Phase transfer catalysis, applications developments

Phase transfer catalysis, applications in heterocyclic chemistry, 36, 175 Phenanthridine chemistry, recent developments in, 13, 315 Phenanthrolines, 22, 1 Phenothiazines, chemistry of, 9, 321 Phenoxazines, 8, 83 Photochemistry of heterocycles, 11, I of nitrogen-containing heterocycles, 30, 239... 

With the discovery of the crowns and related species, it was inevitable that a search would begin for simpler and simpler relatives which might be useful in similar applications. Perhaps these compounds would be easier and more economical to prepare and ultimately, of course, better in one respect or another than the molecules which inspired the research. In particular, the collateral developments of crown ether chemistry and phase transfer catalysis fostered an interest in utilizing the readily available polyethylene glycol mono- or dimethyl ethers as catalysts for such reactions. Although there is considerable literature in this area, much of it relates to the use of simple polyethylene glycols in phase transfer processes. Since our main concern in this monograph is with novel structures, we will discuss these simple examples further only briefly, below. 

The first examples of the application of phase-transfer catalysis (PTC) were described by Jarrousse in 1951 (1), but it was not until 1965 that Makosza developed many fundamental aspects of this technology (2,3). Starks characterized the mechanism and coined a name for it (4,5), whilst Brandstrom studied the use of stoichiometric amounts of quaternary ammonium salts in aprotic solvents, "ion-pair extraction" (6). In the meantime Pedersen and Lehn discovered crown-ethers (7-9) and cryptands (10,11), respectively. 

Phase transfer catalysis (PTC), or more generally, applications of two-phase systems, is one of the most important recent methodological developments in organic synthesis. It is important because it simplifies procedures, eliminates expensive, inconvenient, and dangerous reactants and solvents, and also allows one to perform many reactions that otherwise proceed unsatisfactory or do not proceed at all. PTC has been reviewed,1-12 but only one review concerns the chemistry of heterocyclic compounds.13... 

Thus, dichloro- or dibromomethane in the presence of sodium hydride in solution in N,N-dimethylformamide gives O-methylene derivatives [73,74], Other conditions are also possible, for instance use of potassium hydroxide and dimfethylsulfoxyde [75], but an interesting development is the application of the phase-transfer catalysis technique, by which dibromomethane and sodium hydroxide in water, in the presence of an appropriate ammonium salt, leads to a cis-23-O-methylenation of methyl-4,6-O-benzylidene-a-D-mannopyranoside, [76] and simitar conditions afford the Other examples have been published [78]. 

The aim of this book is to provide a concise and comprehensive treatment of this continuously growing field of catalysis, focusing not only on the design of the various types of chiral phase-transfer catalyst but also on the synthetic aspects of this chemistry. In addition, the aim is to promote the synthetic applications of these asymmetric phase-transfer reactions by giving solid synthetic evidence. Clearly, despite recent spectacular advances in this area, there is still plenty of room for further continuous development in asymmetric phase-transfer catalysis. 

One of the major developments in organic chemistry during the past 15 years has been the application of phase-transfer catalysis to synthesis. These reactions are often effected in an aqueous base-organic two-phase system with an ammonium or phosphonium salt or crown ether as the catalyst. Crown ethers have also been of great utility as catalysts for solid-liquid phase-transfer processes. Some of the more attractive fea-... 

Several good reviews (3-10) and books (11,12) have been published on phase-transfer catalysis, should the reader desire a more detailed examination of the phase-transfer process. Although hundreds of examples of the application of phase-transfer catalysis to organic chemistry have appeared in the literature (13), there were, prior to 1976, no such examples in organometallic chemistry despite acceptance of the pivotal role played by organometallic anions in many stoichiometric and catalytic reactions. Since the first publication on organometallic phase-transfer catalysis (14), the field has developed sufficiently rapidly to justify an account at this time. A brief review was published by Cassar (15), and another by the same author is in press. 

Two novel methodologies termed assymmetric phase transfer catalysis and thermoregulated phase transfer catalysis have been developed readily in the past decade and have broadened greatly the scope of application of PTC. Therefore, it is worthwhile briefly discussing these two techniques. 

The introduction of dipolar aprotic solvents " and the discovery of macrocyclic (crown ethers and macrobicyclic polyethers (cryptands represent some of the more significant steps. Dating from the late sixties, a new general technique was developed phase-transfer catalysis (PTC). PTC has the advantages of being extremely simple and economical and so met with immediate success in industrial applications. 

Phase-transfer catalysis is one of the most practical synthetic methodologies because of its operational simplicity and mild reaction conditions, which enable applications in industrial syntheses as a sustainable green chemical process. As reviewed in this chapter, diverse Cinchona alkaloid-derived quaternaiy ammonium salts have been developed via the modification of Cinchona alkaloids based on steric or electronic factors as highly efficient chiral PTC catalysts and successfully applied in various asymmetric organic reactions. Despite the successful development and application of these catalysts, some problems remain to be addressed. Although Cinchona alkaloids have unique structural features, resulting in the availability of four... 

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