Facile synthesis of methylthiomethyl esters through Pummerer-type rearrangement of carboxylic acids and DMSO under metal-free conditions

Article abstract of DOI:10.1080/00397911.2019.1581894

A green and cost-effective Pummerer-type rearrangement between readily accessible carboxylic acids and DMSO has been achieved under metal-free conditions in satisfactory to excellent yields. The transformation for the synthesis of valuable methylthiomethy

Full text of DOI:10.1080/00397911.2019.1581894

Synthetic Communications
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Chemistry
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Facile synthesis of methylthiomethyl esters
through Pummerer-type rearrangement of
carboxylic acids and DMSO under metal-free
conditions
Shuiliang Wang, Zhengjiang Fu, Yongqing Jiang, Yuxiang Liang & Hu Cai
To cite this article: Shuiliang Wang, Zhengjiang Fu, Yongqing Jiang, Yuxiang Liang & Hu Cai
(2019): Facile synthesis of methylthiomethyl esters through Pummerer-type rearrangement of
carboxylic acids and DMSO under metal-free conditions, Synthetic Communications
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https://doi.org/10.1080/00397911.2019.1581894
Facile synthesis of methylthiomethyl esters through
Pummerer-type rearrangement of carboxylic acids and
DMSO under metal-free conditions
Shuiliang Wang, Zhengjiang Fu, Yongqing Jiang, Yuxiang Liang, and Hu Cai
College of Chemistry, Nanchang University, Nanchang, Jiangxi, China.
ABSTRACT
ARTICLE HISTORY
Received 13 December 2018
A green and cost-effective Pummerer-type rearrangement between
readily accessible carboxylic acids and DMSO has been achieved
under metal-free conditions in satisfactory to excellent yields. The
transformation for the synthesis of valuable methylthiomethyl esters
is shown to be a convenient strategy for various (hetero)aromatic
acids, a,b-unsaturated carboxylic acids and saturated alkyl carboxylic
acids with good functional group tolerance.
KEYWORDS
Carboxylic acids; DMSO;
metal-free conditions; MTM
esters; Pummerer-type
rearrangement
GRAPHIC ABSTRACT
Introduction
Dimethyl sulfoxide (DMSO) is usually utilized as a high-boiling polar aprotic solvent in
organic transformation, furthermore, the “C–S–C”, “C” and “C–S” fragments of DMSO
are able to used as potential synthons in modern synthetic chemistry to construct new
C–C and C–Hetero bonds.[1] It is worth to note that introducing sulfurized moiety of
DMSO into molecular architectures has a dramatic impact on their chemical, physical
and biological properties.[2] Carboxylic acids are widespread and nontoxic compounds,
which can undergo decarboxylative coupling with various nucleophiles or electrophiles
under transition-metal catalysis along with the extrusion of CO2,[3] in this regard, our
group has recently gained extensive knowledge in the realm of copper-mediated
decarboxylative functionalization of aryl carboxylic acids,[4] including decarboxylative
methylthiolation,[4a] arylation,[4b] protonation,[4c] halogenation and cyanation.[4d–f] In
addition, the carboxylic acids can also be easily converted into esters through differ-
ent methods.[5]
CONTACT Zhengjiang Fu
fuzhengjiang@ncu.edu.cncaihu@ncu.edu.cn
College of Chemistry, Nanchang University,
Nanchang, Jiangxi, 330031, China.
Supplemental data for this article is available online at on the publisher’s website
ß 2019 Taylor & Francis Group, LLC

