The Principle of Phase Transfer Catalysis

It is very difficult and probably of little value to try to separate the work of these three pioneers. At the time Makosza reported the two-phase carbene reaction [11] the process was described in a patent issued to Starks [8]. While Brandstrom was pushing back the frontiers of alkylation in nonpolar homogeneous solution [12], Makosza had alkylated a massive number of carbon acids under catalytic conditions [13]. Hennis (Dow, Midland, Michigan) working in a more limited way, was successfully alkylating carboxylates under conditions favorable for in situ quaternary salt formation [14]. And so it goes. It is, in our opinion, of value to credit and compliment these early workers in the field now known as phase transfer catalysis and draw attention to the important work each, in his own way, accomplished. 

An alternative to the use of a dipolar aprotic solvent is to use a nonpolar medium and a cation solvating additive. The use of beta-diamines to solvate and enhance the reactivity of organolithium compounds is well known and documented [15]. Polyethylene glycol derived bases were known to be self-solvating as early as 1963 [16]. 

A suggestion is recorded in a review in 1965 that the hexamethylether of all cis-inositol should be an anion activator by virtue of cation solvation [17]. 

The principle of specific solvation was well established in the literature by the time phase transfer catalysis became a recognized technique. The great beauty of the phase transfer method, however, resides in the fact that the method is general, mild and catalytic. Phase transfer catalysis utilizing either a quaternary ammonium or phosphonium salt as a catalyst works in the following way. In general, there are two immiscible phases. One of these phases (usually aqueous) contains a reservoir of the 

The exchange of anion is of little import, however, if nothing further than that which is formulated occurs. Not only must the anion which will function as nucleophile be paired with Q , but it must find its way into the organic solution. A second equilibrium is therefore a requirement for phase transfer catalysis to be successful namely the phase transfer equilibrium. This is formulated in equation 1.6. 

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Side Note 3.1. The Principles of Phase Transfer Catalysis... 

Prelab Exercise Propose a synthesis of using the principles of phase transfer catalysis. Why is phase transfer catalysis particularly suited to this reaction ... 

A special tribute is due to Dr. Charles Starks of the Continental Oil Company. By the mid sixties, Starks had formulated the principles of phase transfer catalysis and had applied for patents on many reactions that others were later to examine in somewhat greater detail. His mechanistic model of phase transfer catalysis still stands up well today and is a model for much of the thinking in this area. It is fitting that Starks suggested the name phase transfer catalysis by which the whole field is now known. 

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

These principles of phase transfer catalysis allow manyfold applications in orga-nometallic chemistry and, indeed, numerous studies have been published. The following sections concentrate on five subfields that have come to a certain maturity but quite a few other types have been explored. 

This chapter will focus on the basic principles of phase transfer catalysis with major attention devoted to catalysis at liquid interfaces, the topic of this book therefore, important aspects of PTC such as solid-liquid PTC or gas-liquid PTC will only be touched on. 

We have discussed above in general terms the requisites and expectations associated with the basic principles of phase transfer catalysis. In fact, these ideas accord well with what is now known about the mechanism of many phase transfer processes. Detailed work has been carried out by several groups, and their conclusions are in substantial agreement. 

This review will focus on the use of phase transfer catalysis for the chemical modification of pol3nners which already contain reactive groups. As numerous excellent reviews (Ref. 8) of phase transfer catalysis exist, the basic principles of the method will not be included here. In addition no attempt will be made to ensure encyclopedic coverage of the field but selective coverage will be given to areas with which this author is most familiar. 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

With the probable exception of Wohler s isomerization of ammonium isocyanate to urea, most of the major advances in organic chemistry have been preceded and presaged by a number of diverse and perhaps minor advances. In some cases the early work was recognized, extended and built upon. In other cases, early examples of particular phenomena were recognized only after a general statement of principles had been offered. Phase transfer catalysis (ptc) is a major advance which is preceded by earlier examples of related phenomena but most if not all of these examples were recognized as examples of phase transfer catalysis only after the principles had coalesced in several minds across the world. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

A crosslinked rubber particle can be considered as a viscous hydrocarbon phase. In principle, phase transfer catalysis should apply to the chemistry of inorganic ions within such particles,... 

Replacement of the organic phase with surfactants to exploit micellar phase transfer catalysis principles (Battal et al., 1997) for the alkylation of phenol and aniline. This group had previously demonstrated the synthesis of a surfactant by micellar autocatalysis, whereby the surfactant product itself catalyses the reaction (Kust and Rathman, 1995). 

