Chiral catalysis complexes

Using these procedures, many chiral diaminocarbene-transition metal complexes have been synthesized but only a few of them have been used for asymmetric catalysis. The chiral complexes which were isolated but did not receive any application in asymmetric catalysis, are presented at the end of the chapter. 

AT-heterocyclic carbene complexes of Pd(II) or Pd(0) were extensively used in various reactions and several groups have reported syntheses of chiral complexes [5]. However, only a few examples of asymmetric catalysis are... 

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

Gigante, B. Corma, A. Garcia, H. Sabater, M. J. (2000) Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes Catalysis Lett. 68 113-119. 

Molecular sieves, chiral metal complex catalysis, 395... 

Lanthanoid Complexes, Chiral, Asymmetric Catalysis with (Shibasaki and... 

Weak acid-base chiral complex formation represents hydrogen bond catalysis (see Chapter 9) and deprotonation followed by cation/anion association under homogeneous, and also under phase-transfer conditions (see Chapter 4) [14, 65],... 

The principle of enantioselective catalysis (shown in simple terms in Figure 3) uses a chiral complex catalyst which binds the substrate to give different diastereoisomeric intermediates from which different enantiomeric products are formed. The chiral information is generally placed on a ligand. The final... 

The individual chemical species with chiral catalytic properties, such as complex, organometallic compounds, organic ligands or molecules, anchored or grafted into the channels of microporous and mesoporous materials, and some microporous compounds possessing chiral channels or their pore structures composed of the chiral motifs, all promise further development and potential application in microporous chiral (asymmetric) catalysis and separations. It is an important frontier direction in the zeolite catalytic field at present. Therefore, the synthesis and assembly of chiral microporous compounds and materials are of particular interest for researchers engaged in porous materials. This is a research field in rapid development. 

Asymmetric catalysis is one of the most economical processes for the production of chiral compounds, considering the high turnover levels of most homogeneous catalysts and the fact that the optically active catalyst introduces its chiral information during each new catalytic cycle. The asymmetric catalyst molecules are mainly synthesized by coordination of optically active ligands to a metal rather than resolution of complexes in which the optical activity lies at the metal, and which are prone to racemization. These chiral complexes involve only a few metals. 

These complexes also catalyze the enantioselective cyclopropanation of monosubstituted alkynes with bulky diazoesters [939, 1497] (Figure 7.87). Attempts at double diastereodifferentiation ( 1.6) have been carried out in the reaction of styrene with diazoesters of chiral alcohols under chiral Rh-3.61 complexes catalysis [939,1501], but disappointing selectivities were observed. 

If nucleophilic attack constitutes the stereoselective step, special substitution patterns of the intermediate rc-allylpalladium complex are suitable for enantioselective catalysis, Racemic substrates with identical substituents in positions 1 and 3 give rise to chiral complexes incorporating a meso-n- y ligand. In this case, enantioselectivity is determined by discrimination of the nucleophile for the diastereotopic termini of the allylic system. 

BINAP is a bidentate hgand, very highly chelating towards metal centres, particularly palladium (II). Obtaining such chiral complexes is of great interest in the field of enanti-oselective catalysis which will be considered in Chapter 3. 

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