Metal complexes, phase-transfer

Many polypodands having terminal donor groups are also available, e.g. (39)-(41).33 These also exhibit cation complexation and phase transfer properties. KMn04 and aqueous alkali metal picrates are much more readily taken into organic phases in the presence of these podands than with dibenzo-18-crown-6.239 The KSCN complex of (39) exhibits a novel coordination geometry as all 10 donor atoms participate in coordination of the metal cation, and in order to do this the three arms wrap around the cation in a propeller-like fashion.240... 

Edgars Abele gained his habilitation in 1999 and is at present the head of the Modern Catalysis Methods Group of the Latvian Institute of Organic Synthesis in Riga. His areas of interest include the investigation of transition metal complexes and phase transfer catalyzed reactions. 

Alcohols can be dehydrogenated to carbonyl compounds by exposure to a catalytic amount of a rhodium(I) complex under phase-transfer conditions. This reaction is particularly useful for benzylic alcohols such as 1-phenylethanol (50) which gave acetophenone (51) in 78% yield using chlorodicarbonylrhodium(I) dimer as the metal catalyst and benzyltrieth-ylammonium chloride or Aliquat 336 as the phase-transfer catalyst (52). 

H. Alper, Phase transfer reactions catalyzed by metal complexes, 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 2, p. 8. 

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Quite recently, Okahara and his coworkers have extended their method to the formation of long chain N-alkylmonoazacrowns. It was expected that such compounds as N-decylmonoaza-18-crown-6 may be useful as new surfactants with complexing ability with metal salts, phase transfer catalysts and selective ion carriers . ... 

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. 

Generally, stable and well-dispersed metal NPs have been prepared in ILs by the simple reduction of the M(I-IV) complexes or thermal decomposition of the organometallic precursors in the formal zero oxidation state. Recently, other methods such as the phase transfer of preformed NPs in water or organic solvents to the IL and the bombardment of bulk metal precursors with deposition on the ILs have been reported. However, one of the greatest challenges in the NPs field is to synthesize reproducibly metal NPs with control of the size and shape. Selected studies of the preparation of metal NPs in ILs that, in some cases, provide NPs with different sizes and shapes are considered in this section. 

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... 

Belokon et al. (260) reported that Cu(II) complex 426 may function as a chiral phase-transfer catalyst, although enantioselectivities are poor reaching a maximum of 22% based on optical rotation, Eq. 224. The authors suggest that the metal serves to carry the OH as a ligand into the organic phase. 

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