N-heterocyclic carbene catalysed oxidative coupling of aldehydes with alcohols/thiols and one-pot oxidation/esterification of allylic alcohols

Article abstract of DOI:10.1002/ejoc.201301337

A versatile carbene-catalysed oxidation protocol involving N-heterocyclic carbene catalysts and 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) is described for the synthesis of esters, cinnamic acids, and thioesters. A wide range of esters, thioesters, and cinnamic acids were obtained by metal-free coupling of aldehydes with aliphatic, allylic and aromatic alcohols, benzyl mercaptan, and water. In addition to the oxidative coupling of aldehydes with nucleophiles, dehydrogenation of saturated aldehydes and oxidation of allylic alcohols were found under our oxidative coupling conditions. Unlike other TEMPO-mediated oxidative esterification reactions, this reaction does not proceed through a TEMPO ester intermediate to form esters and thioesters. N-Heterocyclic carbene complexes catalyse various oxidative coupling reactions in the presence of TEMPO. A variety of α,β-unsaturated and saturated esters, and aromatic esters and thioesters were synthesised. Copyright

Full text of DOI:10.1002/ejoc.201301337

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301337
N-Heterocyclic Carbene Catalysed Oxidative Coupling of Aldehydes with
Alcohols/Thiols and One-Pot Oxidation/Esterification of Allylic Alcohols
[a]
[a]
[a]
[a]
[a,b]
Miran Ji, Xi Wang, Yu Na Lim, Ye-Won Kang, and Hye-Young Jang*
Keywords: Oxidation / Esters / Thioesters / Carbenes / Dehydrogenation / Reaction mechanisms / Synthetic methods
A versatile carbene-catalysed oxidation protocol involving
N-heterocyclic carbene catalysts and 2,2,6,6-tetramethyl-
piperidine N-oxyl (TEMPO) is described for the synthesis of
esters, cinnamic acids, and thioesters. A wide range of esters,
thioesters, and cinnamic acids were obtained by metal-free
coupling of aldehydes with aliphatic, allylic and aromatic
alcohols, benzyl mercaptan, and water. In addition to the oxi-
dative coupling of aldehydes with nucleophiles, dehydroge-
nation of saturated aldehydes and oxidation of allylic
alcohols were found under our oxidative coupling conditions.
Unlike other TEMPO-mediated oxidative esterification reac-
tions, this reaction does not proceed through a TEMPO ester
intermediate to form esters and thioesters.
Introduction
aldehydes was reported by Studer, wherein TEMPO func-
tioned both as an oxidant and as a substrate to provide
TEMPO esters. Indeed, because of this facile and robust
Oxidation is one of the most important organic transfor-
mations, and abundant examples involving the use of stoi-
chiometric amounts of oxidants with or without catalysts
have been reported.[ Although metal catalysts are known
for selective and efficient oxidation, metal-free oxidation
has recently received great attention because of the associ-
ated economic and environmental benefits.[ In particular,
N-heterocyclic carbenes (NHC) have been utilised as
organocatalysts in the presence of inorganic or organic oxi-
dants for the oxidation of aldehydes.[ Depending on the
reagents used and on the reaction conditions applied, alde-
hydes were transformed into carboxylic acids, esters, thio-
[5a]
formation of TEMPO esters, this reagent has not generally
been used as a stoichiometric oxidant in the carbene-cata-
lysed oxidative coupling of aldehydes with other exogenous
1]
[5]
nucleophiles (alcohols, amines, and thiols). In this study,
the reaction conditions involving TEMPO and N-heterocy-
clic carbenes promoted: (1) the oxidative coupling of vari-
ous aldehydes with allylic, aliphatic and aromatic alcohols,
water, and benzyl mercaptan; (2) the tandem dehydrogena-
tion of saturated aldehydes and oxidative esterification; and
2]
3]
(3) the tandem oxidation of alcohols and oxidative esterifi-
cation to afford a wide range of esters and thioesters. The
conversion of saturated carbonyl compounds into the un-
saturated compounds (dehydrogenation) has been reported
in the Saegusa oxidation, oxonium salt-mediated oxidation,
esters, and amides by using oxidants such as MnO , quin-
2
ones, phenazine, azobenzene, 2,2,6,6-tetramethylpiperidine
N-oxyl (TEMPO), and oxygen.[
4–6]
In terms of cost and en-
vironmental issues, oxidative reactions using oxygen would
be most desirable, but currently reported carbene-catalysed
oxygen-mediated esterification of aldehydes shows high
yields for aromatic aldehydes and electron-deficient alde-
hypervalent iodine-mediated oxidation, and carbene-cata-
[8,9]
lysed oxidation using quinone.
Direct transformation of
alcohols into esters has been achieved either through transi-
tion-metal-catalysed reaction of alcohols or a tandem pro-
[
5l,5o,5q]
hydes only.
Therefore, the search for versatile oxi-
cedure involving metal-promoted oxidation of alcohols and
dants for various carbene-catalysed oxidative couplings is
still interesting and relevant.
[10,11]
subsequent oxidative esterification.
The transition-
metal-free direct conversion of alcohols into esters has ra-
We recently reported the synthesis of optically active α,β-
disubstituted aldehydes involving TEMPO, both as an oxi-
dant and as a substrate.[ In the context of TEMPO-based
organocatalytic oxidation, carbene-catalysed oxidation of
[12–14]
rely been reported.
7]
Results and Discussion
[
a] Division of Energy Systems Research, Ajou University
We used toluene as a solvent at a high temperature
100 °C) to perform an NHC-catalysed oxidative esterifica-
tion using TEMPO. As a result, the desired α,β-unsaturated
esters were obtained, as shown in Table 1. A mixture of
cinnamaldehyde and cinnamyl alcohol was exposed to the
carbene-catalysed oxidative esterification conditions. Both
4
43749 Suwon, South Korea
(
E-mail: hyjang2@ajou.ac.kr
http://ajou.ac.kr/~hyjang2
[b] Korea Carbon Capture & Sequestration R&D Center,
Deajeon 305-343, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301337.
Eur. J. Org. Chem. 2013, 7881–7885
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7881

