Octahedral models

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

In a related study Porter et al. showed that a-bromo-y lactams 185 containing a pyridyl moiety can react with allyltrimethylsilane enantiose-lectively in the presence of chiral Lewis acids derived from zinc and 189 (Scheme 49) [142], In contrast to the above study, the ligand of choice for substrates 185 was found to be the bisoxazoline ligand 189. Excellent ee s were obtained in the presence of two equivalents of the chiral Lewis acid. Under substoichiometric amounts of the catalyst, lower selectivities were obtained. Different substituents on the pyridyl moiety were also examined although no predictable trend was observed. A trans octahedral model simi-... 

An octahedral geometry (Oh symmetry) is, of course, an ideal case, which virtually none of our systems match. All have lower symmetry (typically >3 or Ci), which further splits the dlevels. However, the octahedral model is a starting point. Lowering the symmetries does not affect the basic nature of the types of excited states. Further, such important features as the d state energies are still dictated by the average A of the ligands. 

Figure 1.1 (a) Real heterogeneity of a catalyst from the centimetric level to the nanoscale (atomic) level (b) a cubo-octahedral model of a metal particle and an electron microscope view of a platinum particle covered with n-octylsilyl fragments). (Unpublished results with permission of the Fritz Haber Institute, Berlin.)... 

Fig. 2.10 Structure of bernalite. a) Octahedral model with unit cell outlined, b) Ball-and-stick model (Stanjek, unpubl.)...

Figure 3.9. A structural model of the V2O5 catalyst (a) square pyramids and the idealized octahedral model in the (001) (b) (010) (c) and (100) (d) projections.

Simple tetrahedral or octahedral models are useful in connection with basic structural questions (as, for example, the first time you try to convince yourself that the two enantiomers of CHFCIBr or of [Co(en)3]+3 are really nonsuperimposable). If stick models are rot available, such simple models can be constructed in a few minutes from paper. In addition, models having bond angles not normally found in ball-and-stick kits—for example, the icosahedral boranes and carboranes—can also be reudily constructed from paper. Paper models are especially useful when large numbers of models are necessary as, for example, in constructing models of the iso-end heteropolyanions. 

Octahedral models. Cut out the two sets of eight triangles enclosed by the Oh brackets and marked wilh the horizontal lines in the drawing. Glue or tape tabs onto adjacent faces to form the octahedron. 

Zn(OTf) and afforded product consistent with reaction via an octahedral model, whereas l,3-bis(oxazoline) ligands B were superior to A and C for Mg(OTf)2 and Cu(OTf)2. In addition, among several 1,4-bis(oxazoline) ligands, a non-C2-symmetric bis(oxazoline) D bearing a meso backbone was also found to be highly efficient [35],... 

When the propagating metal carbene complex does not have a predetermined vacant ligand position, but is instead trigonal-bipyramidal or tetrahedral, it may still behave like the octahedral model provided that the ligands other than the carbene offer an asymmetric environment which controls the direction of approach of the monomer. If this is not the case there will not be a favoured direction of approach unless the chain-end effect comes into play. 

The high-c/s polymer of 242 (see above), when made from enantiomeric monomer, has a mainly HH, TT structure and is therefore largely syndiotactic. On the other hand, the 96% Inins polymer made from enantiomeric monomer with RLiC.I(/i-C.I)(r 3 r 3-C. oHi6)]2 as catalyst (C10H16 = 2,7-dimethyloctadienediyl) has an HT structure and is therefore essentially isotactic. These tacticities are as predicted from the pseudo-octahedral model if the ligands are not labile and one site is available for coordination of monomer319 see Section VIII.A.5. 

As for molten ErClj, the edge-sharing and the comer-sharing octahedral models were... 

Octahedral models of the cyclic amine show that four possible strain-free planar conformations are possible, as well as two bent arrangements which lead to cis orientation of the remaining two coordination positions. Because each of these six unhindered configurations is distinct from one another, mirror images of each form are possible—even for the trans structures. The latter phenomenon would be without precedent, and further investigation may open a new chapter in inorganic isomerism. 

Consideration of the octahedral model in accordance with symmetry knowledge also has been used to predict the presence of the mirror isomerism in complex compounds with definite content and structure. Its discovery was made by Werner in 1911, and is a confirmation of the coordination theory. ... 

The presence on the surface of a dispersed metal catalyst of at least three distinct corner sites having different activities is, however, not compatible with the octahedral models of the 3M sites shown in Fig, 3.4 and used in Schemes 3.2 and 3.4 to develop analogies with specific homogeneous catalysts. A more detailed description of these corner atom sites and others present on the surface of metal catalysts is presented in the next chapter in conjunction with a discussion of the surface electronic orbitals of such species. 

As mentioned in Chapter 3, the octahedral models used to describe the active sites on metal surfaces are not compatible with the presence of three different types of saturation sites on a catalyst surface so another model must be developed. On consideration of the fee crystal structure, which is that of most catalytically active metals, it can be seen that the bulk atoms in these metals are bound to twelve nearest neighbor atoms using the lobes of the t2g d orbitals. The octahedraily oriented eg orbitals are directed toward but not bonded to the next nearest neighbors in the crystal lattice as shown in Fig. 4.1.1 This atomic orientation precludes the presence of any octahedral arrangement involving M-M bonds. 

A simple parameterization of these spectral shapes is provided by the exponential healing [73] or NMR layer model [74]. The size of the metal particles (or, more precisely, their size distribution) can be measured by electron microscopy. Using the cubo-octahedral model, we can then calculate what fraction of atoms is in the surface layer, in the subsurface layer, and so on. The local density of states in all sites of the surface layer is not quite the same, but nevertheless clearly different from those in the subsurface layer, and so on. Therefore, the spectrum is decomposed as a superposition of (for convenience) Gaussians, each representing the collection of sites in a layer. The integrals must be proportional to the fraction of atoms in the layer. The maximum of the Gaussian corresponding to the nth layer... 

For work on octahedral c s-models using iron Lewis acids see Corey, E.J. Imai, N. Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728-729. For an octahedral model using Mg Lewis acid see Dcsimo-ni, G. Faita, G. Righetti, P. P. Tetrahedron Lett. 1996, 37, 3027-3030. 

Figure 9 Schematic representation of the chirality at the active site in the case (a) of a C2-symmetric pseudo-tetrahedral metallocene, (b) of a C2-synnmetric octahedral model for heterogeneous catalysts, and (c) of a syndiospecific Cs-synnmetric pseudo-tetrahedral metallocene.

Octahedral and ball models of gibbsite are presented in [54]. Octahedral models of the 0001 and 1-102 faces of a-alumina shown in Figure 3 of [55] and... 

FIGU RE 1.15 An origami by Michal Kosmulski representing the octahedral model. 

Figure 1 of [56] indicate the location of possible adsorption sites for metal cations. The octahedral models shown in Figures 3.10 and 3.15 of [57] illustrate the formation of gibbsite, bayerite, and boehmite from solution monomers. Ball-and-stick models of the 100, 010, and 001 surfaces of gibbsite are shown in Figure 9 of [58]. An original model of different planes of a-alumina was used... 

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