Borazine, 2-vinyl

Ternary materials that incorporate carbon into a boron-nitrogen framework are expected to exhibit extreme hardness as a result of their diamond-like structures. Alkyl-, vinyl- and alkynyl-substituted borazines are potential... 

Although vinyl derivatives of alkylated borazines have been known for some time,15 the methods used for their syntheses cannot be used for the parent B-vinylborazine because of extensive side reactions. The transition metal catalyzed synthetic route described below gives B-alkenyl borazines in high yields.16... 

The procedure18 described below gives lightly cross-linked, pure poly(B-vinyl-borazine) (an inorganic analog of polystyrene) in good yields. 

A 0.056-g sample of AIBN (2,2 -azobisisobutyronitrile) is placed into a one-piece 100-mL flask (such as Chemglass AF-0522-02)t equipped with a Teflon vacuum stopcock and a magnetic stir bar, and the flask is evacuated at — 196°C. A 1.69-g (15.9 mmol) sample of B-vinylborazine (prepared by the procedure described above) is vacuum distilled into the flask. Three freeze-pump-thaw cycles are performed in order to remove any traces of oxygen. The stopcock is closed and the reaction flask is removed to a shielded hood where it is heated in an oil bath at 70°C for about 3 h, at which point the material is sufficiently viscous that the stir bar stops. Then 5 mL of benzene is condensed into the flask and the solution is heated at 70°C for another 9 h. Slow addition of the benzene solution into 40 mL of pentane under inert atmosphere affords the precipitation of 0.73 g (43.2% yield) of poly(B-vinyl-borazine). The polymer is filtered under nitrogen and dried in vacuo for about 5 min. 

The conversion of the vinyl moieties into alkyl substituents gives rise to a signal downfield shift of 5 - 20 ppm in the Si NMR spectra compared to the educt silanes. Varying the silane/borazine... 

Borazine reacts with alkynes (HC H or HC=CR) in the presence of RhH(CO)[P(CeH5)3] to give 2-vinyl- or 2-alkenyl-borazines. The product obtained with propyne consists of a 4 1 mixture of (E)-2-(propen-1-yl)borazine and 2-(propen-2-yl)borazine. In the reaction with ethyne at room temperature the catalyst turnover rate is 293 in a 4 h period, while with propyne it is 419 in a 10 h period [10]. 

Our initial efforts focused on the construction of new types of borazine polymers analogous to poly (styrene) in which the borazine would be bound as a pendant group on a conventional polyethylene polymer backbone. Because of their similarity to conventional organic vinyl polymers, we felt that these types of borazine polymers might be both soluble and processable. 

Thus, our initial goal was to develop new synthetic procedures to allow high yield selective synthesis of vinylborazine. Since our previous work - has demonstrated that transition metal reagents, similar to those widely employed in organic and organometallic chemistry, may be used to catalyze a variety of transformations involving B—H activation reactions, we investigated the use transition metals to catalyze the alkyne-addition and olefin-substitution reactions of borazine. As a result we found, as indicated below, that B-vinyl... 

Polymer precursors fl-alkenylborazines are formed under PdBr2-catalyzed vinylation of borazines with ethylene or propene, and products were accompanied by minor amounts of B-alkylborazines (eq 38). Alkenylpentaboranes are formed under analogous conditions. ... 

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