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S. WANG ET AL.
Methylthiomethyl (MTM) esters are of great interest for their featured chemical prop-
erties under photochemical conditions involving electron-transfer courses,[6] moreover,
they can be served as both rapidly absorbable pro-drugs of nonsteroidal anti-inflamma-
tory agents[7] and flavor additives in dairy products and essential oils.[8] According to
the cleavage under neutral non-hydrolytic conditions, MTM group can also be
employed as a useful carboxyl protecting group in peptide synthesis.[9] Traditional
methods to access MTM ester include the reaction of carboxylate anions with methyl-
thiomethyl chloride (MTM-Cl),[10] treatment of carboxylate salts with DMSO by using
tert-butyl bromide,[11] N-chlorosuccinimide,[12] dicyclohexylcarbodiimide,[13] chlorine or
sulfuryl chloride[14] as activator respectively. DMSO is much more accessible than
MTM-Cl, furthermore, it can be activated to form thionium ion intermediate as electro-
philic reagent via Pummerer-type rearrangement,[15] thus, DMSO is a favorite to
undergo Pummerer reaction with nucleophile to construct C–O bond under certain
conditions. In this context, Ghosh and coworkers have reported a route to synthesize
MTM esters between carboxylic acids and DMSO under Swern oxidation conditions at
[16]
ꢁ
low temperature (ꢀ60 C),
whereas the group of Zimmerman have developed the
preparation of MTM esters from carboxylic acids and DMSO by a microwave-assisted
technique.[17] Yang and Jiang have enabled a Fe-catalyzed Pummerer-type rearrange-
ment of acyl chlorides with sulfoxides to construct alkylthiomethyl ester.[18] In continu-
ation of our interest in diverse functionalization of cheap and abundant carboxylic
acids,[4] herein we reveal an efficient, simple and metal-free approach for the conversion
of a large pool of carboxylic acids including (hetero)aromatic acids, a,b-unsaturated car-
boxylic acids and saturated alkyl carboxylic acids to the corresponding MTM esters
through Pummerer-type rearrangement using Et3N as the sole additive. During the
preparation of this manuscript, Lee and Nam have reported the reaction of carboxylic
acid with DMSO in the presence of Et3N to result in the corresponding MTM
esters,[15d] however, only cinnamic acids and limited benzoic acids were employed as
the coupling partners in their work.
Results and discussion
Initially, we commenced the optimization studies by performing Pummerer-type
rearrangement of cinnamic acid (1a) with DMSO as the model reaction under an inert
atmosphere. Table 1 presented some selected results from the optimization studies,
which showcased the effects of the additive and other parameters on the reaction out-
come. Although we have established protodecarboxylation of o-nitrobenzoic acids with
CuI as catalyst and Et3N as additive in DMSO under an inert atmosphere in recent
exploration,[4c] however, the cinnamic acid (1) in the presence of CuI/Et3N in DMSO
smoothly furnished methylthiomethyl cinnamate (1p) instead of protodecarboxylation
product in good yield (71%) under nitrogen atmosphere (entry 1, Table 1). Much to
our delight, the omission of CuI resulted into an improved yield (77%) under otherwise
identical conditions (entry 2). A brief survey of the N-containing additives revealed that
both monodentate amine (n-Pr3N and morpholine) and bidentate amine (en and
DMEDA) were inferior to Et3N (entries 3–6). Nevertheless, increasing the loadings of
Et3N to two equivalents had no beneficial effect on the reaction (entry 7), while low

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Table 1. Optimization of the model reaction conditions.a
aReaction conditions: 1 (0.2 mmol), DMSO (2 mL), 160 ꢁC, N2 (1 atm), 20 h.
bIsolated yield.
cen ¼ ethylenediamine.
dDMEDA ¼ N,N’-dimethylethylenediamine.
eAir conditions instead of N2.
f10 h instead of 20 h.
g140 ꢁC instead of 160 ꢁC.
yield (37%) was detected when the reaction atmosphere was switched to air (entry 8).
Unfortunately, both shortening reaction time and lowering reaction temperature of the
transformation led to unwelcome reaction conversion under otherwise equal conditions
(entries 9–10). Control experiment exhibited that Et3N was essential to the transform-
ation (entry 11). Weighing the pros and cons, we decided to perform metal-free
Pummerer-type rearrangement of cinnamic acid (1a) with DMSO in the presence of 1
ꢁ
equivalent of Et3N as additive at 160 C for 20 h under nitrogen atmosphere (entry 2)
and use these data as our standard conditions.
Subsequently, the obtained optimality conditions (entry 2, Table 1) inspired us to
explore the scope of this metal-free Pummerer-type rearrangement reaction with respect
to cinnamic acids. As outlined in Table 2, the transformation was shown to be an
effective strategy for a wide range of cinnamic acids containing electron-withdrawing
(fluoro, chloro, trifluoromethyl, cyano and nitro) 2–7 and -donating (methyl, isopropyl,
methoxy, dimethylamino and ketal) substituents 8–16, benzo-fused cinnamic acid 17
and a,b-unsaturated heteroaromatic carboxylic acids 18–20. Moreover, cinnamic acid
substrates bearing electron-donating or -withdrawing substituent directly provided the
corresponding MTM esters in satisfactory to excellent yields, thus, the electronic prop-
erty of substituent on the aromatic ring of cinnamic acid did not significantly affect the
efficiency of transformation. The substituent of chloro-moiety in substrates 3 and 4
were well tolerated in the reaction, which offered the possibility for late-stage function-
alization via the activation of C-Cl bond. Compared to its isomer 8 or 11 respectively,
2-methylcinnamic acid 9 and 2-methoxycinnamic acid 12 were exceptions that showed
moderate Pummerer-type rearrangement yields, indicative of the sensitivity of conver-
sion to steric hindrance. It’s found benzo-fused trans-3-(1-naphthyl)-2-propenoic acid

