Atom transfer coupling reactions performed with benign reducing agents and radical traps

Monobrominated polystyrene (PSBr) was prepared by ATRP, and the resulting chain ends were activated in the presence of radical traps to induce chain end‐coupling. In atom transfer radical coupling (ATRC) with radical trap assistance, to achieve significant coupling requires excess metal catalyst, ligand, and a reducing agent that is often additional metal. In this work, activators generated by electron transfer (AGET) and radical trap assistance are used in the ATRC sequence to successfully lead to chain‐end coupling without the need for the oxidatively unstable copper (I) and with environmentally friendlier agents in place of copper metal. High extents of coupling (Xc) were achieved using ascorbic acid (AA) as the reducing agent and copper(II) bromide as the oxidized version of the catalyst, and when combined with AGET ATRP to prepare the PSBr precursor, only a fraction of the total metal was required compared to traditional atom transfer reactions, while still retaining similar Xc values. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2113–2120     

Diethyl bromodifluoromethylphosphonate: a highly efficient and environmentally benign difluorocarbene precursor

A convenient method for the difluoromethylation of phenols and thiophenols, using diethyl bromodifluoromethylphosphonate (1) as a difluorocarbene precursor, is described. This commercially available phosphonate was found to undergo an extremely facile P-C bond cleavage on basic hydrolysis (−78 °C to rt), presumably leading to the bromodifluoromethyl anion, which subsequently converts to a difluorocarbene intermediate. The latter is trapped by phenolates 2 or thiophenolates 3 to give the corresponding difluoromethyl ethers and thioethers in good to excellent yield. The resulting eco-friendly side product, diethyl phosphate ion, is easily separated from the reaction mixture due to its excellent solubility in water. Due to the mild conditions applied to this reaction, phenolate ions bearing carbonyl or enolate functions are selectively difluoromethylated.     

Copper-catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions in the presence of environmentally benign ascorbic acid as a reducing agent

In recent years, copper-catalyzed atom transfer radical addition (ATRA) has emerged as a viable organic procedure for the formation of carbon-carbon bonds starting from alkyl halides and alkenes. Studies have primarily focused on the use of free-radical initiators to regenerate the copper(I) complex or activator in situ. Although these initiators led to a significant decrease in the amount of metal catalyst, they were much less effective for highly active alkenes that readily undergo free-radical polymerization. In this study, the non-radical reducing agent ascorbic acid (commonly known as Vitamin C) was effectively employed resulting in TONs as high as 15,200 in the homogenous ATRA of polyhalogenated compounds to α-olefins, and enabling selective monoadduct formation for highly active alkenes such as acrylonitrile (TONs as high as 11,800). As low as 7-20 mol% of ascorbic acid relative to substrate was sufficient for all ATRA and ATRC reactions examined. Further, product isolations for all selected syntheses were quite facile and nearly quantitative, requiring only simple liquid-liquid extraction techniques. Reactions in the presence of ascorbic acid were also examined kinetically via (1)H NMR and UV/Vis spectroscopy. A half order rate dependence on reducing agent concentration was observed, but the first order kinetic plots became nonlinear as the concentration of ascorbic acid was increased. Finally, the use of ascorbic acid circumvented otherwise necessary purging techniques, successfully furthering the utility of these reactions in organic synthesis.     

Flavins as organocatalysts for environmentally benign molecular transformations

Recent progress in the development of flavin-catalyzed oxidations and related reactions is described with respect to scope, limitation, and reaction mechanism. The 4a-hydroperoxyflavins, which are the most simplified model compounds of flavoenzymes, act as catalytically active species for the oxidation of organic substrates with the help of H(2)O(2) or O(2) as a mild oxidant. This principle behind the simulation of flavoenzymes led to the discovery of a variety of environmentally benign, oxidative transformations of secondary amines to nitrones, tertiary amines to N-oxides, sulfides to sulfoxides, and Baeyer-Villiger oxidations of ketones. Asymmetric oxidation of sulfides can also be performed with several chiral flavin catalysts. One of the fortunate outcomes of this study is the development of an environmentally friendly ("green") method for the "aerobic hydrogenation" of olefins, which is achieved by in situ generation of diimide with the aid of the flavin-catalyzed oxidation of hydrazine under an O(2) atmosphere.     

Iron-carbon nanohybrid particles as environmentally benign electrode for supercapacitor

Journal of Solid State Electrochemistry - In this work, we report the synthesis and electrode applications of iron-carbon nanohybrid particles prepared by carbonization of a nanocomposite of FeOOH...     

