-Phen)2I]I.S8
HO
KF (4 mmol),
DMF (1.5 mL), 100 °C
18 h, Under air
NO2
NO2
64%
Acknowledgements The authors are thankful to INSF (Grant Number:
95825781) and University of Hormozgan Research Council for the
fnancial support.
As an application of this method to organic synthesis,
we were interested in preparing the benzocaine as an anes-
thetic compound [25, 26]. Unfortunately, the direct con-
version of 4-aminoacetophenone to benzocaine (3c) under
our optimized conditions was unsuccessful (Scheme 1).
Therefore, we decided to prepare this compound from
4-nitroacetophenone.
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