Boron-11 shifts

Comparison of boron-11 shifts of phosphorus containing boron esters with simple boron esters... 

Proton NMR shifts have been interpreted in terms of a ring current, and some coupling constants also indicate aromatic properties. Boron-11 shifts alone can apparently not be used as criteria of aromaticity, in the same way that carbon-13 shifts are not meaningful. 

The activation energies for [l,3]-sigmatropic boron shifts have been determined... 

How does the B-H unit add to the bond The ir bond is electron rich and borane is electron poor. Therefore, it is reasonable to formulate an initial Lewis acid-base complex similar to that of a bromonium ion (Figure 12-3), requiring the participation of the anpty p orbital on BH3. This shifts electron density from the alkene to boron. Subsequently, one of the hydrogens is transferred by means of a four-center transition state to one of the alkene carbons, while the boron shifts to the other. The stereochemistry of the addition is syn. All three B-H bonds are reactive in this way. The boron in the product alkylborane is again electron deficient. The electrostatic potential maps of the general scheme shown below (on a scale that maximizes the desired color changes) show how the boron in borane starts out as an electron-deficient species (blue), becomes more electron rich (red) in the complex, and then loses the electron density gained as it proceeds to product (blue). 

In Summary Hydroboration-oxidation constitutes another method for hydrating alkenes. The initial addition is syn and regioselective, the boron shifting to the less hindered carbon. Oxidation of alkyl boranes with basic hydrogen peroxide gives anti-Markovnikov alcohols with retention of configuration of the alkyl group. 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

The presence of metal salts, particularly those containing alkaline-earth cations and/or haUdes, cause some shifts in the polyborate equiUbria. This may result from direct interaction with the boron—oxygen species, or from changes in the activity of the solvent water (63). 

The second special case is an orthotropic lamina loaded at angle a to the fiber direction. Such a situation is effectively an anisotropic lamina under load. Stress concentration factors for boron-epoxy were obtained by Greszczuk [6-11] in Figure 6-7. There, the circumferential stress around the edge of the circular hole is plotted versus angular position around the hole. The circumferential stress is normalized by a , the applied stress. The results for a = 0° are, of course, identical to those in Figure 6-6. As a approaches 90°, the peak stress concentration factor decreases and shifts location around the hole. However, as shown, the combined stress state at failure, upon application of a failure criterion, always occurs near 0 = 90°. Thus, the analysis of failure due to stress concentrations around holes in a lamina is quite involved. 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

Boron atoms in infinite boron zigzag chains form covalent B—B bonds at 165-190 pm with bond angles of 115°. The boron coordination is trigonal prismatic, giving rise to coordinated rows of face-connected trigonal metal prisms (Table 1). Most typical are the binary transition-metal monoborides such as the FeB, CrB and MoB structure types. Transformations between pairs by simple geometric shifts ... 

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

This solvent is called tetrahydrofuran, or THF for short. Even though it somewhat stabilizes the empty p orbital on the boron atom in BH3, nevertheless the boron atom is very eager to look for any other sources of electron density that it can find. It is an electrophile—it is scavenging for sites of high electron density to fill its empty orbital. A pi bond is a site of high electron density, and therefore, a pi bond can attack borane. In fact, this is the hrst step of our mechanism. A pi bond attacks the empty p orbital of boron, which triggers a simultaneous hydride shift ... 

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