Reviews and Accounts ARKIVOC 2015 (i) 1-28
1. Introduction Thioethers are important class of compounds that are in great demand as intermediates in the synthesis of numerous compounds having biological and therapeutic applications and molecular precursors of material interest.1-4 The transition metal catalyzed cross-coupling reactions offer a powerful strategy for the construction of aryl-sulfur bonds. This approach involves the coupling of organic halides with thiols. Despite the importance of this reaction protocol in organic synthesis, transition metal catalyzed thioetherification received lesser attention, compared to other carbon-heteroatom bond forming reactions. The strong chelating property of sulfur causes catalyst poisoning, rendering the catalytic reaction ineffective. However, recently there has been considerable progress in the area of transition metal catalyzed organosulfur chemistry.
Migita et al. used Pd(PPh3)4 as a catalyst for the coupling reaction between aryl iodides and bromides with thiols under mild conditions.5 Many other Pd based catalytic systems have also been developed, which are based on bidentate phosphines or diverse organophosphane derivatives.6-9 But these systems have limitations since they require the preparation and use of PR3 ligands which are not eco-friendly. Catalytic systems based on other transition metals such as nickel10-13, cobalt14 and iron15 have also been studied. These catalytic systems also suffer from certain disadvantages like metal toxicity, low turnover numbers etc. But there is much interest in the development of copper based catalytic systems because of the low cost of Cu and use of readily accessible and stable ligands. Many attractive Cu-catalyzed coupling processes have been reported by various research groups. However, the Cu-catalyzed C-S coupling alone has not been reviewed yet, although it has been sparsely discussed with other carbon-heteroatom coupling reactions.16-20 This review highlights the existing strategies for copper-catalyzed C-S bond forming reactions and covers the literature from 2004-2014.
The classical Cu-catalyzed reaction between thiols and aryl halides required stoichiometric amounts of copper salts, polar solvents and high temperature.21 In recent years there is an upsurge of reports on C-S bond formation using various ligands such as phosphazene P2-Et base, neocuproine etc., primarily due to the high stability and low cost of copper.
2. Different Catalytic Systems In this review we classify the catalyst systems according to the nature of the ligand present in the complex
2.1. Cu-Nitrogen complexes Venkataraman et al. have used CuI-neocuproine (2,9-dimethyl-1,10-phenanthroline) complex for the coupling of aryl iodides with aryl and alkyl thiols in presence of NaOtBu in toluene at 110 oC, affording the products in excellent yields (Scheme 1).22
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