M. Ji, X. Wang, Y. N. Lim, Y.-W. Kang, H.-Y. Jang
SHORT COMMUNICATION
carbenes A and B successfully promoted the esterification, The cinnamaldehyde possessing a 2-methoxy-substituted
wherein B gave a slightly higher yield (Entries 1 and 2). No phenyl group reacted with benzyl alcohol to afford 7a in
saturated esters were formed through an internal redox pro- 83% yield (Entry 8).
cess.[ Allylic, benzylic, aliphatic, and aromatic alcohols
15]
Next, hydrocinnamaldehyde, a saturated aldehyde, was
all reacted to yield the desired esters in good yields (1a– subjected to the oxidative esterification conditions listed in
a and 7a in Entries 2–6 and 8, respectively). At ambient Table 2. Although saturated aldehydes are vulnerable to an
5
temperature, 2a was obtained with a reduced yield (53%) aldol reaction during the oxidative esterification, esters
compared with that obtained at a higher temperature (En- were formed without any aldol product, similar to the azol-
try 3). Finally, water was also incorporated in cinnamal- ium-catalysed oxidative esterification using MnO2.[
dehyde to form cinnamic acid 6a in 86% yield (Entry 7). Interestingly, under TEMPO oxidation conditions, hydro-
cinnamaldehyde was converted into α,β-unsaturated esters
5b–5d]
2
a and saturated esters 2b (Entries 1 and 2). The respective
Table 1. NHC-catalysed oxidative coupling of α,β-unsaturated al-
yields of the products, 2a and 2b, differed slightly depending
on the amount of TEMPO and the ratio of aldehyde/
dehyde with alcohols and water.[
a]
Table 2. NHC-catalysed oxidative esterification of saturated alde-
hydes with alcohols.[
a]
[
a] Method I: aldehyde (1 equiv.), alcohol (1.5 equiv.); Method II:
[
(
[
a] Reaction conditions: carbene catalyst (5 mol-%), TEMPO
2 equiv.), aldehyde (1 equiv.), alcohol (1.5 equiv.), 4 h, 100 °C.
b] Room temp., 18 h.
aldehyde (1.5 equiv.), alcohol (1 equiv.). [b] TEMPO (2 equiv.):
50% (2a/2b = 0.86:0.14). [c] Reaction time 36 h. [d] TEMPO
(2 equiv.).
Scheme 1. Suggested mechanism for oxidative esterification of cinnamaldehyde.
882
7
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Eur. J. Org. Chem. 2013, 7881–7885

