DOI: 10.1002/cctc.201501140
Communications
Poly(3,4-dimethyl-5-vinylthiazolium)/DBU-Catalyzed
Thioesterification of Aldehydes with Thiols**
Junyong Chung, Ue Ryung Seo, Supill Chun, and Young Keun Chung*[a]
Poly(3,4-dimethyl-5-vinylthiazolium) was synthesized from 3,4-
dimethyl-5-vinylthiazole through free radical polymerization
and was examined as polymer precatalysts in the presence of
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for the thioesterifica-
tion of aldehydes with thiols. The poly(5-vinylthiazolium)/DBU
had excellent catalytic activity and could be reused 10 times
without a considerable loss of activity.
a few studies on N-heterocyclic carbene (NHC)-catalyzed thio-
esterification have been reported.[9] Herein, we communicate
the synthesis of poly(vinylthiazolium) from 3,4-dimethyl-5-vi-
nylthiazolium and its use as a precatalyst for thioesterification
of aldehyde and alkyl thiols. In 1989, Tsuda et al. reported thia-
zolium salt polymers.[10] According to their report, N-methyl-4-
vinylphenylthiazolium iodide could not be not polymerized by
radical initiators. Therefore, 4-vinylphenylthiazole was polymer-
ized, and the resultant polythiazole was quaternized by using
methyl iodide. The degree of quaternization was calculated by
elemental analysis of iodine.
Recently, organocatalysts related to green chemistry have at-
tracted much attention.[1] One of the advantages of organoca-
talysts is that the catalytic reaction does not require a metal
We synthesized poly(vinylthiazolium) from 3,4-dimethyl-5-vi-
nylthiazol-3-ium by using a radical initiator, that is, 2,2’-azobisi-
sobutyronitrile (AIBN) (Scheme 1). 3,4-Dimethyl-5-vinylthiazol-3-
ium (1) was prepared by reaction of 4-methyl-5-vinylthiazole,
catalyst. However, most organic syntheses need
a large
amount of an organic catalyst and are performed in a homoge-
neous medium; hence, recovery and reuse of the catalyst is
often not feasible. To overcome these obstacles, the heteroge-
nization of organocatalysts over organic polymers and meso-
porous materials was recently reported.[2] However, most of
these heterogeneous catalysts suffer from relatively low yields
and poor recyclability, presumably as a result of the poor sta-
bility or degradation of the catalysts under the reaction condi-
tions. We recently developed a highly efficient, recyclable poly-
mer-based organocatalytic system, that is, poly(4-vinylimidazo-
lium),[3] that shows high catalytic activity for benzoin conden-
sation and cycloaddition of CO2 to epoxides. These polymeric
catalysts were successfully recovered and reused over several
cycles without any loss of performance. Our interest in poly(vi-
nylimidazolium)s led us to study poly(vinylthiazolium)s. Thia-
zoles have been widely studied in materials science;[4] however,
to the best of our knowledge, there is no report on the synthe-
sis and chemistry of poly(vinylthiazolium)s. Therefore, we de-
cided to study the polymerization of 4-methyl-5-vinylthiazole,
because it is commercially available and its polymerization to
poly(4-methyl-5-vinylthiazole) has not yet been reported. We
also studied the use of poly(vinylthiazolium) as a precatalyst
for the thioesterification of aldehydes with thiols, because
direct thioesterification of aldehydes has received little atten-
tion relative to direct aldehyde-to-ester conversions.[5] Thioest-
ers are widespread in biochemistry[6] and are versatile building
blocks for the construction of various natural products.[7] Many
useful synthetic routes have been developed.[8] However, only
Scheme 1. Synthesis of poly(vinylthiazolium).
which is commercially available, with methyl iodide. The reac-
tion of 4-methyl-5-vinylthiazole with methyl iodide afforded
3,4-dimethyl-5-vinylthiazol-3-ium in 87% yield. Polymerization
of 1 in the presence of AIBN afforded poly(3,4-dimethyl-5-vinyl-
thiazol-3-ium) (2), which is the organocatalyst precursor, in var-
ious yields depending on the reaction conditions (see the Sup-
porting Information). Broad 1H NMR signals are observed for
this compound, as expected for a high molecular weight poly-
mer. The polymer was slightly soluble in DMSO, DMF, and
water at room temperature but insoluble in other polar sol-
vents, including chloroform, dichloromethane, and acetone.
The weight-average molecular weight (Mw) of 2 is approxi-
mately 42700, as determined by light-scattering experi-
ments.[11]
As a model reaction for thioesterification, we initially exam-
ined the reaction of benzaldehyde with 1-pentanthiol in the
presence of 2 (10 mol%), an oxidant (1.2 equiv.), and DBU
(10 mol%) in DMSO at room temperature for 24 h (Table 1).
The use of oxidants such as phenazine, MnO2, and O2 afforded
the corresponding thioester in yields of 90, 38, and 7%, re-
spectively (Table 1, entries 1–3). However, tert-butyl hydroper-
oxide, 2-iodobenzoic acid, potassium persulfate, diethyl azodi-
carboxylate, 4-phenyl-1,2,4-triazole-3,5-dione, and (diacetoxyio-
do)benzene as oxidants gave only a trace amount of the corre-
sponding thioester (not shown in Table 1). The use of azoben-
zene as an oxidant resulted in a considerable amount of
[a] J. Chung, U. R. Seo, S. Chun, Prof. Dr. Y. K. Chung
Department of Chemistry
Seoul National University
Seoul 151-747 (Korea)
[**] DBU=1,8-Diazabicyclo[5.4.0]undec-7-ene.
Supporting Information for this article is available on the WWW under
ChemCatChem 2016, 8, 318 – 321
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