A Species Containing Trigonal Boron

Ill Monomeric Pyrazol-1-yIboranes ni.A Species Containing Trigonal Boron 

The first reported monomeric pyrazol-l-ylborane, i.e., a species not dimerized to a pyrazabole structure (see Sect. IV.), was 3,5-bis(trifluoromethyl)pyrazol-l-ylbor-ane However, this compound is known only as its 1 1 molar adduct with tri-methylamine, i.e., it contains four-coordinate boron. The first monomeric pyrazol-l-ylborane containing trigonal boron was reported in 1980 l,3-dimethyl-2-(l-pyrazolyl)-l,3,2-diazaboracyclohexane, 4, was obtained on condensation of pyrazole with l,3-dimethyl-l,3,2-diazaboracycIohexane  

Subsequently, several additional monomeric pyrazol-l-ylboranes containing trigonal boron have been described. They are, however, all species in which the boron is incorporated into a 1,3,2-diazaboracycloalkane ring as shown in 5. This feature is believed to provide for a sufficient electronic saturation of the boron by annular 7i-backbonding from the two adjacent nitrogen atoms to reduce the Lewis acidity of the boron and thus prevent dimerization to a pyrazabole. The known species of type 5 are surveyed in Table 1. 

Two principal synthe s of monomeric pyrazol-l-ylboranes containing trigonal boron have been developed. One is the condensation reaction as described above in Eq. (1) (method A). The other process involves the symmetrical cleavage of a pyra-zabole skeleton upon reaction with the appropriate aliphatic a, -diamine (Eq. (2) method fi). 

Although the known monomeric pyrazol-l-ylboranes of type 5 do not dimerize with the formation of pyrazaboles, their boron atom has residual Lewis acidity. Hence, they have been found to form 1 1 molar complexes with various nitrogen ba s (see Sect. IIl.B). In addition, an unusual dimerization has been observed for l,3-dimethyl-2-(l-pyrazolyl)-l,3,2-diazaboracyclopentane (compound Nr. 1 in Table 1) at low temperatures On the basis of NMR data, structure 6 was assigned to this dimer 

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