Examination of the passive control conditions in Eqs. (5.26)-(5.29) shows that there are two values of the sum of phase angles for which zero transfer occurs. In principle, then, one can simultaneously block the transfer of, say, the energy and select the direction of the transfer of the population. One particularly interesting case is the definition of the phase angles for zero total power absorption. Since no energy is absorbed or emitted from the field these conditions define laser catalysis [44]. 

The principles underlying the N- alkylation of indoles are the same as those for pyrroles (67T3771). Development of synthetic techniques for maximizing yields has resulted in procedures using dipolar aprotic solvents, crown ether and phase transfer catalysis, as well as reactions in liquid ammonia. These techniques are illustrated by some representative examples given in Table 8. 

Phase-transfer catalysis is an expedient preparative route for oxidative additions. Thus compound XIa ( / -Cp)Mo(CO)2Cp- is obtained in 95% yield under phase-transfer conditions (5M NaOH, [PhCHjNEtsJCl, benzene). The starting material, -CpMoCl(CO)3, is reduced in situ. The reduction is induced by a hydroxide attack on a coordinated CO (the reducing agent), and a transient species is formed that, upon decarboxylation, yields the reduced, active species [cf. the principles of Eq. (c)]. 

Phase-transfer catalysis (PTC) is the most widely used method for solving the problem of the mutual insolubility of nonpolar and ionic compounds. Basic principles, synthetic uses, industrial applications of PTC, and its advantages over conventional methods are well documented [1-3]. PTC has become a powerful and widely accepted tool for organic chemists due to its efficiency, simplicity, and cost effectiveness. The main merit of the method is its universality. It may be applied to many types of reactions involving diverse classes of compounds. An important feature of PTC is its computability with other methods for the intensification of biphasic reactions (sonolysis, photolysis, microwaving, etc.) as well as with other types of catalysis, in particular, with transition-metal-complex catalysis. Homogeneous metal-complex catalysis under PTC conditions involves the simul-... 

The kinetics and electronic mechanisms of conventional chemical catalysts- are contrasted with those in enzymes. The analogy between certain attributes of surfactants and phase-transfer catalysis and enzyme active sites are made and the limitations of surface catalysts and zeolites are pointed out. The principle features that give enz3nnes their unusual rate enhancements and remarkable specificity are discussed and ways in which these can be realized in man-made catalysts are proposed. The catalytic activation of CO2 by both enzymatic and non-enzymatic means, including a detailed analysis of the electronic reaction sequence for the metalloenzyme carbonic anhydrase, is used to illustrate the above themes. 

Asymmetric phase-transfer catalysis had a very slow development, although the principle is quite simple. A two-phase system is involved, usually liquid/liq-uid (water organic solvent), with a water-soluble base and reactants in the organic phase. A phase-transfer catalyst such as a quaternary ammonium salt with a good balance of hydrophilicity and hydrophobicity transfers the base to the or-... 

Fig-1 General principle of thermoregulated phase-transfer catalysis. The mobile catalyst transfers between the aqueous phase and the organic phase in response to temperature changes. 

The principle of thermoregulated phase-transfer catalysis (TRPTC), originally developed by Bergbreiter et al. [12], which has been applied to two-phase hydro-formylation by Fell, Jin and co-workers [13], which is based on a temperature-controlled switch of the catalyst system from the aqueous phase to the organic phase (see Section 4.6.3). 

In the chemical industry (on the mega- as well as the micro-scale) fine emulsions have many useful applications in, e.g., extraction processes or phase transfer catalysis. Additionally, they are of interest for the pharmaceutical and cosmetic industry for the preparation of creams and ointments. Micromixers based on the principle of multilamination have been found to be particularly suitable for the generation of emulsions with narrow size distributions [33]. Haverkamp et al. showed the use of micromixers for the production of fine emulsions with well-defined droplet diameters for dermal applications [38]. Bayer et al. [39] reported on a study of silicon oil and water emulsion in micromixers and compared the results with those obtained in a stirred tank. They found similar droplet size distributions for both systems. However, the specific energy required to achieve a certain Sauter mean diameter was 3-1 Ox larger for the macrotool at diameters exceeding 100 pm. In addition, the micromixer was able to produce distributions with a mean as low as 3 pm, whereas the turbine stirrer ended up with around 30 pm. Based on energy considerations, the intensification factor for the microstirrer appears to be 3-10. 

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