B Preparative Aspects

It is noteworthy that in this latter reaction no exchange of boron atoms between HjBfpzjjBHj and the BXj occurs, as was demonstrated by employing boron-10 labelled boron tribromide Hence, the central B2N4 ring of the pyrazabole apparently is not opened during the course of this low-temperature substitution process. 

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 

Similarly, the reaction of pyrazaboles with a,co-diaminoalkanes proceeds with symmetrical cleavage of the pyrazabole skeleton. This latter process was used for the synthesis of monomeric pyrazol-l-ylboranes (see Sect. III. A). 

The reaction of H2B(pz)2BH2 with pyrazole has been stopped at the intermediates H(pz)B(pz)2BH2 and H(pz)B(pz)2BH(pz) respectively, by appropriate adjustment of the stoichiometry of the reactants (method ). 

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