Phase-transfer catalysis in organometallic

Y. Goldberg, Phase Transfer Catalysis Selected Problems and Applications, Gordon and Breach, Yverdon, 1989. Specifically Chapters 3 and 4 Phase transfer catalysis in organometallic chemistry, and Metal-complex catalysis under phase transfer conditions, p. 127. 

Alper, H., Phase Transfer Catalysis in Organometallic Chemistry, Adv. Organomet. Chem., 19, 183 (1981). 

One of the most interesting and useful applications of phase transfer catalysis in organometallic chemistry is the catalytic carbonylation reaction of alkyl halides [50]. Palladium tetrakis triphenylphosphine catalyzes the carbonylation of benzyl chloride in a two phase system containing xylene and aqueous sodium hydroxide according to equation 9.21. Phenylacetic acid (4000 moles/mole Pd) is produced in... 

Jin Z, Zheng X (1996) Thermoregulated Phase-transfer Catalysis. In CornUs B, Hermann WA (eds) Aqueous-Phase Organometallic Catalysis, Chap. 4, Sect. 6.3. Wiley, Weinheim, p 233... 

Molecular Clefts and Tweezers, p. 887 Organometallic Anion Receptors, p. 1006 Phase-Transfer Catalysis in Environmentally Benign Reaction Media, p. 1042 Siderophores, p. 1278 Swfactants, Part 1 Fundamentals, p. 1458 Surfactants, Part 11 Applications, p. 1470 The Template Effect, p. 1493... 

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

A comparatively recent and extremely valuable extension of phase-transfer catalysis is to be found in its application to the chemistry of metal carbonyls, which crosses the boundaries of inorganic chemistry, through organometallic chemistry, and into organic chemistry. 

Activated mercaptans undergo desulfurization to hydrocarbons using cobalt carbonyl or triiron dodecacarbonyl as the metal complex, and basic phase transfer conditions (5 ). Acidic phase transfer catalysis has been little investigated, the first example in organometallic chemistry being reported in 1983 (reduction of diarylethylenes)( ). When acidic phase transfer conditions (sodium 4-dodecylcenzenesulfo-nate as the phase transfer catalyst) were used for the desulfurization of mercaptans [Fe3(CO)] 2 the metal complex],... 

Phase transfer catalysis has more recently been applied to the important area of organometallic chemistry(18). Reported applications include both the preparation(19) and the use of transition metal catalysts in isomerizations(20), carbonylations(21) and reductions(22). 

Classical Phase Transfer assisted Organometallic Catalysis588"601 is a further important field which has found industrial applications e.g. in the carbonylation of benzyl chloride to phenylacetic acid using NaCo(CO)4/Bu4NBr catalysts in aqueous NaOH practiced by Montedison.52,466,589,596,601 However, a detailed discussion of classical phase transfer catalysis is beyond the scope of this chapter which is devoted to systems in which the catalytic conversion takes place in the aqueous phase. 

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. 

The first experiments which were carried out in the author s laboratory on organometallic phase-transfer catalysis were concerned with the reduction of nitrobenzenes (4) to anilines (5) by triiron dodecacarbonyl. Such a conversion was reported to occur in benzene containing methanol at reflux for 10-17 h, with the hydridoundecacarbonyltriferrate anion as the likely key intermediate (16). It was our expectation that the trinuclear iron hydride should be generated by phase-transfer catalysis and if so, effect reduction of nitro compounds (4) under exceedingly mild conditions. Indeed this was the case, as illustrated by the results shown in Table I (17). Not only is the reaction complete in 2 h or less using sodium hydroxide as the aqueous phase, benzene as the organic phase, and benzyltrieth-ylammonium chloride as the phase-transfer catalyst, but it occurs at room temperature and requires less metal carbonyl than when the reaction was... 

Phase-transfer catalysis is a versatile and economical synthetic method in both stoichiometric and catalytic organometallic chemistry. New reactions await discovery, and many of those already reported require detailed mechanistic examination. The prospects for this area are excellent. 

To determine the mode of operation (see below) it is useful to group these homogeneous catalysts into aqueous biphasic systems (see Section 3.1.1.1) and nonaqueous biphasic systems (see Section 3.1.1.2). Gas-liquid-liquid reactions are also involved in organometallic phase transfer catalysis, e. g., in biphasic carbonylation of benzyl chloride to phenylacetic acid by the catalyst system NaCo(CO)4/Bu4NBr/NaOH [21]. Here, the biphasic system consists of an organic solvent and aqueous alkali. 

Goldberg Y, Alper H (1996) Phase-Transfer Catalysis and Related Systems. In Cornils B, Herrmann WA (eds) Applied Homogeneous Catalysis with Organometallic Compounds, VCH, Weinheim p 844... 

Finally, extraction of the important reactive species can be executed in the opposite direction, from organic phase to water. This is called inverse phase-transfer catalysis. Catalysts for such processes are mostly cyclodextrins or modified derivatives thereof. Relatively few applications of this type of PTC have been published. Whereas the present section is concerned only with the organic phase as the location of the proper chemical reaction, important contributions of inverse PTC toward organometallic catalysis are detailed in Section 4.6.2. 

R. A. Sawicki, Triphase Catalysis in Organometallic Anion Chemistry, in Phase Transfer Catalysis New Chemistry Catalysts, and Applications (Ed. Ch. M. Starks), ACS Symposium Ser. No. 326, American Chemical Society, Washington, DC, USA, 1987, Chapter 12, p. 143. 

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