Boron substitution

Amine boranes have been examined by a variety of spectroscopic methods (24—29). The boron-substituted alpha-amino acids have been utilized in animal model studies. These compounds along with their precursors and selected derivatives have been shown to possess antineoplastic, antiarthritic, and hypolipidemic activity (30—32). The boron amino acid analogues are also being evaluated for possible utility in boron neutron capture therapy (BNCT) (33). 

Although no 73Ge NMR spectra of stable doubly bonded germanium compounds have been recorded, NMR chemical shift data for the heteroatom of the double bond are available these are listed in Table II. The chemical shift of the, sp2-hybridized carbon atom of dimesitylneopentylger-mene (entry 1, Table II), without any polar substituents, falls at 8 124.2.26 The chemical shifts of the boron-substituted sp2-hybridized carbon atoms of two stable germenes (entry 2, Table II) fall in the upfield region of the... 

Pace, B., and Pace, N.R. (1980) The chromatography of RNA and oligoribonucleotides on boronate-substituted agarose and polyacrylamide. Anal. Biochem. 107, 128-135. 

A variety of boron-substituted stannoles such as those shown in formulas 10 and 12 have been prepared by Wrackmeyer s 1,1-organoboration of dialkynes.234... 

The boron-substituted analogs (THF)2Mg(Me3Si)4(t-BuB)C4B7H7 (143) and (THF)2 Mg(Me3Si)4(MeB)C4B7H7 (144) have also been prepared and structurally characterized in the solid state. Their structures are essentially the same as that of 142, except that one of the borons in 143 and 144 is alkylated. 

The structures and electron delocalization in these boron-substituted derivatives of the cyclobutenyl/homocyclopropenium cations are fully consistent with their designation as homoaromatic systems. 

Frequency differences between the flux-grown crystal and the floating zone-grown crystal are due to hole doping in the latter sample.5 That the dHvA frequencies change with addition of boron indicates that boron substitutes for carbon. Therefore, the dHvA data of the flux-grown crystal was analyzed to construct the Fermi surfaces. 

Terashima, M., Ishikura, M., Boron-substituted Heteroaromatic Compounds, 46, 143. 

Biological pyrimidines, tautomerism and electronic structure of, 18, 199 Bipyridines, 35, 281 Boron-substituted heteroaromatic compounds, 46, 143 Bridgehead nitrogen saturated bicyclic 6/5 ring-fused systems with, 49, 193... 

Also, no solvent or concentration effect has been noted and a spectrum calculated for a planar ring system was consistent with the experimental data. The coupling constant Jmscu in boron-substituted 1,3,2-diazabor-acyclohexanes was found to be about 3 c.p.s. indicating a planar valence arrangement of the nitrogen. These cited results tend to support the assumption of a BN bond order greater than 1. 

In this article all known boranes, carboranes, and metallacarboboranes cannot be treated, and the reader is referred to recent reviews in this field (119-121). Only those cases will be described where the reaction principle of Scheme 1 has been used experimentally. Some 7,8-dicarbollide dianions were employed in reactions with phenyl- or ethylboron dichloride to reconstitute the boron-substituted dicarbaclosodecaborane icosahedrons XXXa-d (122). These carboranes were further degraded to 7,8-dicarbollide dianions, whereby the substituted boron remained within the product. Further reactions with phenyl- or ethylboron dichloride led once more to reconstruction of dicarbaclosoborane species, as indicated in Scheme 2. Further degradation reactions with the carbaboranes XXXIa,b were unsuccessful. 

This protonated metallocarborane is unique in that it undergoes ready substitution at polyhedral boron atoms, whereas the unprotonated species is unreactive in the absence of acid. Reaction of (1,2-C2B9Hn)2FeH- with Lewis bases such as dialkyl sulfides results in the loss of H2 and the formation of boron-substituted complexes in good yields ... 

It is not necessary to start with the protonated Fe(II) complex to isolate these substituted products, however. Reaction of a slurry of the unsubstituted Fe(III) complex with anhydrous HC1 in diethyl sulfide produced a mixture of substitution products, including the species shown in Eq. (12). Although no evidence has been found for a protonated cobalt metallo-carborane analog, boron-substituted complexes of cobalt (III) may also be obtained by reaction of the (1,2-C2B9Hn)2Co- ion with R2S and HC1 52). 

FIGURE 12.1 7 Schematic models of the top (a) and side (b) view for a boron-substituted graphene sheet based on the measured dimensions of Ii-C, and C,-C,. (From Endo, M., et al., J. Appl. Phys., 90, 5670, 2001. With permission.)... 

Carboranyl derivatives of mercury and thallium as synthons for boron-substituted carboranes 91PAC357. 

Due to their great versatility, boron-substituted dienophiles and dienes have emerged as attractive building blocks in cycloaddition reactions. The early works in the field have been reviewed.1... 

Addition and coupling reactions of boron-substituted carbanions... 

Table 1. Chemical Composition of Boron Substituted Mordenite Samples...

Mordenite. It has been reported that partial substitution of boron in mordenite occurs when boric acid is added to the synthesis gel (6.7). Evidence for boron substitution for aluminum in mordenite samples prepared in borosilicate glass reactors has also appeared (8). We sought to determine the extent of boron substitution into the mordenite framework which can be affected by direct synthesis. 

Figure 2. Boron content versus SiC /A Oj ratio for boron substituted mordenites.

A1 saturated mordenite (x = 8 in Formula 1), the product contains only trace amounts of boron. As the Si02/Al2O3 ratio of the gel increases, and insufficient A1 is present to form the aluminous mordenite end-member, the boron content of the product rises dramatically. The upper limit on the Si02/Al2(>3 level at Which pure mordenite forms, which is approximately 25, determines the upper limit on boron substitution level. 

Characterization of Substituted Boron. We used solid state -B NMR and X-ray diffraction data to distinguish occluded borates from boron substituted into the zeolite framework. When an element replaces aluminum or silicon in a zeolite structure, the local coordination environment changes to accommodate the new ion. Since B + is a much smaller ion than Al "1", the unit cell axes contract when boron replaces aluminum in the framework. The ionic radii of trivalent B and A1 in a tetrahedral environment are 0.25 and 0.53, respectively (1). The magnitude of the contraction is dependent upon the level of substitution (17). 

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