N -PEGylated Thiazolium Salt: A Green and Reusable Homogenous Organocatalyst for the Synthesis of Benzoins and Acyloins
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N‑PEGylated Thiazolium Salt: A Green and Reusable Homogenous Organocatalyst for the Synthesis of Benzoins and Acyloins Ali Javaheri Haghighi1 · Javad Mokhtari1 · Khashayar Karimian2 Received: 12 March 2020 / Accepted: 4 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract N-PEGylated-thiazolium salt is used as efficient catalyst for the benzoin condensation. The catalyst was synthesized by reaction of activated polyethylene glycol 10,000 (PEG-10000) with 4-methyl-5-thiazoleethanol (sulfurol). Reaction mixture undergoes temperature-assisted phase transition and catalyst separated by simple filtration. After reaction course, catalyst can be recycled and reused without any apparent loss of activity which makes this process cost effective and hence ecofriendly. Synthesized benzoins and acyloins by this method have been characterized on the basis of melting point and 1H-NMR spectral studies. Graphic Abstract
Keywords N-PEGylated thiazolium salt · Benzoin condensation · Acyloins · Homogenous catalysis · Pegylation · Sulfurol
1 Introduction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-020-03417-3) contains supplementary material, which is available to authorized users. * Javad Mokhtari [email protected] * Khashayar Karimian [email protected] Ali Javaheri Haghighi [email protected] 1
Departments of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
Institute of Biochemistry and Biophysics, University of Tehran, P.O. Box: 13145‑1384, Tehran, Iran
2
The benzoin reaction is one of the oldest reactions in organic chemistry, found serendipitously by Liebig and Wohler in 1832 [l]. They discovered that the cyanide anion can catalyze the union of two molecules of aromatic aldehydes to afford α-hydroxy ketones [1]. More than a century later, Onium salts have been known to catalyze the benzoin reaction by Ukai et al. who discovered catalytic activity of 3-ethylthiazolium bromide [2]. This may be regarded as an early example of organocatalysis using an azolium salt. Breslow, in 1958, investigated the catalyst role of thiazolium salt for the selfcondensation of benzadehyde to benzoin and proposed a mechanism, which generally admitted [3, 4]. He depicted the catalytically active species as a thiazolium zwitterion (the resonance structure of an NHC) and proposed that the reaction proceeds via an enaminol intermediate. However,
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in the recent years, many researchers have challenged this mechanism with evidence [5–7]. Almost three decades later Bertrand et al. used the N-heterocyclic carbene (NHC) as catalytically active species in the benzoin reaction with a stable phosphinocarbene [8]. Stetter’s in 1976 used N-benzyl sulfurol chloride for benzoin condensation and may be regarded as the first report of an NHC-catalysed benzoin reaction on a synthetically useful scale [9]. Since the first thiazolium salt catalyzed benzoin condensation [2] a continuous research
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