Pyrazabole

Pyrazabole is a white crystalline solid, m.p. 80-81°C., with a pungent, camphoraceous odor. It is readily soluble in most organic solvents, is less so in alcohols, and is insoluble in water. Its H1 n.m.r. spectrum consists of a doublet and triplet (J = 2.0) at r values of 2.49 and 3.83, respectively, in 2 1 ratio, whereas the B11 n.m.r. spectrum displays a triplet (J = 108) at +27.1 p.p.m. from trimethyl borate. 

Pyrazaboles substituted on either the ring or the boron may be obtained by reaction between the appropriately substituted pyrazole and the appropriately substituted borane.11 For example the 2,6-dibromo-4,4,8,8-tetraethylpyrazabole can be made by the reaction between 4-bromopyrazole and pyrophoric triethylborane in xylene at reflux temperature. Another route to such compounds involves the dihydrobis(l-pyrazolyl)borate 

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Ferrocene-bridged tris(l-pyrazolylborate) oligomeric systems have been synthesized by Wagner and co-workers.62 The examples are described in more detail in the section on pyrazolylborate and pyrazabole polymers in this chapter. 

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. 

Hi. Organoboron Polymers that Contain Polypyrazolylborate or Pyrazabole Ring Systems... 

Over 2000 papers have appeared on the chemistry of polypyrazolylborate/ pyrazabole ring systems, but there have only been a few examples of polymers containing these ligands.112... 

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

Poly(pyrazabole)s containing an electron withdrawing moiety in the main chain were prepared the same way (scheme 39).69 The coupling reaction between... 

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). 

Pyrazaboles 275 (compound 276 is a particular example of 275 with R3 = R5 = H) were discovered by Trofimenko [216, 217] and thoroughly studied by Niedenzu... 

Fig. 9 Boat (left), chair (middle), and planar (right) conformations of pyrazabole 276 (R1 = H, R2 = F)...

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