The Borazines

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]. 

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. 

Generally, during hydroboration polymerization of dicyano compounds, the formation of the borazine structures that have a six-membered boron-nitrogen ring (scheme 19a) and dihydroborated end groups (scheme 19b) as a structural defect is unavoidable. The borazine cross-linked structures often cause the gelation, and dehydroboration causes a decrease in molecular weight. 

Compounds belonging to the borazine structural class which is based on a six-membered ring of alternating boron and nitrogen atoms are readily prepared by a number of reactions including... 

Therefore, the borazine molecule is considered to be aromatic. Borazine has many properties that are similar to those of benzene, although it is more reactive as a result of the B-N bonds being somewhat polar rather than purely covalent. 

The strong asymmetry of 7Tbn conjugations can also be seen clearly in the form of the borazine NLMOs,... 

Synthesis of B-monosubstituted Borazine Derivatives. The photolytic reaction of borazine with a second reagent is a convenient method for synthesizing a number of B-monosubstituted borazine derivatives. B-monoaminoborazine, produced in the gas phase photolysis of borazine ammonia mixtures with 184.9 nm radiation, was first synthesized by Lee and Porter in 1967. This is the only method currenfly known for generating this compound. A detailed study of the photochemical reaction, under varying conditions of borazine and ammonia pressures, was reported by Neiss and Porter in 1972. The quantum yield for the production of H2 according to the overall Eq. (19) varies from 0.27 and 1.17 when the initial NH3 pressures are varied from 0.1 to 7.0 Torr and the borazine pressure is maintained at 5.0 Torr (Fig. 11). 

Fig. 12. Quantum yield data for CH4 and H2 production from the borazine-CHsBr reaction. In des nated runs added gases were CO2 and Xe. Points with vertical arrows represent true 184.9 nm yields, corrected for the CH3Br absorption at 253.7 nm...

The borazine cation formed by irradiation of borazine reacts with a second molecule in a bimolecular proton transfer reaction ... 

Equation (25) may shed light on the general path of the iminoborane oligomerization. I propose the formation of cyclodimers to be the first stage of such oligomerizations. If a cyclodimer is stable to an excess of iminoborane, it will be isolated (Table III). Otherwise the cyclodimer is attacked by the excess iminoborane according to Eq. (25), and the borazine is formed via the Dewar borazine. In special cases, the Dewar borazine will be the final product. The first step determines the rate of such a sequence of reactions. If the cyclodimerization step becomes relatively fast, so that the first and the second step are comparable in rate, both the cyclodimer and the cyclotrimer will be found this is true for the thermal stabilization of sBuBNsBu. Catalysts for the cyclodimerization make the first step more rapid than the second one. 

A plausible alternative mechanism involves as a first step the formation of a linear dimer, XB=NR—BX=NR, according to the left part of Eq. (21). This linear dimer will be more stable in entropy but less stable in energy than the corresponding cyclodimer. The second step would be addition of iminoborane to the open-chain dimer giving the borazine in either a concerted or a two-step mechanism. The intramolecular cyclization of the open-chain dimer, according to the right-hand side of Eq. (21), would compete addition of an iminoborane. We cannot definitely exclude a mechanism via open-chain dimers. 

Polymers are formed, together with borazines, from iminoboranes RBNR with a-unbranched alkyl groups. By absolute control of the temperature, the stabilization would presumably be directed toward the borazine. Loss of thermal control will cause a loss of kinetic control, so that hot iminoborane molecules will trimerize or polymerize rather unspecifically. 

There is one further remarkable interconversion. The four polymers (RBNR) (mentioned in Section IV,A) are thermally stable, except for (EtBNEt) , which can be transformed into the borazine (EtBNEt)3 at 150°C (18). Such a depolymerization will proceed without considerable change of energy but with a substantial gain in entropy. For kinetic reasons, it cannot proceed with alkyl groups larger than ethyl. 

During a proton NMR examination of a CCI4 solution of MejNsBgFs, an extraneous peak was observed in the spectrum, although the borazine was known to be analytically and mass spectrometrically pure. One microliter of the solution was subjected to GC/MS analysis. Peaks identified as trimethylsilane (TMS) and CCI4 were obtained followed by the expected MesNjBsFs (m/e 177, 176, etc.). An additional component... 

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