Mini-Reviews in Organic Chemistry - Volume 18, Issue 5, 2021

Hypervalent iodine reagents are a class of non-metallic oxidants that have been widely used in the construction of several sorts of bond formations. This surging interest in hypervalent iodine reagents is essentially due to their very useful oxidizing properties, combined with their benign environmental character and commercial availability from the past few decades ago. Furthermore, these hypervalent iodine reagents have been used in the construction of many significant building blocks and privileged scaffolds of bioactive natural products. The purpose of writing this review article is to explore all the transformations in which carbon-oxygen bond formation occurs by using hypervalent iodine reagents under metal-free conditions.

This review discusses an important synthetic tool proposed by K.B. Sharpless in 1980, known as the Sharpless asymmetric epoxidation of allylic alcohols, and examines its use in the total synthesis of representative exponents of biologically active natural products. Focus is given to the synthesis of simple to highly complex secondary metabolites, including lactones, amino acids, diterpenes, and macrolides. The Sharpless approach involves the use of a catalyst, titanium tetra isopropoxide [Ti(OⁱPr)4], dialkyl tartrate as a chiral ligand, and tert-butyl hydroperoxide (TBHP) as an oxidizing agent. The method allows converting allylic alcohols to epoxides, which are chiral building blocks and versatile intermediates in the synthesis of natural products. The biological and synthetic importance of epoxides lies in the susceptibility of the three-membered heterocyclic ring to stereoand regioselective opening by nucleophilic or acidic reagents, providing oxygen adducts.

Karl Elbs published his discovery of the reaction between phenols and peroxydisulfate in 1893. It was not until fifty years later that Boyland extended the reaction to aromatic amines. There have been more than 300 citations of these reactions which, although usually giving only modest yields, are nevertheless useful for their simplicity.

The removal of sulfur compounds from fuel oil has been a major concern of the fuel industry. Hydrodesulfurization (HDS) is the most common desulfurization method currently used in oil refineries. However, HDS requires high temperature and high pressure conditions, consumption of hydrogen, and the removal effect of thiophene sulfides is not ideal. In view of defects of HDS, nonhydrodesulfurization technologies have been developed. Oxidative desulfurization (ODS) is regarded as the most potential desulfurization technology to replace HDS in industry because of mild operating conditions and high desulfurization efficiency. However, most ODS reactions are performed in heterogeneous systems. The common problem is that the oxidant and sulfur-containing compounds cannot contact effectively. Phase Transfer Catalysts (PTCs) are a class of catalysts that can assist in the transfer of reactants, thereby increasing mass transfer and accelerating the reaction rate of the heterogeneous ODS system. In this review, we divide PTCs applied to ODS into quaternary ammonium salts PTCs, ILs PTCs, polymers PTCs, supported PTCs and reaction-controlled PTCs, and will systematically introduce and summarize the research progress of PTCs used in ODS system. It is pointed out that reaction-controlled PTCs have a broad prospect in ODS system. In addition, the synthesis and recovery of PTCs will be briefly described.

With the development of industrialization, global environmental pollution and energy crisis are becoming increasingly serious. Organic pollutants pose a serious health threat to human beings and other organisms. The removal of organic pollutants in the environment has become a global challenge. The photocatalytic technology has been widely used in the degradation of organic pollutants with its characteristics of simple process, high efficiency, thorough degradation and no secondary pollution. However, the single photocatalyst represented by TiO2 has the disadvantages of low light utilization rate and high recombination rate of photocarriers. Building heterojunction is considered one of the most effective methods to enhance the photocatalytic performance of a single photocatalyst, which can improve the separation efficiency of photocarriers and the utilization of visible light. The classical heterojunction can be divided into four different cases: type I, type II, p-n heterojunctions and Z-scheme junction. In this paper, the recent progress in the treatment of organic pollution by heterostructure photocatalysts is summarized, and the mechanism of heterostructure photocatalysts for the treatment of organic pollutants is reviewed. It is expected that this paper can deepen the understanding of heterostructure photocatalysts and provide guidance for high efficient photocatalytic degradation of organic pollutants in the future.