Borazine and Its Derivatives

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

For a review of borazine chemistry see Gmelin Hand bush der Anorganishen Chemie, Borazine and Its Derivatives, Springer-Verlag, New York, 1978, p. 48. 

Borazine is isoelectronic and isostructural with benzene. Poly(borazylene), a polymer of borazine and its derivatives, is reported extensively as a precursor of BN-coated ceramic libers.86 Polymeric borazine oxide derivative is claimed to be flame resistant (Figure 9.10).87... 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

The cyclic borazine (-BH-NH-)3 and its derivatives form one of the largest classes of B-N compounds. The parent compound, also known as inorganic benzene , was first isolated as a colourless liquid from the mixture of products obtained by reacting B2H6 and NH3 (A. Stock and E. Pohland, 1926) ... 

Catecholborane and its derivatives had a pivotal role in establishing the class of boryl complexes, since they proved to be the most versatile starting materials for the synthesis of such compounds. This was demonstrated in many works especially by Baker, Hartwig, Marder, Norman, and Smith. ia In the course of our investigations we aimed at boryl complexes with different substituents attached to boron, and we could show that boranes with B-N bonds are also very well suited for the synthesis of boryl complexes. Over the last three years a wide variety of products was obtained by salt elimination reactions between anionic transition metal complexes and haloboranes. Examples include aminoboryl- 4, 5, diborane(4)yl- 6, 7, and rj -borazine complexes 8 (Figure 4). 

Diazaborolines, isoelectronic with C5H5-, have been obtained by dehydrogenation of diazaborolidines (79) and from a,/3-diimines and halo-genoboranes (80). From UB- and 14N-NMR, UV, and PE studies, it was concluded that compounds of type 13 possess a 7r -electron system and therefore should be able to complex the Cr(CO)3 fragment. This was proved by the formation (91%) of the yellow complex 64 (80a) (air-stable decomp, above 170°C). As observed in borazine and diazadiborine complexes, only a small upfield shift (13, 8 = 26.2 64, 8 = 18.3 ppm) occurs in the B-NMR spectrum. Its constitution has been derived from lH and 13C-NMR, IR [jmass spectra data. (See Scheme 5.)... 

The first s-diazadiborines were described in 1963 16a<187h Since that time, several four-membered ring compounds of alternating B + N atoms with three-coordinated boron have been reported to result from a variety of different condensation, decomposition or rearrangement reactions of B—N derivatives. With two exceptions, these s-diazadiborine compounds are substituted by bulky substituents, and it has been shown 168> that steric hindrance is less significant in four- and eight-membered B—N rings than in the six-membered borazines. 

The gas-phase reactivity of selected ionic species with borazine and borazine-derived ions with selected neutral molecules has been studied by FT-ICR and ab initio calculations. Evidence for an electrophilic substitution on the borazine ring was found in the alkylation by gaseous Me2F+, Me2CH+, and MesC that led to neutral substitution products, as a result of an N-alkylation. Further studies on the protonation, all lation, and nitration of neutral borazine have been compared with the corresponding reactions of benzene, and it was shown that the cyclic B3N3H5+ reacts similarly to the phenylium cation CeHs. In this sense, borazine shows a remarkable similarity to benzene. 

A poly(aminoborazine), precursor for hexagonal boron nitride obtained by reaction of borazine B3N3H6 with ammonia, and its pyrolysis derivatives have been extensively characterised by and B MAS NMR. ... 

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)... 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Benzene may be hydrogenated to produce the saturated compound cyclohexane. Hydrogenation of borazine results in polymeric materials of indefinite compositions. Substituted derivatives of the saturated cycloborazane, 83N3H,2. form readily by addition to borazine (Eqs. 16.39 and 16.41), but special techniques are necessary to prepare the parent compound. It was first synthesized by the reduction of Ihe chioro derivative ... 

These molecules (isoelectronic with ethylene derivatives) are planar and have short B-N bonds. Borazine B3N3H6 is isoelectronic with benzene and has a similar planar, hexagonal structure. It is probably best formulated as ... 

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. 

B-Siloxyborazine derivatives are formed by reaction of B-mercapto-alkylborazines with NaOSi(CH3)3 144> and their thermal stability has been investigated 150>. It was found that borazines with electron-donating groups on boron show reduced thermal stability as compared to hexaorganoborazines. 

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