B Species Containing Four-Coordinate Boron

As noted above, the first monomeric pyrazol-l-ylborane was isolated as its trimethyl-amine adduct The compound (CH3)3N—BH2[pz-3,5-(Cp3)2] (Hpz = pyrazoie) was obtained as a distillable material on reaction of (CH3)3N—BH3 with 3,5-bis(tri-fluoromethyl)pyrazole = H[pz-3,5-(Cp3)2] = Hpz. Surprisingly, when THP—BH3 (THF = tetrahydrofuran) was employed as reagent, the dimeric species H2B(p-pz )2BH2, a pyrazabole (see Sect. IV.), was obtained without difficulty Apparently, the two-coordinate nitrogen of the pyrazolyl group in THP—BH2[pz-3,5-(Cp3)2] is sufficiently basic to displace THP but this base displacement cannot occur in the corresponding (CH3)3N—BH2[pz-3,5-(Cp3)j]. 

In this context it is of interest to note that the mass spectral fragmentation data on (CH3)2HN—B(pz)3 suggest its fragmentation to Hpz and (CH3)2NB(pz)2, which seems to contradict the low-temperature NMR data on the other hand, this observation is quite in consonance with a delocalized proton at higher tefnperatures. 

In an unusual reaction, the 3,5-dimethylpyrazole (= Hpz ) adduct of tris(3,5-di-methylpyrazol-l-yl)borane was obtained in 20% yield when (CjHj)3TiCl2 was reacted with K[HB(pz )3]. The room temperature spectrum of the compound, Hpz —B(pz )3, showed scrambling of the NH proton however, the latter is localized at, —53 °C and also in the solid state as shown by an X-ray diffraction study 

Originating from isolated monomeric pyrazol-l-ylboranes containing trigonal 

The formation of such adducts ms to be a function of the basicity of the donor molecule Neither triethylamine nor pyridine formed an adduct with compound 1 (Table 1), whereas pyrrole or 2,6-dimethylpyridine yielded adducts in equilibrium with the individual components. With diethylamine, a chemical transformation occurred but with pyrazoles, imidazole or 1,2,4-triazole, adduct formation with compound 1 (Table 1) is clearly the favored product Several other such 1 1 molar adducts have been identified and are surveyed in Table 2. However, it should be noted that, based on NMR data, several of these adducts in solution are in equilibrium with the individual components, probably due to steric effects. 

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