Mechanistic aspects, phase-transfer

Reactions performed under two-phase conditions are further complicated by the partitioning of the reactants and catalyst over the two phases. In the case of quaternary ammonium phase-transfer catalysis, the mechanistic aspects have received a great deal of attention (Brandstrom, 1977 Makosza, 1975 Starks and Owens, 1973). In contrast, the mechanism of crown ether-type phase-transfer catalysis has hardly been investigated at all, despite its... 

Rasmussen and co-workers. Chapter 10, have shown that many free-radical polymerizations can be conducted in two-phase systems using potassium persulfate and either crown ethers or quaternary ammonium salts as initiators. When transferred to the organic phase persulfate performs far more efficiently as an initiator than conventional materials such as azobisisobutyronitrile or benzoyl peroxide. In vinyl polymerizations using PTC-persulfate initiation one can exercise precise control over reaction rates, even at low temperatures. Mechanistic aspects of these complicated systems have been worked out for this highly useful and economical method of initiation of free-radical polymerizations. 

Mechanistic Aspects of Phase-Transfer Free Radical... 

The Basic Principle of Phase-Transfer Catalysis and Some Mechanistic Aspects... 

Despite the development of phase-transfer catalysis in organic synthesis, the mechanistic aspects of phase-transfer catalysis remain obscure, due mainly to the... 

The pyridinyl- and 1-oxypyridinyl-substituted silanes and siloxanes were patented as IPTC catalysts in transacylation reactions [178]. In the IPTC nucleophilic substitution reaction of benzoyl chloride with KSCN catalyzed by cyclic and acyclic sulfides such as tetrahydrothiophene and diethyl sulfides, etc., the active ionic intermediate, benzylsulfo-nium ion, formed by benzyl chloride and sulfide in the organic phase, transferred into the aqueous phase to react with thiocyanate ion to produce benzylthiocyanate [179]. In the following discussion, selected IPTC systems are presented, focusing on the kinetic and mechanistic aspects. 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

The difference between well-known SCF antisolvent techniques such as GAS, PCA, and SEDS usually can be attributed to the specific nozzle mixing (or dispersing) technique involved. Enhanced mass and heat transfer can also be achieved by using mechanical and ultrasonic mixers and ultrafast jet expansion techniques. There are new developments for particle formation by means of dispersed systems such as emulsions, micelles, colloids, and polymer matrixes. It should be emphasized that all these processes involve the same fundamental aspects of mass and heat transfer phenomena between an SCF and a subcritical phase. Clearly the ultimate goal of all SCF particle technologies is to achieve predictable, consistent, and economical production of fine pharmaceuticals or chemicals. This is possible only on the basis of comprehensive mechanistic understanding and well-developed scale-up principles. 

The photochemistry of carbonyl compounds has been extensively studied both in solution and in the gas phase. It is not surprising that there are major differences between the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collisions, whereas in the liquid phase, the energy is rapidly transferred to the solvent or to other components of the solutions. Solution photochemistry will be emphasized here, since most organic chemists interested in either mechanistic studies or preparative photochemistry have studied this aspect of the problem. 

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