Borazinic ring

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

Numerous other reactions have been documented, most of which are initiated by nucleophilic attack on B. There is no evidence that electrophilic substitution of the borazine ring occurs and conditions required for such reactions in benzenoid systems disrupt the borazine ring by oxidation or solvolysis. However, it is known that the less-reactive hexamethyl derivative B3N3Mee (which can be heated to 460° for 3 h without significant decomposition)... 

By reaction of 2-alkyl-4,6-dichloro-l,3,5-trimethylborazines (alkyl = methyl, ethyl, i-propyl) with bis(trimethylsilyl)amine the tetrameric borazine ring systems 4-6 are produced (Fig. 2) they can be purified by several successive vacuum sublimations (yields 4-60%). If the borazines carry n-propyl and tert-butyl groups in the 2-position or if methylbis(trimethylsilyl)amine is used to bridge the borazine molecules, the macrocyclic ring formation is inhibited [17, 18]. 

In a similar way, reaction of 2,4-dichloro-l,3,5,6-tetramethylborazine with N,N-dimethylformamide and dimethylamine gives a 36-membered macrocyclic oligoborazine 7 with six borazine rings linked by oxygen bridges (Fig. 2). Again, purification through several vacuum sublimations is necessary and yields are quite low [19]. 

Borazlne synthesis. The ease of borazine ring condensation would be expected to depend on the nature of the substituents. To investigate these aspects, a series of compounds were synthesized, namely B-tr i chloro-N-tr 1 phenyl borazlne, B-triamino-N-trlphenylborazine, B-trianilinoborazine, B-tris[di(trimethylsilyl)amino]borazlne, B-trlchloro-N-tris(trimethylsilyl)borazine, and B-triamino-N-tris-(trimethylsilyl)borazine. 

The nB NMR spectrum of the product showed two broad signals at 27.7 ppm and 24.7 ppm, which are attributed to boron atoms of the borazine ring and the boryl substituents, respectively. These results are consistent with the formation of the /i-tri [bis( methy lam ino (boryl (methyl jam ino]borazine 13 as the main product (Fig. 6). No free B(NHCH3)3 was detected. 

By analogy with B-trialkylaminoborazi ne and polyborazine derived therefrom, the first route envisioned to poly(borylaminoborazine) was the thermal condensation of molecular precursors under a convenient atmosphere. As detailed earlier the innovative idea behind this procedure is to tailor the polymeric precursor structure by increasing the distance between the two borazinic rings. For that purpose, we explored... 

As expected, borylaminoborazines behave like 2,4,6-trialkylaminoborazines. Two competitive mechanisms occurred during the thermal condensation in vacuo the formation of direct B N bonds between two borazinic rings and the formation of three-atom bridges of the N B N type between two rings (scheme 16).53... 

Synthetic methods are available for the incorporation of other p-block elements into a borazine ring. An illustration of the potentially versatile methodology using an NBNBN building block is given for the tellurium-containing borazine 9.11 in eqn (9.5). ... 

Two intriguing examples of this class of compound involve ring systems in which one boron atom of a borazine ring is replaced by the isoelectronic units HC or C , as represented by the generic carbocation A or the carbene B. 

It has been shown that aromatic rings of B-triaryl-A/-triaryl substituted rings are orientated perpendicular to the plane of the borazine ring (114,115). If the aryl rings are substituted it is possible to obtain mixtures of cis and trans isomers (atropisomerism) in which the aryl substituents are on the same or opposite sides of the plane of the borazine ring, respectively (115). 

A decrease in volatility can be achieved by the linkage of borazine rings to form a cyclolinear oligomer or polymer of the type shown in 9.19. Oligomers that approximate to this structure have been produced by the controlled dehydrogenolysis of borazine (reaction (15)) and these yield /-boron nitride when heated to 1,000 °C.55... 

Other approaches to boron nitride, by Paine, Paciorek, and Sneddon and their coworkers are shown in reaction sequences (16) and (17), processes that involve the loss of Me3Si, NH2, or Cl side units from borazine rings as pyrolysis proceeds in the temperature program leading up to 1,000 °C.61-63... 

The cyclodisilazane is not precisely planar. The sum of the angles around the nitrogen atom N(l) is 352.1°. Both of the borazine rings have a boat conformation.49... 

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