Meso catalysts chiral reagents

Next to the above presented use of SiCl for the in situ preparation of a Lewis acid catalyst with a Lewis base for the aldol reaction, it is possible to apply this compound as a reagent in the ring opening of epoxides leading to chlorinated alcohols. Denmark [104] reported that the chiral phosphoramide 38 catalyzed the asymmetric ring opening reaction of meso-epoxides in the presence of tetrachlo-rosilane. Similar examples were provided by Hashimoto in 2002 [105], applying the A -oxide 39 as catalyst (Scheme 30). 

Precursor of Useful Chiral Ligands. OPEN is widely used for the preparation of chiral ligands. Organometallic compounds with these ligands act as useful reagents or catalysts in asymmetric induction reactions such as dihydroxylation of olefins, transfer hydrogenation of ketones and imines, Diels-Alder and aldol reactions, desymmetrization of meso-diols to produce chiral oxazolidinones, epoxidation of simple olefins, benzylic hydroxylation, and borohydride reduction of ketones, imines, and a,p-unsaturated carboxylates. ... 

NaBr/H20, LiBr on Amberlyst-15 resin, TiCU-LiCl, " SiCL, I2 with a Sml2 catalyst, and Lil on silica gel. Epoxides can be converted directly to 1,2-dichloro compounds by treatment with SOCI2 and pyridine, or with Ph3P and CCl4. These are two-step reactions a halohydrin is formed first and is then converted by the reagents to the dihalide (10-48). As expected, inversion is found at both carbons. Meso epoxides were cleaved enantioselectively with the chiral B-halodiisopinocampheylboranes (see 15-16), where the halogen was Cl, Br, or I. ° Diatomic iodine gives an iodohydrin with a 2,6-bis[2-(o-aminophenoxy) methyl]-4-bromo-l-methoxybenzene catalyst. ... 

So far we have looked largely at substrates which are flat achiral objects that have chiral centres introduced by means of a reagent. Desymmetrisations are slightly different. Molecules which are desymmetrised tend to be of a meso type. That is, they are achiral because they have a mirror plane and the sides of the molecule contain left and right handed portions 219. This is in contrast to the C2 axis present in many catalysts such as the TADDOLate 218. Desymmetrisations are powerful because there may be several chiral centres embedded in an achiral molecule which suddenly become much more useful in the newly formed chiral molecule. There are a large variety of symmetrical substrates that have been enantioselectively desymmetrised48 and we look at a few of the more important classes. 

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