Boron model structure

Muetterties and Schunn (1966) suggest that certain boron hydride structures, e.g. 26 and 22 or 23 (C, symmetry, when X=Y), might be used as models for electrophilic substitution. In general, if 21 or 22 were the preferred C3 transition states or intermediates, the reaction would go with retention. Note that these are the analogs of the two-electron three-center species A3 discussed earlier by HMO theory. If, however, X, Y, and R were permuted, there would be several stereochemical possibilities, two of which are indicated in Table 7. To decide what kind of geometry is assumed in SE2 transition states, which are pentacoordinated and electron- and orbital-deficient, calculations on model species are needed to establish preferred geometries. 

Scheme 8.3 General assembly of asymmetric carbon chains via pinanediol boronic esters. Structures 13-20 are drawn as slightly distorted planar projections of a three-dimensional computer model of the rigid terpenoid unit. For detailed reaction conditions, see Scheme 8.1.

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

Assuming perfect stoichiometric structures, the stabilization of the boron frameworks of MB2, MB4, MBg, MBj2 and elemental B requires the addition of two electrons from each metal atom. Whatever the Bj2 unit, icosahedron or cubooctahe-dron, 26 electrons are required for internal bonding and 12 for external bonding. Since the 12 B possesses only 36 electrons, the metal must supply two electrons to each Bi2 group. The results for YB,2 are consistent with this model measurements indicate that one electron per Y is delocalized in the conduction band. ... 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

The interesting structures of the Lasioderma compounds have been the subject of many syntheses, serving as models for stereocontrolled approaches. More recent syntheses of serricornin form two groups those using chiral auxiliaries (oxazolidinone [250],boronic esters [251],and SAMP/RAMP [252]) and those involving chemoenzymatic steps ([253-255]). 

Hoggard PE (2003) Angular Overlap Model Parameters 106 37-57 Hopfl H (2002) Structure and Bonding in Boron Containing Macrocycles and Cages. 103 1-56... 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

A quite new type of antibiotic and one of the few naturally-occurring boron compounds is boromycin (86). Hydrolytic cleavage of D-valine with the M(7) hydroxides gave caesium and rubidium salts of this antibiotic, and crystal structure analysis established the formula as (XIIT). The rubidium ion is irregularly coordinated by eight oxygen atoms. Experiments with models showed that the cation site would be the natural place for the—NH3+ end of the D-valine residue, and the whole structure raises the possibility that transport of larger alkali metals is related to the N-ends of peptides and proteins. 

The strongest influence of configuration has been observed for Pi -substituted diastereomers of Z-Phe-(aTfm)Ala-Ala-NH2. The crystal structures of both dia-stereomers have been solved, which enables a better interpretation of this rather interesting effect. While the (S,S,S)-diastereomer has been shown to be almost as stable as the Aib-substituted peptide, the (S,R,S)-diastereomer was hydrolyzed very quickly within the same time range. Molecular modeling studies readily support the formation of hydrogen bonds as a possible explanation for this effect [18,54]. With the known crystal structure of the a-chymotrypsin/phenyl boronic... 

---