Copper-catalyzed cross-coupling of thiols, alcohols, and oxygen for the synthesis of esters

Article abstract of DOI:10.1002/ejoc.201403311

Copper-catalyzed, one-pot, three-component coupling reactions using thiols, alcohols, and oxygen to form a variety of esters in good yields were studied. In the presence of easily oxidized benzylic and allylic alcohols, thiols were selectively oxidized to form thionoesters, which underwent facile S/O exchange to afford esters. Thiols may be used as an alternative benzoyl source under mild aerobic conditions.

Full text of DOI:10.1002/ejoc.201403311

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DOI: 10.1002/ejoc.201403311
Copper-Catalyzed Cross-Coupling of Thiols, Alcohols, and Oxygen for the
Synthesis of Esters
Seungyeon Lim,[a] Miran Ji,[a] Xi Wang,[a] Chan Lee,[a] and Hye-Young Jang*[a,b]
Keywords: Synthetic methods / Oxidation / Desulfurization / Thiols / Esters / Copper
Copper-catalyzed, one-pot, three-component coupling reac- oxidized to form thionoesters, which underwent facile S/O
tions using thiols, alcohols, and oxygen to form a variety of
esters in good yields were studied. In the presence of easily
oxidized benzylic and allylic alcohols, thiols were selectively
exchange to afford esters. Thiols may be used as an alterna-
tive benzoyl source under mild aerobic conditions.
Introduction
Esters are one of the most important organic functional
groups, and there are many synthetic methods to convert
carboxylic acids, aldehydes, and alcohols into esters.[1] In an
effort to synthesize esters efficiently and economically, di-
rect alcohol into ester conversion approaches have been de-
veloped by using transition-metal catalysis,[2,3] transition-
metal-mediated oxidation,[4] and transition-metal-free reac-
tions.[5] Transition-metal-catalyzed aerobic one-pot esterifi-
cation of alcohols that use ruthenium, iridium, palladium,
and gold complexes have also been studied.[2] These reac-
tions only require small amounts of chemicals and are
therefore considered green chemical processes. Considering
the cost of the currently used catalysts (ruthenium, iridium,
palladium, and gold complexes), more economical catalysts
should be employed, for example, copper complexes.
Although copper catalysts and oxygen have been widely
utilized in various oxidative coupling reactions, to our
knowledge, they have not been used in the aerobic one-pot
oxidation/oxidative esterification of alcohols.[6,7]
In addition to examining inexpensive metal sources for
these one-pot ester-forming processes, the control of homo-
coupling and cross-coupling of alcohols is an important is-
sue in ester synthesis. A synthetically useful cross-esterifica-
tion of activated alcohols (benzylic and allylic alcohols) and
less reactive or nonreactive alcohols (methanol, aliphatic
alcohol, and phenol derivatives) under oxidation conditions
was carried out successfully;[2] however, selective cross-
esterification of activated alcohols such as benzylic alcohols
or allylic alcohols was not possible (Scheme 1).
Scheme 1. Metal-catalyzed synthesis of esters.
Recently, we have reported a copper-catalyzed oxidative
coupling of thiols and amines to produce thioamides.[8] Al-
though sulfur and oxygen atoms belong to the same group
in the periodic table, the reactivity of thiols is not analogous
to that of alcohols.[9] This fact can be used to identify the
more susceptible species between thiols and alcohols under
oxidation conditions. To test this hypothesis, we carried out
a cross-coupling of thiols and alcohols. In this study, we
developed the first copper-catalyzed, three-component
cross-coupling of thiols, alcohols, and oxygen to form a
range of esters, whereby benzyl thiols were selectively oxid-
ized in the presence of activated alcohols (Scheme 1). Under
copper-catalyzed aerobic oxidation conditions, an oxygen
atom from an oxygen gas molecule participated in the ester
synthesis.[10]
[a] Department of Energy Systems Research, Ajou University,
Suwon 443-749, Korea
E-mail: hyjang2@ajou.ac.kr
Results and Discussion
http://ajou.ac.kr/~hyjang2/
[b] Korea Carbon Capture & Sequestration R&D Center,
Deajeon 305-343, Korea
The optimization results are given in Table 1. Benzyl
mercaptan 1a (2 equiv.) and cinnamyl alcohol 1b (1 equiv.)
were subjected to aerobic oxidation conditions. In the pres-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403311.
