Trichloroborazines

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

The synthesis of borazine (Fig. 1) was first described in 1926 by Stock and Pohland.12 More recently, Wideman and Sneddon reported interesting three one-step synthetic procedures using various starting compounds including 2,4,6-trichloroborazine, metal borohydrides, and ammonia-borane.13 The B N bond is isoelectronic with the C C bond, which explains why borazine is often presented as the inorganic analog of benzene, that is, borazine has almost the same colligative properties as benzene. 

Paciorek et al. have investigated the direct formation of a polymer from the reaction of 2,4,6-trichloroborazine and hexamethyldisilazane.29,30 This one-step synthetic procedure also has been studied by Paine et al.31 The reaction in principle is depicted in scheme 5, and its main interest is that the driving force is the formation of stable and volatile Me3SiCl. 

In order to increase the organic solvent solubility of the polymers formed by reaction of 2,4,6-trichloroborazine with a disilazane, Paine et al. proposed the selective use of alkyl- substituted R-chloroborazines as monomers.32 These polymers are intended for materials applications, where the methyl substituent is used to ensure... 

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... 

B,j3,B"-Trichloroborazine forms white crystalline needles, m.p. 83.9 to 84.5°. It is extremely sensitive to moisture and decomposes violently in water. It is soluble without decomposition in anhydrous nonprotonic organic solvents such as benzene, ethyl ether, and carbon tetrachloride. 

Borazine originally was obtained by the reaction of ammonia with diborane.1 Mixtures of lithium or sodium tetrahydro-borate with ammonium chloride also have been pyrolyzed to yield this product.2 More recently, the reduction of B,B, B"-trichloroborazine with alkali metal hydroborates has proved to be a convenient laboratory-scale method for the preparation of this compound.3-7 The procedure described herein is a variation of the last method as reported by Dahl and Schaeffer.7 This method is effective for the synthesis of iV-substituted alkyl- and arylborazines, i.e., compounds of the formula (HBNR)3 where R is CH3, C2H6, C6H , C6H5, p-C6H4CH3, or p-C6H4OCH3.5... 

The trichloro derivative having a chlorine atom bound to each of the boron atoms is known as B-trichloroborazine. Borazine can be prepared by several different methods, one of which is the reduction of the trichloro compound ... 

On a larger scale, the reaction of (NH2)2CO with B(OH)3 at high temperature in an atmosphere of ammonia is used to produce borazine. Trichloroborazine, B3N3CI3H3, has the structure... 

The compound [Q2C=N—BC12]2 is formed on photochemical chlorination of l,3,5-trimethyI-2,4,6-trichloroborazine 3°) or by interaction of trichlorobor-ane with thiocyanogen trichloride. The reaction of the latter compound with tribromoborane leads to [C12C=N—BBr2]2 21). These processes are summarized in Eq. (7). 

The observation that l,3,5-trimethyl-2,4,6-trichloroborazine yields some dimeric (dichloromethyleneamino)dichloroborane upon photochemical chlorination 30> (c.f. Eq. (7)) prompted the synthesis of this compound by the reaction of BC13 with C1SNCC12 2I> (c.f. Eq. (7)) and also by the 1,2-addition of trichloroborane to chlorocyanide 23> as depicted in Eq. (15). 

Synthesis. The parent compound, borazine [6569-51-3], is best prepared by a two-step process involving formation of B-trichloroborazine followed by reduction with sodium borohydride. These reactions have been studied in some detail (96). 

Hydrolysis. Borazine is slowly hydrolyzed by water at ambient or higher temperatures to boric acid. Substituted borazines react with water to give ring-cleavage products, eg, boronic acids and amines (122). B-Trichloroborazine reacts with water to give boric acid and chloramine (123). 

Addition Reactions. In general, polar molecules such as hydrogen halides add across the B—N bonds, the more electronegative group bonding to boron (91). The adducts are cydotriborazanes such as the product formed by reaction of B-trichloroborazine and hydrogen chloride (eq. 35). X-ray crystal analysis shows the structure exists in a chair conformation (124). 

Complex Formation. B-Trichloroborazine was reported to readily form crystalline adducts of uncertain structure with pyridine (131). The Lewis acids aluminum tribromide or gallium trichloride form 1 1 adducts with hexamethylborazine (eq. 36) in which the metal atom coordinates with a nitrogen with loss of planarity of the ring (132,133). 

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