4
S. WANG ET AL.
Table 2. Substrate scope of cinnamic acids.
a8 mmol substrate was used.
b170 ꢁC instead of 160 ꢁC.
17 was a suitable substrate to furnish hoped-for product in synthetically useful levels.
Although the substrates of a, b-unsaturated heteroaromatic carboxylic acids including
3-(3-pyridyl)acrylic acid 18, 3-(2-furyl)acrylic acid 19 and 3-(2-thienyl)acrylic acid 20
had not been displayed in previous Pummerer-type rearrangement protocols under
Swern oxidation[16] or microwave-assisted conditions,[17] it’s observed for the first time
to undergo the transformation with DMSO to afford desired products in satisfactory
yields. The yields of the corresponding products in conversion were determined via the
1
average of two runs, and the products were unambiguously characterized by H and 13C
NMR spectral analyses to be an exclusive E stereochemistry.

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Table 3. Substrate scope of other carboxylic acids.
a8 mmol substrate was used.
The feasibility of Pummerer-type rearrangement of cinnamic acids 1–20 with DMSO
promoted us to further evaluate the generality of other carboxylic acids substrates with
diverse structure, and the results were compiled in Table 3. A wealth of benzoic acids
bearing methyl and methoxy substituents at different positions 21–33 as well as mor-
pholino group at the para position 34, a-naphthoic acids 35–36, N- and S-containing
heteroaromatic carboxylic acid 37–39, unsaturated carboxylic acid containing a conju-
gated diene unit 40, alkyl carboxylic acids 41–50 and alkyl dicarboxylic acid 51 were
favorably carried out with DMSO under the standard conditions to afford the desired
MTM esters products in satisfactory to excellent yields. Although the Pummerer-type
rearrangement of ortho-toluic acid 21 was implemented in previous method,[16] it’s also
identified to be an effective substrate to afford the target product with high yield in this
protocol. Similar to 2-methylbenzoic acid 21, an array of benzoic acids bearing methyl
or methoxy group 22–33 successfully participated in the process to give the hoped-for
MTM esters. It’s noteworthy benzoic acid tolerated methyl group at the ortho-position
21 gave superior yield to that of benzoic acid with methoxy substituent at the ortho-
position 25, likewise, 4-methoxy-2-methylbenzoic acid 31 was more efficient to deliver
anticipated product than its isomeric compound 33 with methoxy substituent at the
ortho-position, presumably due to the subtle electronic effect on the reactivity for ben-
zoic acids toward this transformation. For the substrates of N- and S-containing hetero-
aromatic carboxylic acids 37–39, they were detected for the first time to undergo this