Henry reaction in environmentally benign methods using imidazole as catalyst

Abstract

Imidazole has been found to be an efficient Lewis base catalyst for the synthesis of 2-nitroalkanols from nitro-aldol reaction in aqueous medium as well as using solvent-free grinding methods. In both cases, the products have been found in good to moderate yields without formation of any side product. Recycling of the catalyst is also possible with imidazole containing aqueous phase reaction medium (after extraction of product).     

N-Acyltrifluoromethanesulfonamides as new chemoselective acylating agents for aliphatic and aromatic amines

This work describes advances in the study of the internal condensation of ammonium salts of N-acylsulfonamides. N-Acyltrifluoromethanesulfonamides show considerable advantages over the non fluorinated analogues by virtue of their higher reactivity and acidity. The reaction chemoselectivity has been investigated using a wide range of amines. The sensitivity of the reaction to steric and electronic effects confirms the potential application of these reagents in chemoselective acylation of polyamines.     

Solid-phase reducing agents as alternative for reducing disulfide bonds in proteins

Disulfide reduction of Kluyveromyces lactis and Aspergillus oryzae β-galactosidases and β-lactoglobulin was assessed. Reduction was performed using one of two thiol-containing agents: dithiothreitol (DTT) or thiopropyl-agarose with a high degree of substitution (1000 μmol of SH groups/g of dried gel). Both reductants allowed an increase of three- (for K. lactis β-galactosidase) and fourfold (for A. oryzae β-galactosidase) in the initial content of SH groups in the lactases. Nearly sevenfold fewer micromoles of SH groups per milligram of protein were needed to perform the reduction of K. lactis β-galactosidase with thiopropyl-agarose than for the same reduction with DTT. However, for A. oryzae β-galactosidase, nearly twice as many micromoles of SH groups per milligram of protein were needed with thiopropylagarose than with DTT. Disulfide bonds in β-lactoglobulin were not accessible to thiopropyl-agarose, since this reduction was only possible in the presence of 6 M urea. These results proved that highly substituted thiopropyl-agarose is as good a reducing agent as DTT, for the reduction of disulfide bonds in proteins. Moreover, excess reducing agent was very simply separated from the reduced protein by filtration, making it easier to control the reaction and providing reduced protein solutions free of reductant. All these advantages substantially cut down the time required and therefore the cost of the overall process.     

A new highly chemoselective isomerization of allylamides

This work describes the first iridium-catalyzed isomerization of N-allylamides into enamides. This strategy allows the chemoselective preparation of (E)-N-(1-propenyl)-enamides and can be applied for the selective deprotection of N-allylamides.     

Decomposition of N-cyclopropyl-N-nitrosourea in the presence of reducing agents as a new way of generating the cyclopropyl radical

A new way of generating cyclopropyl radicals in the base-catalyzed decomposition of N-cyclopropyl-N-nitrosourea in the presence of organic reducing agents (1-phenylpyrazolidin-3-one and 4-methoxyphenol) was developed. The cyclopropyl radical generated under these conditions can not only abstract a proton from the substrate to give cyclopropane but also form C-C or C-Br bonds in reactions with aromatic substrates or polybromomethanes. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1934–1938, November, 2006.     

A highly efficient and environmentally benign synthesis of 6,8-dibromoflavones, 8-bromoflavones, 5,7-dibromoaurones and 7-bromoaurones

Various ring substituted 6,8-dibromoflavones (3a-e) as well as 8-bromoflavones (3f-j) can be synthesized easily from the corresponding 2′-hydroxychalcones (1a-j) in good yields and in two steps under environmentally benign reaction conditions. This can be achieved by bromination with concomitant cyclization by using a combination of vanadium pentoxide, hydrogen peroxide and ammonium bromide in dichloromethane-water at 0-5 °C, followed by dehydrobromination of the brominated products 2a-j using 0.2 M ethanolic KOH solution at ice-bath temperature. On the other hand, various 5,7-dibromoaurones and 7-bromoaurone derivatives 6a-c can be obtained, exclusively, from the corresponding 2′-acetoxychalcones (4a-c) in good yields and in two steps by tuning the reaction conditions.     

Bis-arene-vanadium anions as reducing agents towards carbon dioxide and monoxide

A novel way of activating an alkali metal for the reduction of CO₂ and CO involves dissolving potassium in tetrahydrofuran containing [V(C₆H₆)₂], to give a solution of K[V(C₆H₆)₂] as the actual reducing agent. In contact with CO₂ or CO, the formal V⁻¹ derivative is immediately re-oxidized to the V⁰ complex and C₁ and C₂ products are formed.     

