Chirality octahedral molecules

The high regiospecificity of the template encapsulation reaction should be noted. Eleven molecules (three en, six HCHO, and two NH3 or two CH5NO2) participate in the formation of the chelate rings of the final macrocyclic product. In the absence of a matrix a cage-like molecule is believed to be formed in 0.1% yield at best [10], Moreover, if a chiral octahedral (Table 4-1) ethane-1,2-diamine... 

With chiral enol species (/ )-silylketene acetal derived from (1 R,2S)-N-methyl ephedrine-O-propionate, both the aldehyde carbonyl and the ephedrine NMe2 group are expected to bind to TiCU, which usually chelates two electron-donating molecules to form ra-octahedral six-coordinated complexes.25 Conformational freedom is therefore reduced, and the C-C bond formation occurs on the six-coordinated metal in a highly stereoselective manner.18... 

A chiral bis(oxazoline) ligand forms effective catalysts in combination with magnesium salts for enantioselective Diels-Alder addition of acryloyl oxazolidinone L32] (Eq. 8A.17), Recendy, Desimoni found that both Diels-Alder adduct enantiomers could be obtained by using the same bis(oxazoline)/MgC104 catalyst, that is, in the presence of water (2 equiv.), the other enantiomer was obtained selectively, which can be rationalized by taking into account the coordination of two water molecules to the metal, changing its geometry to octahedral [33],... 

Denmark introduced an array of efficient chiral phosphoramides as nucleophilic activators (Fig. 7.3) for the enantioselective C-C bond formation, and also carried out a detailed mechanistic investigation [56, 59, 60]. Bidentate (e.g., 60) and smaller monodentate catalysts (58) have been shown by Denmark to react via a cationic chair-like transition state 56 with octahedral extracoordinate silicon (Scheme 7.10). Following this manifold, (Z)-enol ethers 55b and 55c produced syn-adducts 57b and 57c, whereas (E)-derivatives 55a and 55d furnished anti-diastereoisomers 57a and 57d. By contrast, with a bulky monodentate activator (e.g., 59), where coordination of the second catalyst molecule is precluded by steric factors, the reaction exhibits opposite diastereoselectivity, presumably due to the cationic boat-like transition state, where silicon is pentacoordinate. Along this manifold, cyclohexanone-derived enol ether 55d with the fixed (E)-... 

Among the commonest types of diketo complex are those with the stoichiometries M(dike)3 and M(dike)2. The former all have structures based on an octahedral disposition of the six oxygen atoms. The tris(chelate) molecules then actually have >3 symmetry and exist as enantiomers. When there are unsymmetrical diketo ligands (i.e., those with R R"), geometrical isomers also exist, as indicated in (11-XXVI). Such compounds have been of value in investigations of the mechanism of racemiza-tion of tris(chelate) complexes. Chiral diketonate complexes have found many... 

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

MacGillivray and Atwood have demonstrated the ability of six resocin[4]arenes to self-assemble to give a very large molecular capsule of octahedral symmetry.67 Specifically, six molecules of tetramethylresorcin[4]arene assembled with eight water molecules via 60 hydrogen bonds to form a capsule, approximately 2.5 nm in diameter, that accommodated multiple solvent molecules as guests. The assembly was chiral, possessing a structure that conformed to an Archimedean solid known as a... 

Use of the chiral iron catalyst 28 for asymmetric sulfide oxidation was recently reported by Fontecave [48]. The complex is a dimer with two iron centers which are connected via a r -oxo bridge. Two bidentate (-)-4,5-pinene-bipyridine hg-ands 29 and one molecule of water are coordinated to each iron atom thus completing the octahedral environment of this diferric complex. Complex 28 can be isolated as a green solid with four perchlorates as counterions. 

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