NHC-Catalysed Oxidative Coupling
Scheme 2. Suggested mechanism for tandem dehydrogenation and oxidative esterification.
alcohol. The formation of unsaturated esters was favoured Table 3. NHC-catalysed tandem oxidation/oxidative esterification.
(Entries 1 and 2). The reactions of hydrocinnamaldehyde
with cinnamyl alcohol and ethanol afforded predominantly
unsaturated esters in 66 and 54% yield, respectively (Entries
3
and 4). Unlike in the aforementioned examples yielding
α,β-unsaturated esters, the oxidative reaction of hydro-
cinnamaldehyde with phenol afforded saturated ester 5b as
a major product (Entries 5–7). When phenol was used as a
limiting reagent, the desired saturated esters were obtained
in good yield. The yield of 5b increased upon the use of
lower amounts of TEMPO (Entries 6 and 7). The reaction
of octanal with benzyl alcohol also provided saturated ester
8b in 52% yield (Entry 8).
To account for the results summarised in Tables 1 and
2, two mechanisms have been proposed (Schemes 1 and 2).
Presumably, cinnamaldehyde reacted with the carbene cata-
lyst to form the Breslow intermediate, which underwent oxi-
dation by TEMPO and deprotonation to provide intermedi-
ate II (Scheme 1). The latter either reacted with benzyl
alcohol to afford 2a or was converted into a TEMPO ester,
which underwent carbene-catalysed transesterification to
[
16]
form 2a. To probe the mechanism, an independently pre-
pared TEMPO ester was subjected to carbene-catalysed
transesterification conditions, but this did not afford any
[
[
a] 3a was isolated in 11% yield. [b] 3a was isolated in 3% yield.
c] 3a was isolated in 1% yield.
2a.
A plausible mechanism for tandem dehydrogenation and
oxidative esterification is presented in Scheme 2. Intermedi-
ate III, derived from hydrocinnamaldehyde and a carbene
We speculated that cinnamyl alcohol might have con-
under oxidation conditions, underwent deprotonation to af- verted into cinnamaldehyde under our reaction conditions,
ford an enolate intermediate. Further oxidation followed by because TEMPO is known to oxidise alcohols. Interestingly,
deprotonation induced dehydrogenation of the saturated al- cinnamyl alcohol underwent one-pot oxidation, followed by
dehyde. The reaction of phenol and hydrocinnamaldehyde oxidative esterification to afford 1a (Table 3). The yield of
provided oxidative esterification product 2b as a major 1a was not improved by varying either the amount of
product, whereas the reactions of hydrocinnamaldehyde TEMPO or the catalysts (Entries 1–5). Upon addition of
with cinnamyl alcohol, benzyl alcohol, and ethanol pro- 2 equiv. of ethanol (EtOH), 3a and 1a were isolated in 11
vided dehydrogenation/oxidative esterification products 1a, and 68% yield, respectively (Entry 6). When the amount of
2a, and 3a (Table 2). Considering the basicity of alcohols, ethanol was reduced to 0.2 equiv., the yield of 1a remained
phenol is less basic than benzyl alcohol, cinnamyl alcohol, constant at 59% (Entries 7 and 8). Addition of methanol
and ethanol. In the case of octanal (Table 2, Entry 8), dehy- also increased the yield of 1a to 55% (Entry 9). The mecha-
drogenation of the aldehyde did not occur. Accordingly, the nism is not clear, but ethanol and methanol might help the
acidity of the β-position of aldehydes and the basicity of deprotonation of the α-proton of cinnamyl alcohol, leading
alcohols are important factors that promote dehydrogena- to facile oxidation of cinnamyl alcohol. Various allylic
tion.
alcohols were exposed to the tandem oxidation/oxidative es-
Eur. J. Org. Chem. 2013, 7881–7885
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Ji, X. Wang, Y. N. Lim, Y.-W. Kang, H.-Y. Jang
SHORT COMMUNICATION
terification conditions, yielding the desired α,β-unsaturated good to modest yields. Although many methods can be
esters in modest-to-good yields (9a, 10a, and 11a; Table 3). used to prepare synthetically and biologically important
The reaction of cinnamyl alcohol (1 equiv.) and benzyl thioesters, efficient catalytic systems, especially metal-free
[
5k]
