Cycloaddition reactions in aqueous systems: A two-decade trend endeavor

Abstract

Cycloaddition reactions are an integral and weighty part of organic chemistry in pedagogy and research as well. The wealthy literature on cycloaddition reactions from their birth up to now, unequivocally witnesses to their leading chemistry. The so-called ''conventional solvents'' are organic solvents that have indubitably promoted their success. Yet, the toxicity facet of these solvents impedes their use freely and with no fear. Not only is the operating chemist uncomfortable while experimenting, but also the environment is equally threatened. Working out the cycloaddition reactions and other organic ones in aqueous systems would certainly bring some relief to the chemist and to the environment as well. Unusual outcomes in terms of yield, reactivity and selectivity compared to those performed in organic solvents were commonly observed, and have overwhelmed the chemists with surprise indeed. In this review, homo Diels-Alder reactions in aqueous media include those involving the following dienophiles: maleimides, a,b-unsaturated esters, p-benzoquinones, vinyl ketones, phenyl-1-(2-pyridyl)-2-propen-1-one, a,bunsaturated esters. A special case is the organocatalysis of Diels-Alder cycloaddition of a,b-unsaturated ketones (aldehydes). Of no less importance, some hetero Diels-Alder cycloaddition reactions in water systems are delineated. The impact of additives (salts, organic and inorganic chemicals), micellar catalysis and Lewis/Brønste ¨d acid catalysis on outcomes of such cycloaddition reactions is discussed. The 1,3-dipolar cycloaddition methodology applied to aqueous media has brought forth a number of heterocyclic compounds, usually with a regio-and stereoselectivity pecularity. These heterocycles include triazoles, tetrazoles, pyrazoles, isoxazoles, isoxazolidines, pyrroles and pyrrolidines. The superiority of copper(I) catalysis in the azide-alkyne cycloaddition (Huisgen cycloaddition) in water is endorsed by a number of examples.

Hydrogel synthesis by aqueous Diels‐Alder reaction between furan modified methacrylate and polyetheramine‐based bismaleimides

“…5 The DA reaction using Nsubstituted maleimides as dienophiles has been reported to be fast, efficient, and chemoselective in aqueous media. 6 DA hydrogels in water have already been reported by Nimmo et al introducing furan groups in hyaluronic acid and performing the subsequent cross-linking with an oligoethylene oxide bismaleimide. 4 Hyaluronic acid has been also employed for hydrogel formation through its modification with furan and maleimide groups.…”

“…5,6 The role of water in accelerating the DA reaction seems to be related to the hydrophobic effect displayed by this solvent which provides a driving force for the packing of the diene and the dienophile. 6 One of the classical examples of the DA reaction in organic chemistry is the coupling between furan, which posses a strong dienic character, with maleimide as the dienophile. 7 Among the relevant aspects of employing this furan/ maleimide couple the renewable character of furan derivatives should be pointed out.…”

First example of stepwise, zwitterionic mechanism for bicyclo[2.2.1]hept-5-ene (norbornene) formation process catalyzed by the 1-butyl-3-methylimidazolium cations

“…The Diels–Alder reaction (DA) is generally the most universal synthesis method of bicyclic carbon skeletons [ 1 – 3 ]. In particular, using cyclopentadiene (Cp) and ethylene derivatives, it is possible to synthesize a wide range of compounds from the bicyclo[2.2.1]hept-5-ene (norbornene) group.…”

Covalent Functionalizations of Poly(vinyl chloride) in Tune with Applications: An Update

“…The conventional as well as the newly developed chemistries were adequately referred to for the chemical modifications of poly(vinyl chloride). Indeed, since its emergence in 2001 [9], click chemistry has opened new avenues not only in related chemistry synthesis [10,11], but also in several other research aspects [12,13]. Its application was even extended to polymer field for specific uses [14][15][16], and, more interesting, to biopolymers [17].…”