Pyrazabole derivatives

Figure 50 Fluorescent organoboron polymers (81) obtained by Heck-Sonagashira coupling between diyne monomers and pyrazabole derivatives. (Adapted from ref. 115.)...

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

The condensation of pyrazole (=Hpz) or C-substituted derivatives thereof with a trigonal borane (which may be employed as its adduct with a Lewis base) proceeds readily to yield N-borylated pyrazoles. However, the resultant species generally exist in the dimeric pyrazabole structure, i.e., R2B(p-pz)2BR2 =... 

The pyrazaboles constitute the bulk of the presently known neutral boron derivatives of pyrazoles more than 70 different species have been described. 

Similar reactions of B-hydropyrazaboles with active hydrogen compounds such as pyrocatechol phenol thiols or additional pyrazole are equally facile but require high temperatures (method D). Therefore, the reaction of pyrazabole with o-phenylenediamine may proceed via the expected substituted pyrazabole as an intermediate. However, the latter is unstable under the reaction conditions and condenses further to yield the borazine derivative 10 with the elimination of pyrazole... 

Pyrazaboles of the general types represented by (47) and (48) have been under study for a number of years, and the syntheses of numerous derivatives have been reviewed. The compounds are relatively stable, and a significant body of C-atom organic substitution chemistry has been developed. In addition, the anionic character of (47) has made these species particularly attractive hgands for both metal and nonmetal cations. That topic is summarized in Section 10 of this article. 

A,7V-polyazolylmethanes can be prepared by reaction of azoles with methylene chloride in an autoclave at 150 °C (Scheme ll).20 At 200 °C this reaction leads to 4,4 -dipyrazolylmethane, which upon reaction with boranes gives a pyrazaboles polymer. Some derivatives have been prepared from the reaction of potassium salts of the azole and methylene iodide. The ligand (tetrakis-(l-pyrazolyl)methane was prepared analogously. In selected cases a basic medium was employed.21... 

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