Deep eutectic solvent mediated controlled and selective oxidation of organic sulfides and hydroxylation of arylboronic acids Apurba Dutta 1 , Anirban Garg 1 , Jyotsna Borah , Ramyata Priyam Borah , Diganta Sarma * Department of Chemistry, Dibrugarh University, Dibrugarh, 786 004, Assam, India ARTICLE INFO Keywords: Oxidation Sulfoxides Phenols DES ABSTRACT A mild and efficient protocol has been developed for the controlled oxidation of wide variety of organic sulfides to corresponding sulfoxides using deep eutectic solvent (DES) as catalyst and hydrogen peroxide as oxidant under ambient condition. In presence of benzyl alcohols along with sulfides, DES selectively oxidizes only the sulfides to corresponding sulfoxides. The DES is prepared by using zirconium oxychloride and urea (1:5 M ratio) which is thermally stable up to 213  C. The prepared DES can also be effectively applied for hydroxylation of arylbronic acids without using any additional catalyst and solvent. 1. Introduction In modern era, for sustainable growth of human race the most important factor that needed to be incorporated in both industry and academia is green chemistry philosophy. On that context, preventing waste but not treating or cleaning it up, is a basic requirement for pollution prevention [1]. The most frequently encountered reaction in organic chemistry is oxidation. Among the oxidation reactions, oxidation of sulfide to sulfoxides is very important in pharmaceutical research considering broad application of sulfoxides in pharmaceuticals as anti-ulcer, antibacterial, antifungal, antihypertensive and cardiotonic agents, even applied in the activation of certain enzymes [2]. Typically conversion of sulfides into sulfoxides with organic or inorganic catalysts [3–7], is found to be ineffective for large scale preparations and some- times over-oxidation of sulfoxides to sulfones take place. In spite of proper control of reaction parameters like temperature, time and the relative amounts of oxidants, over-oxidation cannot be completely avoided. Phenols and their derivatives are abundant in nature as they are key structural unit in natural antioxidants [8], variety of natural products like terpenoids, alkaloids, aromatic steroids, macrolides, quinines etc [9]. Moreover, phenolic compounds are widely utilized in polymer chemistry, chemical industries, pharmaceuticals and natural product chemistry [10, 11]. Typical synthesis of phenol involves pyrolysis of sodium benzene sulfonate or hydrolysis of benzene diazonium salts. Further, laboratory grade synthesis of phenols is afforded by nucleophilic substitution of aryl halides activated by electron withdrawing substituents or with Pd and Cu catalysts [12,13]. However, hydroxylation of arylboronic acids to afford variously substituted phenols is recently growing as a popular method to afford phenols using diverse catalyst e.g. peroxymonosulfate [14], amberlite IR 120 resin [15], cellulose [16], calcined burnt peel ash (CBPA) [17] etc. Likewise, many green synthetic routes have been devised using photoinduced condition [18], microwave irradiated con- dition [19], bio based solvent [20] and solvent- and catalyst-free con- dition [21]. Furthermore, Zhang et al. recently reviewed recent advances in green condition to access functionalized phenols from reaction of ar- omatic boronic acid [22]. However, sustainable protocols for oxidation of sulfides as well as arylboronic acids are still in great demand. Deep Eutectic Solvents (DESs) which are fluids in nature composed of different Lewis or Bronsted acids and bases. Their preparation involve the simple mixing of two components generally at moderate heating that are inexpensive, non-toxic and biodegradable. They are cheaper and safer for their uses as solvents (as well as catalysts) as compared to ionic liquids in synthetic transformation. In 2003 [23], Abbott and his co-workers had shown that it was possible to lower the melting point of high melting salts, such as quaternary ammonium salts and metal salt/metal salt hy- drate, by mixing them with hydrogen bond donors (HBD). This melting point depression was ascribed due to the charge delocalization occurring through hydrogen bonding between them. Their formation is based on the depression in freezing point by the addition of a solute to a solvent * Corresponding author. E-mail address: dsarma22@dibru.ac.in (D. Sarma). 1 Both the authors contributed equally to this work. Contents lists available at ScienceDirect Current Research in Green and Sustainable Chemistry journal homepage: www.elsevier.com/journals/ current-research-in-green-and-sustainable-chemistry/2666-0865 https://doi.org/10.1016/j.crgsc.2021.100107 Received 24 December 2020; Received in revised form 3 April 2021; Accepted 12 April 2021 Available online 8 May 2021 2666-0865/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). Current Research in Green and Sustainable Chemistry 4 (2021) 100107