Selenium-Epoxy ‘Click’ Reaction and Se-Alkylation—Efficient Access to Organo-Selenium and Selenonium Compounds

Thiol-epoxy 'click' chemistry: a focus on molecular attributes in the context of polymer chemistry

Base-catalyzed ring-opening reaction of epoxides with the thiol nucleophiles is useful in the preparation and post-polymerization modification of synthetic polymers. Due to its many beneficial characteristics, this process is referred to as the thiol-epoxy 'click' reaction. In this article, our aim is to discuss the fundamental attributes of this process by tracing our own steps in the field. We initially address the aspects of efficiency, regio-selectivity, stoichiometry, and reaction conditions with the help of linear, hyperbranched, graft, dendritic, and cross-linked poly(β-hydroxy thioether)s. A special emphasis is placed on hydrogel synthesis and photopolymerization on surfaces. Subsequently, quenching of the alkoxide anion is considered which is a critical step in the formation of the β-hydroxy thioether linkage upon completion of reaction. The amenability of further reaction on the hydroxy and thioether groups through esterification and sulfur alkylation is then discussed. Initially, post-gelation/fabrication modification of sulfide linkages is considered to obtain cationic sulfonium hydrogels and zwitterionic photopatterned networks with antibacterial and antibiofouling properties, respectively. A post-synthesis functionalization strategy is then described to access same centered and segregated main-chain poly(β-hydroxy sulfonium)s as potent antibacterial materials. In side-chain polysulfides, the sequential post-synthesis modifications involving poly(glycidyl methacrylate) scaffolds can lead to the formation of amphiphilic homopolymers. The application of such materials is discussed in the arena of siRNA delivery. Finally, concerns relating to the formation of disulfide defects and open research goals such as study of the orthogonality of the reaction are addressed.

Regioselective One-Pot Synthesis of Novel Functionalized Organoselenium Compound by Bis-Alkoxyselenenylation of Alkenes with Selenium Dibromide and Alcohols

The one-pot efficient synthesis of novel functionalized organoselenium compound by bis-alkoxyselenenylation of alkenes with selenium dibromide and alcohols was developed. The reaction of the selenium dibromide with cyclopentene or cyclohexene in the system alcohol/sodium bicarbonate/methylene chloride at room temperature afforded bis(2-alkoxycycloalkyl) selenides in 90–99% yields. The regioselective and efficient method for bis-alkoxylation of terminal alkenes was developed based on the addition of selenium dibromide with 1-alkenes in acetonitrile followed by refluxing of addition products in alcohols in the presence of traces of sulfuric acid. This method made it possible to selectively obtain bis(2-alkoxyalkyl) selenides in 94–98% yields.

Regio- and Stereoselective Synthesis of (Z,Z)-Bis(3-amino-3-oxo-1-propenyl) Selenides and Diselenides Based on 2-propynamides: A Novel Family of Diselenides with High Glutathione Peroxidase-like Activity

The efficient regio- and stereoselective syntheses of (Z,Z)-bis(3-amino-3-oxo-1-propenyl) selenides and diselenides in high yields based on the nucleophilic addition of sodium selenide to 2-propynamides and sodium diselenide to 3-(trimethylsilyl)-2-propynamides have been developed. The first examples of the addition of a selenium-centered nucleophile to 2-propynamides with a terminal triple bond and diselenide anion to 3-(trimethylsilyl)-2-propynamides have been carried out. Bis(3-amino-3-oxo-1-propenyl) diselenides are a novel family of compounds, none of which has yet been described in the literature. The glutathione peroxidase-like activity of the obtained compounds has been evaluated and products with high activity have been found. It was established that diselenides are superior to selenides with the same substituents in glutathione peroxidase-like activity. The results of the structural studying of products by single-crystal X-ray diffraction analysis and 77Se-NMR data are discussed.

Micellar Assembly and Disassembly of Organoselenium Block Copolymers through Alkylation and Dealkylation Processes

The aim of this work is to demonstrate that the alkylation and dealkylation of selenium atoms is an effective tool in controlling polymer amphiphilicity and, hence, its assembly and disassembly process in water. To establish this concept, poly(ethylene glycol)-block-poly(glycidyl methacrylate) was prepared. A post-synthesis modification with phenyl selenolate through a base-catalyzed selenium-epoxy 'click' reaction then gave rise to the side-chain selenium-containing block copolymer with an amphiphilic character. This polymer assembled into micellar structures in water. However, silver tetrafluoroborate-promoted alkylation of the selenium atoms resulted in the formation of hydrophilic selenonium tetrafluoroborate salts. This enhancement in the chemical polarity of the second polymer block removed the amphiphilic character from the polymer chain and led to the disassembly of the micellar structures. This process could be reversed by restoring the original amphiphilic polymer character through the dealkylation of the cations.

Selenonium Polyelectrolyte Synthesis through Post-Polymerization Modifications of Poly (Glycidyl Methacrylate) Scaffolds

Atom transfer radical polymerization of glycidyl methacrylate monomer with poly(ethylene glycol)-based macroinitiators leads to the formation of reactive block copolymers. The epoxide side-chains of these polymers can be subjected to a regiospecific base-catalyzed nucleophilic ring-opening reaction with benzeneselenol under ambient conditions. The ß-hydroxy selenide linkages thus formed can be alkylated to access polyselenonium salts. 77Se-NMR indicates the formation of diastereomers upon alkylation. In such a manner, sequential post-polymerization modifications of poly(glycidyl methacrylate) scaffolds via selenium-epoxy and selenoether alkylation reactions furnish practical access to poly(ethylene glycol)-based cationic organoselenium copolymers.

Polyselenonium salts: synthesis through sequential selenium-epoxy 'click' chemistry and Se-alkylation

With the help of amphiphilic homopolymers, this work explores three new avenues in polymer chemistry: (i) the 'click' nature of the selenium-epoxy reaction, (ii) alkylation of the seleno-ethers as a means to prepare cationic polyelectrolytes, and (iii) the antibacterial activity of polyselenonium salts.