Boron heterocycles borazines

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

Keywords Borazines Boron heterocycles Boroxins Cyclocondensation Heteraborines Heteraboroles Isoelectronic Isosteric Macrocycles... 

The original method1 reported for the preparation of trichloroborazine involves the use of borazine. However, this procedure is not recommended because of the difficulty of preparing this heterocycle and its hydrolytic instability. More recently, this trichloroborazine has been prepared directly from ammonium chloride and boron trichloride at elevated temperatures with or without a solvent.2-8 Ammonium chloride may be replaced by a primary aliphatic or aromatic amine or its hydrochloride, and organodihaloboranes can be used in place of the boron trichloride to give the respective N- and B-substituted borazines.6 Boron tribromide has been used in analogous reactions to yield -B,/ , .B"-tribromoborazines.6... 

Spin-coupled calculations, analogous to those for the organic heterocycles, have been carried out for both molecules. Three of the spin-coupled n orbitals for borazine are related to each other by successive C3 rotations and they take the form of fairly localized, slightly distorted N(2p) functions. The other three orbitals are also transformed into one another by successive C3 operations. These are also based on N(2p) functions but clearly show significant delocalization onto neighboring boron centres. Nonetheless, most of the electron density remains on N. 

The heterocyclic compounds occupy an important place amongst boron-containing ligands [8], It is well known that borazines, having a six-member ring, form ri6(7i)-complexes, for example 554 [953], It was recently shown that cr(B)-metal-coordinated compounds of type 555 may also be obtained on the basis of this ligand system [954a] ... 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

This boron-nitrogen heterocycle is a convenient route to the synthesis of borazine and -substituted borazines.1-6 The procedure described here is a modification of the reaction of boron trichloride with ammonium chloride.7-9... 

Heating the inorganic heterocyclic ring B3N3H6 leads to the loss of hydrogen to afford a polymer where the borazine rings are interconnected with each other [27] (see Eq. 1.12). These types of polymers have been found to be useful as precursors for the preparation of the ceramic boron nitride. 

---