Asymmetric Reactions in Organic Solvents

In this chapter, recent development of rare earth metal-catalyzed organic transformations, mainly highly controlled catalytic asymmetric reactions, in organic solvents and in aqueous media are highlighted. For more or other examples using rare earth metal catalysts, see reviews and references cited therein. ... 

MTO [methyltrioxorhenium(VII), cf. Chapter 3.3.13] can be used as a catalyst for the epoxidation of olefins with urea hydroperoxide in [EMIMJBF4 [19]. The activity is reported to be comparable with the reaction in organic solvents but side reactions are suppressed. The use of an ionic liquid as a co-solvent in CH2CI2 for the enantioselective Mn-salen complex-catalyzed epoxidation of olefins with Na(OCl) was reported to result in enhanced reaction rates at no loss of enantioselectivity [20]. Cr-salen complexes can further be used for the asymmetric kinetic resolution of epoxides by ring-opening with azide [21]. 

The first example of the use of rare earth metal complexes for asymmetric catalysis in organic solvents was reported in 1983 in chiral europium-catalyzed hetero Diels-Alder reactions. As for scandium catalysts, the first chiral catalyst was reported in 1994. Diels-Alder reactions using a chiral catalyst prepared from Sc(OTf)3, (/ )-BINOL, and an amine afforded the desired products in up to 97% ee. Following these results, many chiral rare earth metal catalysts have been developed. 

Chiral Induction.—As with many synthetic techniques in organic chemistry, PTC methods have been under investigation to determine whether asymmetric induction (chiral selection) can be achieved. In this case chiral onium salts are necessary, and the systems investigated to date have been quaternary derivatives (23) of A -methyl ephedrine. These salts have been used to catalyse two-phase metal boro-hydride reductions of ketones, and the chiral tetra-alkylammonium borohydride ion pair does accelerate this reaction " in organic solvents. However, either no asymmetric reduction is observed, " or low optical purity of products is achieved (less than 15% enantiomer excess). - A low chiral selectivity is also observed in... 

It seemed likely that mechanism B would produce a symmetrical trigonal bipyramid and thus lead to racemization if an optically active substrate was used in the reaction. Mechanism A would certainly lead to total racemization, but mechanism C would not cause loss of optical activity. On the other hand it is possible to draw an asymmetric trigonal pyramid or an asymmetric tetragonal pyramid as a five-coordinate intermediate in mechanism B. The latter seem unlikely in organic solvents with weak donor properties. Furthermore, recent evidence suggests a symmetric trigonal pyramid as an intermediate in the racemization of trisacetylacetonate (50). 

The structures of the polymers obtained according to Scheme 3.6 are shown in an idealised form since reactions can actually take place at a variety of positions on the starting tetramine monomer [28]. The polymers are characterised by a high solubility in organic solvents (NMP, DMF, DMAC, DMSO, w-cresol, TCE, chloroform and cyclohexanone) resulting from an asymmetric polymer... 

Very recently, Belokon and North have extended the use of square planar metal-salen complexes as asymmetric phase-transfer catalysts to the Darzens condensation. These authors first studied the uncatalyzed addition of amides 43a-c to aldehydes under heterogeneous (solid base in organic solvent) reaction conditions, as shown in Scheme 8.19 [47]. It was found that the relative configuration of the epoxyamides 44a,b could be controlled by choice of the appropriate leaving group within substrate 43a-c, base and solvent. Thus, the use of chloro-amide 43a with sodium hydroxide in DCM gave predominantly or exclusively the trans-epoxide 44a this was consistent with the reaction proceeding via a thermodynamically controlled aldol condensation... 

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