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ence of CuCl2 (2 mol-%), a solution of 1a, 1b, and 1,5,7- captans were treated with cinnamyl alcohol to form the cor-
triazabicyclo[4.4.0]dec-5-ene (TBD, 1 equiv.) in toluene responding esters 3c and 4c in 69 and 65% yield, respec-
(0.5 m) was stirred at 100 °C under one atmosphere of oxy- tively (entries 2 and 3). The coupling of furyl mercaptan
gen, to afford ester 1c in 32% yield (entry 1). Initially, it and cinnamyl alcohol formed the desired ester 5c, albeit in
was expected to form thionoester 1d; however, it was found low yield (entry 4).
that the sulfur atom was displaced by an oxygen atom (see
below). Although both starting materials were consumed
Table 2. Examples of esters formed from substituted mercaptans
in 2 h at 100 °C, the yield of 1c was not high. At a lower
temperature (50 °C), the yield of 1c increased to 63% (en-
try 2). When the amount of TBD was increased to
1.5 equiv., a slightly higher yield was obtained (78%, en-
try 3). We also screened several copper and iron complexes.
CuII complexes, CuBr2, Cu(OTf)2, and Cu(OAc)2 catalyzed
the ester formation to afford 1c in 77, 57, and 79% yield,
respectively (entries 4–6). CuI complexes, CuCl, CuI, and
CuOAc catalyzed the reaction to form 1c in 79, 78, and
69% yield, respectively (entries 7–9). FeCl3·6H2O was also
checked for this oxidative coupling reaction; however, a
lower yield (41%) compared with those from copper-cata-
lyzed reactions was found (entry 10). In the absence of
metal complexes, 1c was formed in much lower yield (21%;
entry 11). Finally, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
was tested instead of TBD, which led to the formation of
1c in 11% yield (entry 12).
and cinnamyl alcohol.
Table 1. Optimization of the formation of ester 1c.
[a] Reaction performed at 50 °C. [b] Reaction performed at room
temp.
A variety of alcohols were then treated with benzyl mer-
captan (Table 3). Interestingly, electron-rich methoxy-sub-
stituted benzyl and cinnamyl alcohols reacted with benzyl
mercaptan to form esters in better yields at room tempera-
ture than at 50 °C, along with the isolation of thionoesters
(entries 1–5). Benzyl alcohols substituted with p-, o-, and
m-methoxy groups underwent esterification with benzyl
mercaptan to form the desired esters/thionoesters in 68/7%
(6c/6d), 61/9% (7c/7d), and 62/11% (8c/8d) yield, respec-
tively (entries 1–3). The position of the methoxy substituent
did not affect the yield or the ratio of esters and thiono-
esters. Benzyl alcohol possessing two methoxy groups was
also used to form 9c (60% yield) and 9d (7% yield) (en-
try 4). In addition to benzyl alcohol derivatives, methoxy-
substituted cinnamyl alcohol was submitted to the reaction
To investigate the substrate scope of the reaction, a di- conditions, to afford 10c (65%) and 10d (5%) (entry 5). In
verse range of thiols and alcohols were subjected to the aer- the case of the reaction of cinnamyl alcohols, thionoester
obic oxidation conditions involving CuCl2 (2 mol-%) and 1d was not formed even at room temperature. The reaction
TBD (1.5 equiv.) (Tables 2 and 3). In Table 2, the results of of electron-rich alcohols led to the formation of a small
the reaction with benzyl mercaptan derivatives and furyl amount of thionoesters 6d–10d, which presumably served
mercaptan with cinnamyl alcohol are shown. Electron-rich as the intermediates for conversion into the major product,
tert-butyl-substituted benzyl mercaptan was treated with esters. Electron-deficient benzyl alcohols were tested.
cinnamyl alcohol to form 2c in 66% yield (entry 1). Elec- p-Bromobenzyl alcohol was subjected to the reaction condi-
tron-deficient fluoro- and chloro-substituted benzyl mer- tions to provide 11c in 58% yield (entry 6). p-Nitrobenzyl
2
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Copper-Catalyzed Cross-Coupling for the Synthesis of Esters
Table 3. Examples of esters formed from benzyl mercaptan and alcohol did not form the oxidative coupling product under
various alcohols.
our reaction conditions. Hexenol and geraniol were con-
verted into the corresponding products 12c and 13c in 70
and 66% yield, respectively (entries 7 and 8). Additionally,
aliphatic alcohols reacted to form esters 14c (74% yield)
and 15c (56% yield) (entries 9 and 10).
We propose a possible reaction mechanism, which is
shown in Scheme 2. Thiol 1a is converted into disulfide 1aЈ
under oxidation conditions.[11] In Scheme 3, Equation (1),
the reaction of 1aЈ with 1b to provide 1c in 68% yield is
shown; this is comparable to the yield obtained from 1a.
