Applications of phase-transfer catalysis

Application of phase-transfer catalysis in heterocyclic chemistry 99MI33. 

M Makosza at the Technical University in Warsaw led the field in a study of the application of phase-transfer catalysis, particular into the generation of dihalocarbenes. 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

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. 

The first application of phase transfer catalysis in metal carbonyl chemistry was reported by Alper in 1977(23). It was found that metal carbonyl anions could be readily generated by this technique and used to prepare pi-allyl, cluster, and ortho-metalated complexes(24). 

Applications of Phase Transfer Catalysis in Heterocyclic Chemistry... 

Applications of Phase Transfer Catalysis in Heterocyclic Chemistry Roger J. Gallo, Mieczyslaw Makosza, Henri J.-M. Dou,... 

R. J. Gallo, M. Makosza, H. J.-M. Dou, and P. Hassanaly present an authora-tive review of applications of phase-transfer catalysis to heterocyclic chemistry that will be of great benefit to all involved with the synthesis of heterocycles. 

Dehmlow, E.V., Thieser, R., Zahalka, H.A. and Sasson, Y. (1985) Applications of phase transfer catalysis, part 30. The... 

Freedman HH (1986) Industrial applications of phase transfer catalysis (PTC) past, present and future. Pure Appl Chem 58(6) 857-868... 

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. 

All these observations were empirical, individual results of unsystematic experiments. Since water had been judged, as mentioned, to be incompatible with the metal carbonyl catalysts of the oxo process, this solvent was not a seriously considered alternative. This paper points the way to the introduction of water as a future-oriented solvent for industrial homogeneous catalysis. Applications of phase transfer catalysis will not be considered here (since they require additional, cost-increasing phase transfer agents), but the emphasis will be placed on aqueous biphasic homogeneous catalysis and its status and possibilities. 

Section 7 reviews non-aqueous biphase processes and their variations. Sections 4.5 and 4.6 deal with micellar systems and various applications of phase transfer catalysis in relation to aqueous biphase catalysis. Interestingly, biphase techniques are also being utilized from the other side, that of heterogeneous catalysis [35]. 

Shenoy, G., and D. Rangnekar, Synthesis of Desylesters by the Application of Phase Transfer Catalysis and a Study of theRuores-... 

In general it can be stated, that although there are numerous industrially important applications of phase-transfer catalysis, only a few mathematical models involving explicitly mass transfer phenomena have been reported. 

There are hundreds of commercial applications of phase-transfer catalysis and they were commercialized due to the competitive advantages which they truly provide. Following is only a selection of industrial processes using PTC. 

A) Synthesis of ether compound catalyzed bv quaternary ammonium salts (TSiPTCl One of the most useful synthesis applications of phase transfer catalysis (PTC) is in the preparation of ether according to the following general equation... 

Many studies of asymmetric chemical conversions through the catalysis of Cinchona-derived PTC catalysts have been performed to expand the application of phase-transfer catalysis to various organic reactions. In addition to the reactions classified above, some selected examples of asymmetric phase-transfer reactions are shown below. 

Since the discovery of phase transfer type phenomena in the late 1960s by workers such as Starks [2], Brandstrom [3], and Makosza [4], there has been a flood of papers and patents [5], and several books [6]. This chapter will briefly outline the concepts and applications of phase transfer catalysis, and will highlight a few of the more interesting recent advances in the subject. This can only be a distillation of the thousands of publications already produced on the subject, and the interested reader should note that there are already several excellent reviews of the subject [6, 7, 8]. 

In the first example of the application of phase-transfer catalysis to early-transition metal organometallic chemistry, reaction of (q -cyclopentadienyOtetracarbonylvanadium with 5 N sodium hydroxide, in benzene and in the presence of tetrabutylammonium hydrogen sulphate as a phase-transfer catalyst, afforded the (q -cyclopentadienyl)tricarbonylhydridovanadate anion (Scheme 8) [75] ... 

---