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Polyethylene glycol and solutions of polyethylene glycol as green reaction media

Among the cyclic compounds of carbon, cyclopropane and its derivatives are outstanding by virtue of their unusual structural, spectroscopic, and chemical properties. The cyclopropane ring more closely resembles the CC double bond than the cyclobutane ring: it is a small ring with “double bond character”. Cyclopropyl and vinyl groups interact with neighbouring π-electron systems and p-electron centers; both cyclopropane derivatives and olefins form metal complexes, and add strong acids, halogens, and ozone; they both undergo catalytic hydrogenation and cycloadditions. While distinct differences in reactivity do exist–the double bond usually being more reactive than the three-membered ring–there are no fundamental differences in behavior.–Although cyclopropane derivatives have been known for more than 90 years, intensive studies have been limited to the past 25 years. The development of carbene chemistry has rendered cyclopropane derivatives far more readily accessible. In recent years, the synthetic potential of the small-ring function has been increasingly exploited. A considerable number of newly developed methods utilizing this approach clearly demonstrates that the reactivity of the cyclopropene ring, like that of the CC double bond, qualify it as a “functional carbon group”. This development is in full swing; we may therefore justifiably devote considerable effort to the study of cyclopropane chemistry.

Bonding Properties of Cyclopropane and Their Chemical Consequences

Organocatalyzed coupling of carbon dioxide with epoxides for the synthesis of cyclic carbonates: catalyst design and mechanistic studies

The shift of paradigm in combinatorial chemistry, from large compound libraries (of mixtures) on a small scale towards defined compound libraries where each compound is prepared in an individual well, has stimulated the search for alternative separation approaches. The key to a rapid and efficient synthesis is not only the parallel arrangement of reactions, but simple work-up procedures so as to circumvent time-consuming and laborious purification steps. During the initial development stages of combinatorial synthesis it was believed that rational synthesis of individual compounds could only be achieved by solid-phase strategies. However, there are a number of problems in solid-phase chemistry: most notably there is the need for a suitable linker unit, the limitation of the reaction conditions to certain solvents and reagents, and the heterogeneous reaction conditions. Further disadvantages are: the moderate loading capacities of the polymeric support and the limited stability of the solid support. In the last few years several new separation techniques have been developed. Depending on the chemical problem or the class of compounds to be prepared, one can choose from a whole array of different approaches. Most of these modern separation approaches rely on solution-phase chemistry, even though some of them use solid-phase resins as tools (for example, as scavengers). Several of these separation techniques are based on liquid-liquid phase separation, including ionic liquids, fluorous phases, and supercritical solvents. Besides being benign with respect to their environmental aspects, they also show a number of advantages with respect to the work-up procedures of organic reactions as well as simplicity in the isolation of products. Another set of separation strategies involves polymeric supports (for example, as scavengers or for cyclative cleavage), either as solid phases or as soluble polymeric supports. In contrast to solid-phase resins, soluble polymeric supports allow reactions to be performed under homogeneous conditions, which can be an important factor in catalysis. At the same time, a whole set of techniques has been developed for the separation of these soluble polymeric supports from small target molecules. Finally, miscellaneous separation techniques, such as phase-switchable tags for precipitation by chemical modification or magnetic beads, can accelerate the separation of compounds in a parallel format.

https://www.chem.ccu.edu.tw/~joyce/referance/gump/IL/Angew.%20Chem.%20Int.%20Ed.%202002,%2041,%203964.pdf

Modern Separation Techniques for the Efficient Workup in Organic Synthesis

The “ligandless” palladium acetate-catalyzed Suzuki cross-coupling reaction of ArX with aryl- and vinylboronic acids in water without organic cosolvent in the presence of tetrabutylammonium bromide is reported. Aryl bromides give high yields and considerably accelerate the coupling. A wide variety of functional groups can be tolerated. Aryl iodides, however, give incomplete conversion and aryl triflate coupling shows no improvement over reported conditions.

Highly Efficient Palladium-Catalyzed Boronic Acid Coupling Reactions in Water: Scope and Limitations.