This result confirms that disulfide can be a key intermediate
of this transformation. Thiobenzaldehyde I is assumed to
be formed through the reaction of disulfide 1aЈ with
TBD.[8,12] Thiobenzaldehyde I reacts with cinnamyl alcohol
1b to afford thioacetal II. Similar to the formation of I,
thioacetal II undergoes disulfide formation and deproton-
ation to generate thionoesters, which were isolated in the
reactions of methoxy-substituted benzyl alcohols. To ac-
count for the formation of an ester as the major product,
we conducted a number of control experiments, see
Scheme 3 and Equations (2)–(5). To determine the oxygen
source in the S/O exchange, independently prepared 18O-
labeled benzyl alcohol 16b was submitted to the reaction
conditions to form ester 16c having a single 18O atom, see
Equation (2). Upon the addition of 18O-labeled water, no
18O-incorporated 16c was observed by mass spectrometry,
see Equation (3), implying that the formation of 1c is not
induced by simple water hydrolysis of 1d. The reactions of
benzyl mercaptan 1a and dimethoxy substituted benzyl
alcohol were then performed independently in the absence
of either O2 or copper catalysts, see Equations (4) and (5).
In the absence of O2, ester 9c was not formed and thionoes-
ter 9d was formed in 7% yield, see Equation (4). In the ab-
sence of copper catalysts, both ester 9c and thionoester 9d
were formed, albeit in low yields, see Equation (5). Because
esters were formed in the absence of copper catalysts with
low yields, the copper complex is presumed to accelerate
the oxidation of 1a and II.
Recent reports on copper-catalyzed aerobic desulfuriza-
tion involving the conversion of thiocarbonyl into carbonyl
compounds have proposed that the reduced oxygen species
O2– was an active species.[13] Similarly, reduced oxygen spe-
cies generated during the formation of disulfides 1aЈ or III,
might induce the desulfurization of 1d to afford 1c. Finally,
to compare the reactivity of alcohols with thiols under our
reaction conditions, we tested ester formation by using
benzyl alcohol instead of thiols; unfortunately, the ester was
formed in very low yield at 100 °C and was not formed at
all at 50 °C, see Equation (6).
During the reaction, the oxidative dimerization of benzyl
mercaptan occurred as a side reaction to form dithioester
1e and thioester 1f. In the absence of alcohols, 1a was con-
verted into dithioester 1e in 47% yield and 1f in 5% yield
(Scheme 4). Dithioester 1e was formed from the reaction of
thioaldehyde I with 1a, and thioester 1f was transformed
from 1e by the same mechanism forming ester 1c from 1d
(Scheme 2). Compared with thionoesters, dithioesters ap-
[a] Reaction performed at 50 °C. [b] Reaction performed at room
temp. [c] Reaction performed at 100 °C.
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Scheme 2. Plausible reaction mechanism for the formation of ester 1c.
pear to be less susceptible to aerobic desulfurization and
they form thioester 1f as the minor product.
Conclusions
We have reported an efficient copper-catalyzed oxidation
for the one-pot cross-coupling of thiols, alcohols, and oxy-
gen. Under our reaction conditions, the reactivity of benzyl
thiols was significantly higher than that of allylic and benz-
ylic alcohols, which allowed the selective oxidation of
benzyl thiols in the presence of alcohols. In this reaction,
the oxygen molecule functions as both a mild oxidant and
a reactant to form esters. A possible mechanism for the for-
mation of esters was investigated by conducting several con-
trol experiments including 18O-labeling studies. The cross-
coupling of thiols and alcohols first provides thionoester
intermediates, which undergo the S–to–O exchange by oxy-
gen to form various esters.
Experimental Section
General Procedure for the Reaction: Copper(II) chloride (1.3 mg,
0.01 mmol) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 105 mg,
0.75 mmol) were added to
a solution of benzyl mercaptan
(124.2 mg, 1.0 mmol) and cinnamyl alcohol (67 mg, 0.5 mmol) in
toluene (0.5 m, 1 mL). A slow stream of O2 was passed through this
solution for 10 min. The reaction mixture was then stirred at 50 °C
for 18 h under an O2 atmosphere. The solvent was removed under
vacuum, and the residue was purified by flash silica gel column
chromatography (ether/hexane, 1%) to obtain cinnamyl benzoate
1c (92.4 mg, 78%). In a larger scale reaction, the yield of 1c was 63,
63, and 46%, respectively, using 1.0, 1.5, and 2.0 mmol of cinnamyl
alcohol.
Scheme 3. Control experiments.
Acknowledgments
This study was supported by the Korea Research Foundation
(grant numbers 2009-0094046 and 2013008819) and a Korea CCS
Scheme 4. Oxidative dimerization of benzyl mercaptan.
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Received: October 7, 2014
Published Online: ᭿
[7] For selected recent articles of Cu-catalyzed oxidative aerobic
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Synthetic Methods
Copper-catalyzed coupling of thiols, al-
cohols, and oxygen provided a variety of
esters through S/O exchange of thionoester
intermediates
S. Lim, M. Ji, X. Wang, C. Lee,
H.-Y. Jang* ...................................... 1–6
Copper-Catalyzed
Cross-Coupling
of
Thiols, Alcohols, and Oxygen for the Syn-
thesis of Esters
Keywords: Synthetic methods / Oxidation /
Desulfurization / Thiols / Esters / Copper
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