Borazine , derivatives

One of the first reports related to the preparation of macrocyclic ring systems with boron atoms deals with the transformation of m-difunctionalized borazines into cyclic di-, tetra-, or hexaborazines [16-19]. 

The cyclic dimeric borazine derivatives 2 and 3 can be prepared by reaction of 2,4-dichloro-l,3,5,6-tetramethylborazine with aromatic 1,3-dihydroxy and l-amino-3-hydroxy compounds (Fig. 2). The products are obtained in yields 

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]. 

3-(Diethylboryl)pyridines and (3-Aminophenyl)boronic Acid Derivatives 

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B. tris(2 -Alkylamino)borazine-Derived Poly Z -(alkylamino)borazines]... 

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). 

Observation of the list of borazine derivatives produced photochemically (Table 3) reveals that all of the reagents yielding photochemical products have an electronegative element (either 0,N,F,C1, or Br) which substitutes at the boron site. No B-C bonds have been formed photochemically, even when, as in the CH3Br and the HFA reactions, methyl or perfluoromethyl radicals are present Also, no B-C bonded compounds were formed when borazine was photolyzed with benzene, pyridine, cyclopentadiene, allene, ketene, acetylene, or acetone. 

Renewed interest in borazine derivatives has resulted from their possible application as precursors to boron nitride ceramics. For example, the inorganic analogue of styrene, (H->C=CH)B3NjH5i has been polymerized and decomposed to produce BN.72... 

The electronegatitives of B and P are similar, unlike those of B and N. As a result, polarization should be less extensive in this compound than in borazine. The B3P3 ring is planar, with equal BP bond lengths and shortened BP bonds, suggesting significant aromaticity. Even more recently the boron of borazine derivatives has been replaced with aluminum to give "alumazenes."74... 

Hydrogels and amphiphilic membranes Poly(carbophosphazenes) and poly(thiophosphazenes) New condensation syntheses NLO and high refractive index polymers Microencapsulation of mammalian cells (PCPP) Polyphosphazene polymer blends and IPN s Borazine derivatives Poly(phosphazophosphazenes)... 

Borazole (borazine), derivatives of, from boron halide-amine coordination compounds, 5 28 Boric acid, H3BO3, esters of, 6 29 Boron bromide, 3 27, 29 Boron chloride, 3 27, 28, 29 Boron chloride-trimethylamine, 6 27... 

It is essential in this preparation to start from the preformed tri-ethylamine-trihaloborane complex. Addition of triethylamine as a hydrogen halide acceptor at a later stage of the reaction (i.e. to the pentafluotoanilino-dihaloborane) does not yield the N-tris(perfluoroaiyl) compound. Rather, a mixture of products is obtained from which the desired compound cannot be isolated in a pure state. Fluoroorgano-borazine derivatives described so far are listed in Table 1. The table also includes pentafluorophenoxy- and pentafluoroanilinoborazines. 

N-tris(Y-fluoroorgano)borazine compounds can be made either by the standard Brown-Laubengayer method or by interacting the corresponding nitriles with diborane in dimethoxyethane. Several N-tris(penta-fluorobenzyl)borazine derivatives have been described 74>. 

A borazine derivative has been obtained from the reaction of pyraza-bole with o-phenylenediamine [Eq. (27)]. The resultant product is characterized by high stability U2). 

As noted above, (cf. II. 2.5) a s-tetrazatetraborine, (C6H5BN—i C4H9)4, has been reported to be formed along with the corresponding borazine derivative when the isobutylamine-phenyldichloroborane adduct was treated with triethylamine 49>. 

Unless the nitrogen is protected by an organic exocyclic substituents, 1,3,2-diazaboracycloalkanes with reactive groups at the boron atom tend to undergo intermolecular condensation reactions. For example, 2-di-methylamino-1,3,2-diazaboracycloalkanes have not yet been isolated in pure form. Rather, condensation to the borazine derivative 11 seems to occur at relatively low temperatures 21> as illustrated in Eq. 10. 

Similar reactions of B-hydropyrazaboles with active hydrogen compounds such as pyrocatechol phenol thiols or additional pyrazole are equally facile but require high temperatures (method D). Therefore, the reaction of pyrazabole with o-phenylenediamine may proceed via the expected substituted pyrazabole as an intermediate. However, the latter is unstable under the reaction conditions and condenses further to yield the borazine derivative 10 with the elimination of pyrazole... 

Alternative synthetic approaches to Si-B-C-N-H polymers starting from functionalized borazine derivatives were published by Srivastava et al. " and Haber-echt et al." Srivastava et al. published a method for synthesizing a soluble borazine-based Si-B-C-N-H polymer by reacting B-chloroborazines with LiSi(Si(CH3)3)3 followed by subsequent polymerization with hexamethyldisilazane. 

The unsymmetrical B -substituted borazine derivatives H(X)(Y)B3N3H3, where X = C1, Y=OCN X = C1, Y = CN X = CN, Y = OCN X=OCN, Y = OCN X = CN, Y = CN, have been prepared by the reaction of the appropriate B-substituted chloroborazine with an Ag1 salt.295 It was found that H atoms in B -substituted borazines, unlike those in H3B3N3H3, are inert to attack by an Ag1 salt, giving the first evidence for a substituent effect in borazine chemistry. 

These two compounds are among the thermally more stable boron halide-amine compounds. Compounds derived from certain primary and secondary amines lose hydrogen halides at elevated temperatures to give condensed boron-imide or borazole (borazine) derivatives. 

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