Oligomerization of Iminoboranes

The activation barrier for the oligomerization of alkynes may be overcome thermally or catalytically. Because the classical thermal transformation of ethyne into benzene in a hot tube is rather ineffective (47), more emphasis has been placed on working out catalytic routes for the synthesis of linear oligomers, cyclooligomers, and polymers. Transition metal compounds have proved to act as effective catalysts in homogeneous as well as in heterogeneous processes (48). 

The stabilization of iminoboranes can yield five different tj ies of products cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr ), cyclotetramers (octahydro-l,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po) these substances are isoelectronic with cyclobutadienes, benzenes, Dewar benzenes, cyclooctatetraenes, and polyalkynes, respectively, which are all known to be products of the thermodynamic stabilization of alkynes. 

A correlation between the iminoboranes and their thermal stabilization products is given in Table III. Thermal, in this context, means at room temperature. Mixtures of two products can be separated either by extraction of the soluble component (Tr/Po) or by distillation 

Ab initio calculations were reported for the parent compound, (HBNH)2 (23, 56). A rhombic Cjy structure with a B—N bond length of 147 pm and a B—N—B angle of 87° was predicted. The jr-bonding 

X R Mean ring bond lengths (pm) Mean ring angles (degree) Reference 

---