Heterocyclic boron compounds

B n.m.r. spectra of heterocyclic boron compounds containing B—or B—O bonds are inconsistent with their having aromatic character. 

Another type of anion, confined for practical purposes to boron compounds, has no unshared electrons at the anionic site, and must be thought of as being formed by addition of hydride to a boron atom (or other atom with an incomplete valence shell). Such structures were not anticipated at the time general heterocyclic nomenclature was developed, and they are only recently being fitted into systematic nomenclature (lUPAC Provisional Recommendation 83.2). Proposals for a suffix to indicate such structures are under consideration (1982). 

Complexes of N-heterocycles with boron compounds 99JOM(581)129. 

One chemical property quoted in support of aromatic character is kinetic stability towards hydrolytic break-down. Many boron heterocycles have been stated to be more stable than expected for organoboranes. On the other hand, several saturated cyclic boron compounds are stable as well, suggesting that the cyclic structure itself is favorable. 

Nucleophiles other than hydride can be added to support-bound imines to yield amines. These include C,H-acidic compounds, alkynes, electron-rich heterocycles, organometallic compounds, boronic acids, and ketene acetals (Table 10.9). When basic reaction conditions are used, stoichiometric amounts of the imine must be prepared on the support (Entries 1-3, Table 10.9). Alternatively, if the carbon nucleophile is stable under acidic conditions, imines or iminium salts might be generated in situ, as, for instance, in the Mannich reaction. Few examples have been reported of Mannich reactions on insoluble supports, and most of these have been based on alkynes as C-nucleophiles. 

For the last four decades, there has been an exponential increase in the synthesis and usage of a number of organic and inorganic boron compounds in industry and academia. Due to the explosive growth of heterocyclic boron-containing compounds, this topic is covered as a separate chapter here. Previously, the subject of this chapter was covered in a subsection of Chapter 4.24 in CHEC-II(1996) (Sections 4.24.1.3.6 and 4.24.1.3.7). This chapter covers as much literature as possible, but the space restraints did not permit a comprehensive coverage of all literature. 

So far, only one detailed discussion of boron-11 nuclear magnetic resonance spectra of aminoborane systems has been reported 31>. It was found that the 1 lB chemical shifts of aminoborane systems can be described fairly well in terms of a set of additive substituent contributions. In consonance with earlier work on trisubstituted boron compounds 35> these contributions depend on the mesomeric effects of substituents rather than their electronegativity. 1,3,2-diazaboracycloalkanes can be considered as aminoborane derivatives and in the case of the known heterocycles the exocyclic boron substituent will govern primarily the boron chemical shifts and will do so by mesomeric effects. However, the available data are rather limited and it may be possible that additional factors must be considered. Steric effects appear to be negligible, however, since the heterocycles with either six or seven annular atoms have almost identical shifts (Table 5). 

Caution. l,3-Diaza-2-boracycloalkanes are moisture-sensitive and react readily with protonic materials. Alcohols, in particular, may occasionally cause an explosive solvolysis reaction on contact with these heterocycles. The compounds should be handled under dry inert gas (nitrogen or argon) to avoid their contamination. The possible toxicity of boron compounds has not yet been explored. 

The SjSii ring system is susceptible to attack by nucleophiles, as is illustrated for 636. Examples of nucleophiles are alcohols (best with primary and secondary), 1563-1538 phenols,hydroxide ion, water, " and carboxylic acids. The boron compound 641 reacts with 636 to give the five-membered heterocycle 642. The electrophilic reagents, acetyl chloride and benzoyl chloride, react with tetraphenylcyclodisiladithianes to give the diacyl sulfide and dichlorophenyl-silane. ... 

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

A large number of heterocyclic boronates and, to a lesser degree, stannanes are available conunercially. Included in the suppliers lists are many compounds, the preparations and properties of which do not appear in the literature. They are often noted in papers just as reagents for syntheses of libraries for biological testing and, although this may be a practical approach, it is scientifically unsatisfactory because there is no trail of characterization, particularly as some of these compounds, for example azine a-boronates, are significant from a theoretical viewpoint. 

Heterocyclic organoaluminums are prepared by the transmetallation of the corresponding boron compounds , e.g. for the 3-methyl-l-ethyl-l-aluminaindane ... 

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