Chiral metal complexes metals

Many chiral metal complexes with Lewis acid properties have been developed and applied to the asymmetric Diels-Alder reaction. High enantioselectivity is, of course, one of the goals in the development of these catalysts. Enantioselectivity is not, however, the only factor important in their design. Other important considerations are ... 

Auxiliary-bourtd Chiral Michael Acceptors and Auxiliary Chiral Metal Complexes... 

A number of enantioselective hydrogenation reactions in ionic liquids have also been described. In all cases reported so far, the role of the ionic liquid was mainly to open up a new, facile way to recycle the expensive chiral metal complex used as the hydrogenation catalyst. 

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general processes should be atom efficient and have low E factors, i.e. involve catalytic methodologies. This is reflected in the increasing attention being focused on enantioselective catalysis, using either enzymes or chiral metal complexes. 

Enantiometrically pure alcohols are important and valuable intermediates in the synthesis of pharmaceuticals and other fine chemicals. A variety of synthetic methods have been developed to obtain optically pure alcohols. Among these methods, a straightforward approach is the reduction of prochiral ketones to chiral alcohols. In this context, varieties of chiral metal complexes have been developed as catalysts in asymmetric ketone reductions [ 1-3]. However, in many cases, difficulties remain in the process operation, and in obtaining sufficient enantiomeric purity and productivity [2,3]. In addition, residual metal in the products originating from the metal catalyst presents another challenge because of the ever more stringent regulatory restrictions on the level of metals allowed in pharmaceutical products [4]. An alternative to the chemical asymmetric reduction processes is biocatalytic transformation, which offers... 

Experimental evidence of the —S03" H0Si— interaction have been obtained from IR, Rh K-edge EXAFS, and CP MAS 3 IP NMR studies. These supported catalysts have been tested for the hydrogenation and hydroformylation of alkenes. No Rh leaching was observed.128-130 An extension to the immobilization of chiral metal complexes for asymmetric hydrogenation is reported below. 

Complexation with Chiral Metal Complexes. This idea was first suggested by Feibush et al.44 The separation is realized by the dynamic formation of diastereomeric complexes between gaseous chiral molecules and the chiral stationary phase in the coordination sphere of metal complexes. A few typical examples of metal complexes used in chiral stationary phase chromatography are presented in Figure 1-13.45... 

Many chiral compounds can be used as selectors, for example, chiral metal complexes, native and modified cyclodextrins, crown ethers, macrocyclic antibiotics, noncyclic oligosaccharides, and polysaccharides all have been shown to be useful for efficient separation of different types of compounds. 

Cyanohydrination (addition of a cyano group to an aldehyde or ketone) is another classic reaction in organic synthesis. Enantioselective addition of TMSCN to aldehyde, catalyzed by chiral metal complexes, has also been an active area of research for more than a decade. The first successful synthesis using an (5,)-binaphthol based complex came from Reetz s group142 in 1986. Their best result, involving Ti complex, gave 82% ee. Better results were reported shortly thereafter by Narasaka and co-workers.143 They showed that by... 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

Highly enantioselective hydroformylation catalyzed by chiral metal complexes has been obtained with only a few catalytic systems. Many chiral phosphorus ligands have been used in Pt(II) and Rh(I) systems in the asymmetric hydroformylation of styrene. The first highly enantioselective examples of the asymmetric hydroformylation of styrene were reported by Consiglio et al. in 1991 and used Pt-Sn systems. ligand 1 achieved an ee of 86% (Fig. 1) [10-12],... 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

When a chiral metal complex forms a complex with a prochiral alkene , either because it contains a chiral ligand or a chiral metal centre, the resulting complex is a diastereomer. Thus, a mixture of diastereomers can form when the chiral complex coordinates to both faces of the alkene. As usual, these diastereomers have different properties and can be separated. Or, more interestingly, in the catalytic reactions below, the two diastereomers are formed in different amounts and their reactivities are different as well. 

Over the last years, one of the most studied DCR has been the asymmetric version of the cycloaddition of nitrones with alkenes. This reaction leads to the construction of up to three contiguous asymmetric carbon centers (Scheme 4). The resulting five-membered isoxazolidine derivatives may be converted into amino alcohols, alkaloids, or p-lactams. Several chiral metal complexes have been used as catalysts for this process [13-15, 18-22]. However, the employment of iridium derivatives is very scarce. 

Typically chiral metal complexes catalyzed asymmetric ring opening of achiral / racemic and chiral epoxides with various nucleophiles conveniently produce enantioenriched 1,2-... 

Kinetic resolution of racemic secondary hydroperoxides rac-16 can be effected by selective reduction of one enantiomer with employing either chiral metal complexes or enzymes (equation 10). In this way hydroperoxides 16 and the opposite enantiomer of the corresponding alcohols 19 can be produced in enantiomerically enriched form. As side products sometimes the corresponding ketones 20 are produced. 

Chiral metal complexes, aUylic alcohol epoxidation catalysis, 394-401 Chiral naphthalene derivatives, singlet oxygen cycloaddition, 271... 

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

The asymmetric fluorination of enolates by means of chiral metal complexes has been reported with Selectfluor in the presence of a chiral Lewis acid derived from TADDOL (TiCl2/TADDOL), or with F-A-sulfonimide (NFSI) with palladium complexes and chiral phosphines. 

Doyle et al. have demonstrated the catalyst-dependent diastereoselectivity in Rh(ii) complex-catalyzed reaction of cinnamyl methyl ether 36 and ethyl diazoacetate 35 (Scheme 6). " The change of the diastereoselectivity of the products 38a and 38b with different Rh(ii) catalyst provides strong evidence that Rh(ii) catalyst is associated with the ylide in the rearrangement process. The moderately high level of asymmetric induction (4-69% ee) is also observed with allyl iodide (Equation (4)). In this case, the chiral metal complex must be in the product-forming step, because free iodo ylide is achiral. 

Asymmetric induction in the ylide formation/[l,2]-shift has also been studied with chiral metal complexes. Katsuki and co-workers examined the reaction of ( )-2-phenyloxetane with 0.5 equiv. of /< //-butyl diazoacetate in the presence of Gu(i) catalyst. With chiral bipyridine ligand 53, trans- and m-tetrahydrofurans 54 and 55 are obtained with 75% and 81% ee, respectively (Equation (6)). This asymmetric ring expansion was applied by the same group to their enantioselective synthesis of translactone. 

Reagent-controlled Asymmetric Reactions 11.10.4.2.1 Use of chiral metal complexes... 

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