alcohol (2 equiv.) affords cross-ester 2a (23%) and 12a catalytic reactions, have rarely been reported.
25%) under NHC-catalysed oxidative conditions. work, we present versatile metal-free oxidative couplings of
Finally, aromatic aldehydes were subjected to the carb- aldehydes with alcohols and benzyl mercaptan, to afford a
ene-catalysed oxidative esterification and thioesterification wide range of esters and thioesters.
Table 4). In the case of the oxidative esterification with
In this
(
(
benzyl alcohol, aromatic and heteroaromatic aldehydes
were smoothly converted into esters in good yields (En- Conclusions
tries 1–4). The carbene-catalysed thioesterification was per-
We have established a set of conditions that can be used
formed under the optimised conditions for the oxidative es-
terification of aldehydes with alcohols. As shown in Table 4
for efficient NHC-catalysed oxidative coupling with
TEMPO. Even though TEMPO was used as a stoichiomet-
ric oxidant, various exogenous alcohols, water, and benzyl
mercaptan reacted with aldehydes to afford esters, cinnamic
acid, and thioesters. In particular, our oxidative catalytic
system promoted two unexpected transformations; (1) the
dehydrogenation of saturated aldehydes; and (2) the conver-
sion of allylic alcohols into α,β-unsaturated esters in a one-
pot reaction. In the presence of TEMPO, a range of oxidat-
ive coupling reactions involving oxidative esterification, oxi-
dative thioesterification, one-pot oxidation of alcohols/oxi-
dative esterification, and tandem dehydrogenation/oxidative
esterification were realised. In this study, we expand the
utility of the TEMPO oxidant to various oxidative pro-
cesses under metal-free carbene-catalysed conditions.
(Entries 5–10), various aromatic thioesters were formed in
Table 4. Carbene-catalysed esterification and thioesterification.[a]
Experimental Section
General: Anhydrous solvents were transferred by using an oven-
dried syringe. Dichloromethane was distilled from calcium hydride.
1
H NMR spectra were recorded with a Varian Mercury plus
(
400 MHz) spectrometer. Chemical shifts (δ) are reported in parts
per million (ppm) downfield from trimethylsilane. Coupling con-
1
3
stants are reported in Hertz (Hz). C NMR spectra were recorded
with a Varian Mercury plus (100 MHz) spectrometer. Chemical
shifts are reported relative to the centre of the triplet at δ =
77.00 ppm for CDCl
3
. TEMPO and 1,3-bis(2,6-diisopropylphenyl)-
1,3-dihydro-2H-imidazol-2-ylidene were purchased from Aldrich
and used without purification.
General Procedure: TEMPO (156.3 mg, 1.0 mmol) and 1,3-bis(2,6-
diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (B) (9.7 mg,
0
0
.025 mmol) were added to a solution of cinnamaldehyde (66.1 mg,
.5 mmol) and cinnamyl alcohol (101.0 mg, 0.75 mmol) in toluene
(
0.5 m, 1 mL). The reaction mixture was stirred at 100 °C for 4 h
until the starting aldehyde was consumed. The solvent was removed
under vacuum, and the residue was purified by flash silica gel col-
umn chromatography (diethyl ether/hexane, 1%) to obtain cinn-
amyl cinnamate (1a) (110.3 mg, 84%).
Supporting Information (see footnote on the first page of this arti-
1
13
cle): Experimental procedures and H and C NMR spectra.
Acknowledgments
This study was supported by the Korea Research Foundation (No.
[
a] Reaction conditions: To a solution containing carbene catalyst
2009-0094046) and a Korea CCS R&D Center (KCRC) grant from
B (5 mol-%) and TEMPO (2 equiv.), aldehyde (1 equiv.) and either
alcohol (1.5 equiv.) or thiol (2 equiv.) were added. The solution
mixture was stirred at 100 °C for 6 h. [b] 18 h.
the Korea Government (Ministry of Education, Sicence and Tech-
nology) (No. 2012-0008935).
7884
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Eur. J. Org. Chem. 2013, 7881–7885

NHC-Catalysed Oxidative Coupling
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Published Online: November 11, 2013
Eur. J. Org. Chem. 2013, 7881–7885
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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