6
S. WANG ET AL.
Scheme 1. Proposed mechanism for this transformation.
conversion to give the desired products.[16–17] Cinnamic acids effectively took place in
the transformation as illustrated in Table 2, furthermore, cinnamalacetic acid 40 com-
prised a conjugated diene unit to generate the end product under the standard condi-
tions. Delightfully, the protocol was also efficiently applied to primary, secondary, and
tertiary alkyl monocarboxylic acids 41–50 that tolerated different types of functional
groups including aryl, C=C double bond, CꢂC triple bond, cyclohexyl and adamantyl.
Moreover, for the case of hexanedioic acid 51, it worked well in the optimized system
to supply the desired product in 51% yield.
Furthermore, a 40-fold scale-up of the synthesis was also carried out to illustrate the
practicality of this protocol. As the isolated yields were showed in parentheses in Tables
2 and 3, employing 8 mmol of carboxylic acid (1, 27 and 48) as starting material
with Et3N (1 equiv) in DMSO under the standard reaction conditions, as a result, the
transformation of the carboxylic acids reached nearly the same efficiency as the reaction
conducted on a 0.2 mmol scale, which declared a great potential of the method on
large-scale synthesis.
Although the proposal of a detailed mechanism need to be awaited further investiga-
tion, basing on the above results and related reports,[16–18] a plausible mechanism for
this transformation was hypothesized in Scheme 1. Initially, the resonance hybrid B of
DMSO nucleophilic attacked carbonyl carbon of starting substrate carboxylic acid A to
produce intermediate C followed by losing hydroxyl ion OHꢀ to give acylated DMSO
adduct D. Once formed, deprotonation of a-H of the resultant intermediate D to afford
thionium salt E in the presence of Et3N, followed by combination of the thionium inter-
mediate E with carboxylate anion to generate the final MTM ester F, while the Et3N
was regenerated with the assistance of liberated OHꢀ, and that’s why increasing the
amount of Et3N to two equivalents had no beneficial effect on the transformation.
Conclusion
In summary, we have established a simple and practical method for the Pummerer-type
rearrangement of easily available carboxylic acids with DMSO under metal-free condi-
tions. The transformation demonstrated a highly efficient protocol to synthesize the cor-
responding MTM esters in satisfactory to excellent yields from a wide range of
carboxylic acids substrates. There are several distinguishing features in the protocol:
extensive substrate scope including readily available (hetero)aromatic acids,
a,b-unsaturated carboxylic acids and saturated alkyl carboxylic acids with good

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7
functional group tolerance; a simple reaction system to efficiently afford MTM esters
products with cheap Et3N as the sole additive under metal-free conditions. Thereby,
this environment-friendly and robust method will attract much attention in academic
and industrial fields, and further investigation to extend the practical application of this
method is currently in progress.
Experimental
General information
The reagents used for experiments were commercially available and were used as
received unless otherwise noted. DMSO was distilled from CaH2 under reduced pres-
sure and stored under nitrogen. All reactions were performed under nitrogen with the
strict exclusion of moisture using Schlenk techniques. Column chromatography was per-
formed on silica gel 300–400 mesh. The yields reported are the isolated yields and the
1
average of two runs. H, 13C and 19F NMR spectra of compounds were recorded at 400,
100 and 377 MHz with CDCl3 as solvent respectively. All coupling constants (J values)
were reported in Hertz (Hz). HRMS was performed by Comprehensive Laboratory
Center of the College of Chemistry, Nanchang University.
Typical procedure for the synthesis of MTM ester
An oven-dried Schlenk tube equipped with a stir bar was charged with carboxylic acid
(0.2 mmol) and Et3N (28 mL, 0.2 mmol, 1 equiv). The tube was fitted with a rubber sep-
tum, and then it was evacuated and refilled with nitrogen three times. Under nitrogen,
DMSO (2 mL) was added via syringe. The rubber septum was replaced with a Teflon
screwcap under nitrogen flow, and the Schlenk tube was pressurized to 1 atm. With
ꢁ
stirring, the reaction mixtures were heated at 160 C for the indicated amount of time
(unless otherwise specified). After cooling to room temperature, the reaction mixtures
were diluted with ether (10 mL) and filtered through a pad of silica gel that was then
washed with ether (10 mL ꢃ 3). The combined organic phase was washed with brine
(20 mL ꢃ 2), dried over Na2SO4, filtered and concentrated in vacuo. The resulting resi-
due was purified by flash column chromatography over silica gel to provide the corre-
sponding product with ethyl acetate/hexane as eluent.
(E)-(methylthio)methyl cinnamate (1p)[16]
1
Colorless oil, 77% yield. H NMR (400 MHz, CDCl3): d 7.73 (d, J = 16.4 Hz, 1H),
7.53–7.51 (m, 2H), 7.38 (t, J = 2.8 Hz, 3H), 6.46 (d, J = 16.0 Hz, 1H), 5.27 (s, 2H), 2.28
(s, 3H). 13C NMR (100 MHz, CDCl3): d 166.5, 145.6, 134.2, 130.5, 128.9, 128.2, 117.4,
68.3, 15.5.
1
Full experimental detail, H, 13C and 19F NMR spectra data, and references for all the
products. This material can be found via the “Supplementary Content” section of this
article’s webpage.

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S. WANG ET AL.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the National Natural Science Foundation of China (NSFC)
(21761021 and 21571094), Natural Science Foundation of Jiangxi Province (20161BAB203074 and
20171BAB203002), Sci & Tech Project of Education Department of Jiangxi Province (60007).
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