Boron placement

In comparison to other dialkylboranes, 9-BBN is more sensitive to electronic factors (in the absence of steric factors). The above data indicate that the TMS group provides an electronically based preference for the placement of boron to the a position, a phenomenon consistently observed in the ionic additions to unsaturated organosilanes [16]. However, because of the limited ionic nature of the hydroboration process, this effect is less important in these reactions. Steric effects easily overcome the weak electronic effects of the TMS group so that P-boron placement becomes the dominant process. 

Table 5.24 Influence of the silyl substitution (11) with 9-BBN [14] on the boron placement in different alkynylsilanes ...

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

It is argued that the poor diastereoselectiyity in the reactions of the (Z)-l-methyl-2-butenyl-boronate reflects the competition for placement of R versus methyl in an axial position in transition states analogous to 6. 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

The 1971 hypothesis 164) (throughout this paper referred to as rule 1) may now be expanded by the addition of supplementary rules that deal with the placement of BE hydrogens (rule 2), the placement of carbon and other heteroelements (rule 3), and the consequent coordination situation of boron (rule 4). 

The following sections expand on the deltahedron-deltahedral fragment hypothesis (structural rule 1), and in the order of decreasing importance add three additional rules, one involving the placement of BE hydrogens (rule 2), next the placement of the various heteroelements (with emphasis on carbon) (rule 3) 172), and finally the structural accommodations of boron, including the influences of endohydrogens (rule 4). All are shown to have their roots firmly and simply embedded in CNPB, considerations. 

The controlled asymmetric placement of a deuterium label is particularly easy to accomplish with a-halo boronic ester chemistry. 

Nuclear power plants are designed to prevent accidents such as meltdown by careful control of fuel-rod placement and positioning of control rods made of boron or other materials that have high affinity for neutrons. If the core of the reactor should become overheated, the fuel rods are... 

One of alternative syntheses of allylic boranes is their synthesis via hydroboration of alkynes followed by one carbon chain homologation using thiomethoxymethylli-thium. This provides exclusive placement of boron atom at the terminal position as shown in Eq. 44 97). 

The symbol (sp01)1 denotes a hybrid AO pointing in the same direction as (sp2)t but whose s/p character ratio, a, differs from 2 by the factor (c2/c6)2 which is slightly different from 1. The first two terms represent the intuitive localized picture, namely, that one boron sp hybrid (a 2) is bonded to one H AO which implies that c9 and c10 are both positive. The last two terms are small corrections because cn = (ci — c5)/ /3 orthogonality requires furthermore that they be negative. These corrections could have been anticipated from the placement and sizes of the plus and minus lobes of the hybrid GO as shown in Fig. 14 (b). If these correction terms are neglected, we arrive at the conventional picture of localized bonds in terms of hybrid AO s. 

Functionalized alkenes and alkynes can be hydroborated . Directive effects are discussed in 5.3.2.5.1(i). As follows from Table 2, hydroboration of vinylic and allylic derivatives leads to the placement of boron in the a., fi or y position to the substituent. The -substituted organoboranes are prone to uncatalyzed cis elimination and acid- or base-catalyzed trans- elimination The rate of elimination depends on the substituent at... 

Table 1 Boron Atom Placement in the Hydroboration of Simple Alkenes...

Poly(vinyl chloride) prepared with boron alkyl catalysts at low temperatures possesses higher amounts of syndiotactic placement and is essentially free from branches. " ... 

Several reaction mechanisms were also proposed to explain stereospecific placement with insoluble catalysts. Furukawa [46] suggested that here the mechanism for cationic polymerization of vinyl ethers depends upon multicentered coordinations. He felt that coordinations of the polymeric chains and monomers with the catalysts are possible if the complexed counteranions have electrically positive centers. This can take place in the case of aluminum alkyl and boron fluoride ... 

The design of the reactor internals has not been addressed yet, but they likely will be made of graphite or carbon composites to accommodate the high-core outlet temperature required by the NGNP (1000°C). It is possible that carbon-insulated metallic alloy will be used for the core support structure, although this has not been evaluated yet. Control rods will be required to provide for reactor startup, normal operation, and shutdown. The munber and placement of control rods has not been evaluated yet, but the rods will be constructed from carbon composites for the drive shafts and absorber casing and boron carbide or other high-temperature absorber for the neutron absorber. The control rod drive mechanisms will be located above the reactor enclosure head. 

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