Synthesis of borazines

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. 

Apart from a real interconversion, cyclodimers can be transformed into borazines, however, by addition of iminoboranes [Eq. (25)]. In order to carry out such a reaction, a solution of the iminoborane in a dropping funnel, kept at — 80°C, is slowly dropped into a solution of the cyclodimer at 50°C. The yield of borazines is quantitative. The procedure can be applied to components with the sEime set of ligands, but different sets may also be applied, permitting the synthesis of borazines with an unsymmetric arrangement of more than two different ligands (13.19). 

Borazine, B3N3H6, was first prepared by thermolysis of the diborane ammonia adduct [(BH2)(NH3)2] [BH4] . More convenient procedures for the laboratory preparation of this important ring system in multigram quantities involve either (a) the decomposition of ammonia- borane [eqn (9.1)] or (b) the reaction between ammonium sulfate and sodium borohydride [eqn (9.2)]. The latter method provides a convenient and economical synthesis of borazine. 

Figure 1. Apparatus for the synthesis of borazine. (A) nitrogen supply from tank (B) oil bubbler (C) valve (D) 2-L, three-necked, round-bottomed flask (E) thermometer (F) distilling column (G) vacuum tubing (H) mercury manometer (I) four traps for the collection of the borazine product (J) valve to mechanical vacuum pump.

This boron-nitrogen heterocycle is a convenient route to the synthesis of borazine and -substituted borazines.1-6 The procedure described here is a modification of the reaction of boron trichloride with ammonium chloride.7-9... 

SCHEME 18.25 Synthesis of borazine-based Si-B-C-N-H polymers by hydrosilylation of B-triethynylborazine with trichlorosilane and subsequent hydrogenation with LiAlH4 or aminolysis with methylamine, respectively." - 2 ... 

Transient monomeric intermediates may be formed during the pyrolytic synthesis of borazines. The pathway shown in Eq. (x) has been postulated for the reaction of ammonia with diborane ... 

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. 

Equation (aj) also allows the synthesis of unsymmetrieally substituted borazines, e.g., (EtBMe2B2NjMe3)Cr(CO)3 or (PhBMe2B2N3Me3)Cr(CO)3 . The reactions take place in dioxane at 30-40°C with reduced pressure to pump off eliminated nitrile. The dioxane complexes (C4Hg02)2M2(C0)5 are intermediates. If liquid borazines are used additional solvent becomes unnecessary. 

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 synthesis of B-trichloro-N-tris(trimethylsilyl)borazine was much more complicated than that of the other borazines. The overall scheme is given below ... 

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... 

With the development of CO 2 lasers, work on the infrared photochemistry of boron compounds is now appearing in the literature. Future woric on these compounds with UV laser sources is also expected. In this review the effect of radiation on boron compounds in the photon energy range 0.1 eV (CO2 laser) to 10.2 eV (H-a line) is examined. The range of tropics extends from the use of photochemical techniques for synthesis of new compounds to the production and isolation of reactive photochemical intermediates. The photochemistry of borazine is most extensively discussed. 

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). 

An upsurge of interest in the N-methylborazines in the early 1970 s was coupled with a convenient method of synthesis and purification for these compounds The photoelectron spectrum of N-trimethylborazine has been reported. Table 6 summarizes the theoretical and experimental data comparing the location of the molecular orbitals of N-trimethylborazine with those of borazine. The HOMO is predicted and observed to be an e" (w) orbital as in borazine The methyl substitution on nitrogen destabilizes the e" and the a2 jr-orbitals, but does not signiBcantly effect the e (a) orbital. The result is a lowering of the ionization potential for electrons in the two TT-orbitals. This effect, predicted in the dieoretical calculations, was also verified experimentally. 

N or B-substituted borazines may be made by appropriate substitution on the starting materials prior to the synthesis of the ring ... 

Another example is the poly(borazine) chain, with repeat unit -BRNR-.53,65 The synthesis of tractable, unbranched polymers of this type is complicated by the formation of borazines, which are very stable cyclic molecules, [(RBRN)3], This problem can... 

In addition to information compiled elsewhere 1-7> the preparation of N-trimethylborazine has been described in Inorganic Syntheses 20> and the standard procedures have been used to make several isotopically labeled species of borazine 21> and N-methylborazine 22.28). Recently, the direct synthesis of N-organoborazines by the interaction of nitriles with diborane or alkali metal hydroborates, rather than the two-step procedures involving the reduction of the corresponding B-chloroborazi-nes, has received much attention. Besides the extensive study by Wade et al. 24>, it has been shown 25> that borazines can be prepared in high yield according to the reaction depicted in [Eq. (4)]. 

The tetramer is said to be irreversibly converted into the trimeric product by heating above its melting point. In contrast to an earlier report claiming the synthesis of linear N-isobutyl-B-phenyl-polybora-zynes 86> only the normal borazine-formation could be observed on the pyrolysis of bis(isobutylamino)phenylborane 87> ... 

Direct synthesis of unsymmetrical substituted borazines has been reported infrequently. The feature of borazines with unsymmetrical substitution at the boron atoms resides in the fact that 2-organo- or 2,4-diorgano-borazines may be utilized in polycondensation reactions to yield insoluble compounds of macroscopic linear structure. 

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