Oxone as an inexpensive, safe, and environmentally benign oxidant for C-H bond oxygenation

[reaction: see text] This paper describes the application of peroxide-based oxidants in the Pd(OAc)(2)-catalyzed acetoxylation and etherification of arene and alkane C-H bonds. Oxone in acetic acid and/or methanol proved particularly effective, and these transformations were applied to a wide variety of substrates.     

A simple and environmentally benign synthesis of polypyridine-polycarboxylic acids

An oxidation method using dilute nitric acid solutions under solvothermal conditions has been developed to synthesise a series of polypyridine-polycarboxylic acids. It has been successfully applied to a range of methyl substituted polypyridines including symmetrical and asymmetrical 2,2′-bipyridines; 2,2′:6′,2″-terpyridines and; 2,2′:6′,2″:6″,2?-tetra-pyridines and yields crystalline polypyridine-polycarboxylic acids in a single step. Simple product recovery through filtration yields a recyclable filtrate. More forcing conditions led to demethylation of the polypyridine ligand most probably via decarboxylation. This simple approach avoids potentially harmful metal-based oxidants and negates any issues associated with the disposal of their resultant (hazardous) waste.     

Cobalt-catalyzed chemoselective dehydrogenation through radical translocation under visible light

The transformations that allow the direct removal of hydrogen from their corresponding saturated counterparts by the dehydrogenative strategy are a dream reaction that has remained largely underexplored. In this report, a straightforward and robust cobaloxime-catalyzed photochemical dehydrogenation strategy via intramolecular HAT is described for the first time. The reaction proceeds through an intramolecular radical translocation followed by the cobalt assisted dehydrogenation without needing any other external photosensitizers, noble-metals or oxidants. With this approach, a series of valuable unsaturated compounds such as α,β-unsaturated amides, enamides and allylic and homoallylic sulfonamides were obtained in moderate to excellent yields with good chemo- and regioselectivities, and the synthetic versatility was demonstrated by a range of transformations. And mechanistic studies of the method are discussed.

The dehydrogenative reactions proceeded through selective 1,n-hydrogen atom transfer (n = 5–7) for remote C–H activation by cobaloxime catalysis.     

Silicon-based cross-coupling reaction: an environmentally benign version

Much attention has been paid to the cross-coupling reaction of organosilicon compounds due to their stability, non-toxicity, and natural abundance of silicon. In addition, the silicon-based cross-coupling has many advantages over other cross-coupling protocols. Successful examples of the silicon-based cross-coupling reaction are reviewed, focusing especially on the advances made in the last decade. Having had a number of highly effective palladium catalysts developed mainly for other cross-coupling reactions, the development of the silicon-based protocol owes heavily to the design of organosilicon reagents which effectively undergo transmetalation, a key elemental step of the silicon-based cross-coupling reaction. This tutorial review thus classifies various organosilicon reagents depending on substituents on silicon and surveys their cross-coupling reactions with various electrophiles.     

Synthesis of borosiloxane/polybenzoxazine hybrids as highly efficient and environmentally friendly flame retardant materials

A series of novel borosiloxane/polybenzoxazine hybrids were synthesized through the copolymerization of 3,3′‐phenylmethanebis(3,4‐dihydro‐2H‐1,3‐benzoxazine) and phenol‐functionalized borosiloxane (BSi‐OH) oligomer. The structures were characterized using nuclear magnetic resonance and fourier transform infrared. The thermal and flame retardant properties of hybrids were investigated by dynamic mechanical analysis, thermogravimetric analysis, and oxygen index instrument. The results showed that the addition of BSi‐OH oligomer is not only highly efficient in environmentally friendly flame retardancy of polybenzoxazine, but also enhances its thermal property. Only 25 wt % content of BSi‐OH oligomer was able to increase the glass transition temperature, 5% weight loss temperature (T_d5), 10% weight loss temperature (T_d10), and limited oxygen index (LOI) value from original 211 °C, 374 °C, 395 °C, and 29.5 °C to 244 °C, 408 °C, 448 °C, and 40.1, respectively. This work provides a facile and useful method for the preparation of new polybenzoxazines possessing highly efficient and environmentally friendly flame retardance as well as heat resistance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2390–2396     

Trivalent molybdenum as volumetric reducing agent for admixed oxidizing agents

Summary The aim of this study was to test the applicability of trivalent molybdenum as a volumetric reducing agent for mixtures of two and three oxidizing agents. The oxidizing agents used were ceric, vanadate dichromate and ferric solutions. All possible combinations of two and three ions were attempted. With the exception of a few cases, successful results were obtained with mixtures of two ions. Accurate determinations with three ions admixed could not be achieved; however, a close estimation was possible in some cases.