Preface. Part 1: Basic concepts in phase-transfer catalysis: Phase-transfer-catalyzed reactions Basic steps of phase-transfer catalysis The PTC reaction rate matrix Anion transfer and anion activation Effect of reaction variables on transfer and intrinsic rates Outline of compounds used as phase-transfer catalysts. Part 2: Phase-transfer catalysts -- Fundamentals I: Introduction Structural factors affecting the distribution of anions between aqueous and organic phases Structural factors affecting the distribution of phase-transfer catalyst cations between the aqueous and organic phases Effects of the organic phase polarity on the distribution of phase-transfer cation-anion pairs Effects of changes in organic phase polarity during reaction Factors affecting the distribution of phase-transfer catalyst cation-anion pairs between an organic phase and an aqueous phase containing hydroxide ion Effect of hydration of the transferred anion and the effect of inorganic salt and/or hydroxide concentration in the aqueous phase. Part 3: Phase-transfer catalysis -- Fundamentals II: Introduction Liquid-liquid PTC Solid-liquid PTC. Part 4: Phase-transfer catalysts: Introduction Use of quaternary salts as phase-transfer catalysts Macrocyclic and macrobicyclic ligands PEGs, Tris (3, 6-dioxahepty)amine (TDA-1), and related ethoxylated compounds as phase-transfer catalysts Other soluable polymers and related multifunctional compounds as phase-transfer catalysts Use of dual PTC catalysts or use of cocatalysts in phase-transfer systems Catalysts for transfer of species other than anions Separation and recovery of phase-transfer catalysts. Part 5: Insoluable phase-transfer catalysts: Introduction PTC catalysts bound to insoluable resins Phase-transfer catalysts bound to inorganic solid supports PTC catalysts contained in a separate liquid phase (Third-liquid-phase catalyst). Part 6: Variables in reaction design for laboratory and industrial applications of phase-transfer catalysis: Choice of catalyst Choice of solvent Presence of water Agitation Choice of anion, leaving group, and counteranion Choice of base Guidelines for exploring new PTC applications. Part 7: Phase-transfer catalysis displacement reactions with simple anions: General considerations Behavior of various anions in PTC displacement reactions. Part 8: Phase-transfer catalysis reaction with strong bases: C-Alkylation N-Alkylation O-Alkylation -- Etherification S-Alikylation -- Thioetherification Dehydrohalogenation Carbene reactions Condensation reactions Deuterium exchange, isomerization, and oxidation. Part 9: Phase-transfer catalysis -- Polymerization and polymer modification: Introduction Polymer synthesis Chemical modification of polymers. Part 10: Phase-transfer-catalyzed oxidations: Introduction Permanganate oxidations Oxidations with hyperchlorite and hypobromite PTC oxidations with hydrogen peroxide PTC air or oxygen oxidations Oxidations by persulfates PTC oxidations with nitric acid PTC carbon tetrachloride/sodium hydroxide oxidations PTC oxidations with perborate PTC oxidations with ferrate and ferricyanide PTC oxidations with superoxide PTC electrochemical oxidations PTC oxidations with other oxidents. Part 11: Phase-transfer-catalyzed reductions: Sodium borohydride reductions Lithium aluminum hydride reductions Reductions with sodium formate Reductions with sulfur-containing anions Hydrogenation Reductions with formaldehyde Electrochemical reduction Photochemical reduction Wolff-Kishner reduction Reduction by dodecarbonyltriiron and related species. Part 12: Phase-transfer catalysis -- Chiral phase-transfer catalyzed formation of carbon-carbon bonds: Introduction Alkylation reactions. Part 13: Phase-transfer catalysis - Transition metal cocatalyzed reactions: Introduction Carbonylation and reactions with carbon monoxide PTC reduction and hydrogenation

Phase-Transfer Catalysis: Fundamentals, Applications, and Industrial Perspectives

Phase transfer catalysis: Principles and techniques

Critical to the success of phase-transfer catalytic (PTC) processes are (1) the maximization of the rate of transfer of reactant anions from the aqueous or solid phase to the organic phase, (2) the maximization of the rate of transfer of product anions from the organic phase to the aqueous or solid phase, and (3) the related equilibrium partitioning of the reactant and product anions between the organic and aqueous or solid phases. The common organic solvents employed in phase-transfer processes are usually relatively nonpolar and usually aprotic. Because anions do not have a great affinity for such solvents and prefer to reside in an aqueous environment, the desired transfer is not a particularly favorable process. The transfer of anions from an aqueous to an organic phase, however, may be achieved by choosing a phase-transfer cation that is not strongly solvated by water and that has organic-like characteristics and is thus compatible with the organic phase. For instance, the volume-to-charge ratio (as well as the organic-like nature) of quaternary ammonium and phosphonium salts can be adjusted over a wide range of values by simply changing the length of the alkyl (or aryl) substituents bonded to the quaternary heteroatom. Tetramethylammonium salts are highly soluble in aqueous media and only slightly soluble in most organic solvents, whereas tetradoecylammonium salts are soluble in most organic media but only slightly soluble in water.

Phase-Transfer Catalysis: Fundamentals I

Phase-transfer catalysis (PTC) is a powerful tool in many areas of chemistry. It is a technique for conducting reactions between two or more reagents in two or more phases, when reaction is inhibited because the reactants cannot easily come together. A “phase-transfer agent” is added to transfer one of the reagents to a location where it can conveniently and rapidly react with another reagent. It is also necessary that the transferred species be in a highly active state when transferred; otherwise large amounts of phase-transfer agent will be required. This activation function, plus the transfer function, allows phase-transfer catalysis to occur with only a catalytic amount of phase-transfer agent.

Basic Concepts in Phase-Transfer Catalysis

In previous chapters we learned that a phase-transfer catalyst must have two particular chemical functions to be successful, that is, it must rapidly transfer one of the reactant species into the normal phase of the other reactant, and second it must make the transferred species available in a highly reactive form. The need for one or the other of these two functions to be strongly catalyzed often plays an important role in the selection of the best transfer catalyst for a particular reaction, as, for example, to pick a phase-transfer catalysis (PTC) catalyst that is especially useful for activating an anion, or for a catalyst that is especially good for facilitating anion transfer to the organic phase. Catalyst structure may also be of great significance when more than one product can be formed, to cause one product to be favored over another. Sometimes catalyst structure is not important at all, and almost any PTC catalyst will perform satisfactorily for some reactions, whereas for others it may be desirable to have two (or more) catalysts to facilitate each of the various steps in a reaction sequence. Some examples of the effect of catalyst structure are discussed in this chapter to illustrate these points.

Charles M. Starks

Papers

Phase-Transfer Catalysis: Fundamentals, Applications, and Industrial Perspectives

Phase transfer catalysis: Principles and techniques

Phase-Transfer Catalysis: Fundamentals I

Basic Concepts in Phase-Transfer Catalysis

Phase-Transfer Catalysts