Metal complexes, phase-transfer catalysis

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

This chapter focuses exclusively on microwave heterogeneous catalysis. Microwave homogeneous catalysis by transition metal complexes is treated in Chapt. 11, phase transfer catalysis in Chapt. 5, catalytic reactions on graphite in Chapt. 7, photocataly-tic reactions in Chapt. 14, and catalytic synthesis oflabeled compounds in Chapt. 13. 

We have exploited this base catalysis of the oxygen exchange process to effect oxygen lability in the less electrophilic carbonyl sites of neutral metal carbonyl species. Because [MCOOH] intermediates are readily decarboxylated in the presence of excess hydroxide ion, in order to observe oxygen exchange processes in neutral metal carbonyl complexes it was convenient to carry out these reactions in a biphasic system employing phase transfer catalysis () (16, 17. 18). Under conditions (eq. 7) the... 

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],... 

In conclusion, phase transfer catalysis is a method of considerable potential for metal complex catalyzed reduction, oxidation and carbonylation reactions. 

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). 

The term Counter Phase Transfer Catalysis (CPTC) was coined by Okano214,215 to describe biphasic reactions catalysed by water soluble transition metal complexes which involve transport of an organic-soluble reactant into the aqueous phase where the catalytic reaction takes place. Similarly, Mathias and Vaidya564,565 gave the name Inverse Phase Transfer Catalysis to describe this kind of biphasic catalysis which contrasts with classical Phase Transfer Catalysis where the reaction occurs in the organic phase and does not involve formation of micelles.389,564... 

In the following sections, progress made in asymmetric phase-transfer catalysis using chiral crown ethers, taddolates, Nobin and metal(salen) complexes is surveyed. Each section is further subdivided according to the reaction being catalyzed. 

These reactions are often reversible and depend on temperature, concentration, and nature of the base. In addition, since this reaction involves changing from a metal-metal bonded system that is soluble in organic solvents to an ionic complex that is water soluble, the solubility patterns change greatly. Phase transfer catalysis see Phase Transfer... 

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-... 

Scrutiny of the literature data shows that numerous transformations of diverse organic molecules are readily catalyzed by homogeneous metal complexes in two-phase systems. The joint use of transition metals and phase-transfer agents considerably expands the synthetic possibilities of both metal-complex catalysis and PTC. The combination of these two different types of catalysts in many cases leads to higher reaction rates, higher product yields, milder conditions, higher selectivities, and simpler reaction execution and product isolation. Biphasic reactions in the presence of transition-metal complexes and PT agents have been... 

This is a case where chiral and achiral metal complexes compete with each other (>600 1 by rate). Asymmetric autocatalysis is certainly an attractive, yet still largely unexplored field. Two-phase processes for stereoselective syntheses are under investigation [38], and phase transfer catalysis must be mentioned in this context. 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

Hj/CO) phase transfer catalysis supported metal complexes, liquid biphasic catalysis C0/H,0... 

Other interesting systems have been employed, such as CO/HjO (water gas) or CO/Hj (syngas) as reducing mixtures [49, 50], phase transfer catalysis [37], and more recently, aqueous [46, 47] and non-aqueous ionic liquid [48] biphasic catalysis which offer more promise for practical uses. Some interesting examples of metal complexes grafted onto oxides [55, 56] or supported metals [38, 39] as arene hydrogenation catalysts have been provided. 

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. 

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