Current Organic Chemistry - Volume 22, Issue 13, 2018

Transition metal-mediated atom transfer radical polymerization (ATRP) is well known as one of successful reversible deactivation radical polymerizations for synthesizing various designable structures. However, the resultant polymers are usually contaminated or colored by large amounts of metal catalyst, which does not facilitate its industrialization. How to efficiently separate and recycle the metal catalyst has triggered lots of interest in green chemistry and ATRP. In this review, we mainly focus on a recyclable liquid/liquid thermo-regulated phase separable catalysis (TPSC) strategy applied in ATRP, which provides a facile and efficient method combining homogeneous reaction with heterogeneous separation just by simply changing temperature. TPSC-based ATRP systems include but not limit to the following systems: fluorous biphasic system, lower alcohol/alkane biphasic system, poly(ethylene glycol) (PEG) biphasic system and thermo-regulated ionic liquids biphasic system and so on.

Nitroxide mediated polymerization (NMP) is one of the main reversible deactivation radical polymerization (RDRP) techniques and was one of the earliest discovered. However, other RDRP processes like atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer polymerization (RAFT) surpassed NMP, as it was largely restricted to styrenic polymerizations. Recently, with second-generation nitroxides, controlled homopolymerizations (predictable degree of polymerization with conversion, high chain end fidelity and narrow molecular weight distributions) of acrylates, acrylamides and dienes are now possible. Methacrylic ester homopolymerizations have remained somewhat elusive with commercially available alkoxyamines, but with a low concentration ~ 1-10 mol% of a controlling co-monomer, methacrylate-rich polymers are accessible. Recent reports have shown promising possibilities for methacrylate homopolymerizations. This review will summarize the progress made in the diversity of monomers polymerizable by NMP and an outlook of the ability of NMP to polymerize new families of monomers or to use under-utilized monomers for emerging applications will be given.

The reversible addition-fragmentation chain transfer polymerization is one of the most important techniques to control the molecular weight and the chain distribution of the polymers made via radical chemistry. Its application to the aqueous dispersed systems is an important step to improve the control of relevant polymers due to the advantages in the control of the reaction and to avoid the environmental issues related to the use of organic solvents. In this review, a brief overview on different techniques used to perform a well-controlled RAFT polymerization in aqueous dispersed systems is critically revised.

Natural polymers are unique materials due to inexpensive, easily available, reproducible sources, hydrophilic and stable biopolymers, safe and devoid of side effects and flexible polymers. In addition, they have excellent adsorption specifications to remove pollutants including dyes from colored wastewater. Adsorption is a preferred method compared to other methods due to its fast and comfortable and inscrutable to toxic contaminants. The natural polymers also have excellent properties such as low initial cost, production of nontoxic by-products, relatively simple design and productivity in time. Properties of an ideal and suitable absorbent for removal dye include the following: extensive surface, high capacity and ability to absorb, the pores with appropriate size and volume, comfortable accessible, efficient, economical, high mechanical stability, compatibility, easy regenerable, environmental friendly, high selectivity to elimination various dyes and needless of high processing techniques. Therefore, researchers have lately centralized on expanding compounds based on natural polymers. The current review provides information about the application of natural polymers as adsorbents to remove dyes from colored wastewater. Also, some useful information about the influence of dye removal process variables, dye adsorption mechanism, adsorption isotherms, kinetics, and thermodynamics are presented.

Click chemistry has emerged as a valuable synthetic tool for the assembly of diverse molecular units. In particular, the copper-catalyzed azide–alkyne cycloadditions (CuAAC) revealed to be extremely versatile and are currently used for the synthesis of molecules with a wide range of applications. Accordingly, CuAAC reactions have been extensively used for the functionalization of phenolic compounds, many of them with well-known biological properties. Herein, an overview of the most important advances in the application of CuAAC reactions to chalcones and chromones, including flavones, since 2002 is presented. In particular, this review discusses the use of azidochalcones, azidochromones, alkynylchalcones and alkynylchromones in 1,3-dipolar cycloadditions. The biological properties of some of the resulting conjugates, where the two units are linked by a 1,2,3-triazole bridge, are also presented. In some reported examples, the new conjugates revealed additive activities which represent a promising step forward in medicinal chemistry.

A novel pyridine base modified core-shell (γ-Fe2O3@Hap (Hap: Hydroxyapatite)) inorganic-organic hybrid magnetic nanocatalyst has been introduced. The catalyst was fully characterized by spectroscopic analyses (FT-IR, FESEM, EDX, XRD) and its efficiency evaluated as a basic catalyst in one-pot multicomponent reaction of aryl aldehydes, rhodanine and piperidine under ultrasonic irradiation to obtain thiazole derivatives. This green, fast and straightforward protocol produced the products in short reaction times (14-40 min) and high to excellent yields (75-95%).

An efficient and eco-friendly process has been developed for the synthesis of α- aminophosphonates containing aminothiazole moiety via Kabachnik-Field reaction catalyzed by H6P2W18O62.14H2O as reusable catalyst, by condensation of aromatic aldehydes, 2-aminothiazole and triethylphosphite under solvent-free conditions. All the newly synthesized compounds were systematically characterized by IR, 1H NMR, 13C NMR and 31P NMR analysis. The X-ray crystallographic data of compounds 3f and 3j were provided. Simplicity, inexpensive catalyst, milder, short reaction times and easy workup and purification of the final product are the main advantages of this method.

A novel C-glycosyl rhodamine-based fluorescent sensor with a thiourea receptor has been readily synthesized and its fluorogenic behavior towards various metal ions was fully studied. It was found that the obtained target sensor exhibited good selectivity for Hg2+ over many other metal ions, which has been attributed to a binding mode of thiourea with thiophilic Hg2+, the sensor showed high selectivity and sensitivity to Hg2+ with a violent fluorescence enhancement at 580 nm. The sensing mechanism was explored by spectroscopic techniques and the IR analysis results demonstrated that a 1:1 mixture was formed between Hg2+ and the sensor.