Aromaticity of borazine

The aromaticity of such inorganic ring systems as borazine, the phosphonitrilic halides (69), and the thiazyl halides (70) has been studied extensively from a theoretical viewpoint. ... 

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... 

The gas-phase chemistry of borazine B3N3H6 (147) and the conjugate N-protonated acid B3N3H7+ indicates analogies with benzene,189 although the aromatic stabilization energy of neutral borazine is only 30% that of benzene, and the reactivities of benzene and borazine are not similar (Scheme 62). Comparable conclusions were reached when HOMA and Iq aromaticity indices were used.190a... 

Analogues of borazine are shown in Scheme 63 the planarity of 148 indicates some aromatic character. Some compounds with P and As were found to prefer... 

In particular, iminoboranes (XBNR) are isoelectronic with alkynes (XCCR). Well-known comparable pairs of isoelectronic species are aminoboranes (X2BNR2) and alkenes (X2CCR2), amine-boranes (X3BNR3) and alkanes (X3CCR3), borazines [(XBNR)3] and benzenes [(XCCR)3], etc. The structure of aminoboranes, amine-boranes, and borazines is well known from many examples. It has turned out that these BN species are not only isoelectronic, but also have structures comparable with the corresponding CC species. In the case of borazines, the aromatic character was widely discussed on the basis of theoretical and experimental arguments. The structural and physical properties of... 

Borazine is isoelectronic with benzene, as B=N is with C=C, (Fig. 16.21). in physical properties, borazine is indeed a close analogue of benzene. The similarity of the physical properties of the alkyl-substituted derivatives of benzene and borazine is ever more remarkable. For example, the ratio of the absolute boiling points of the substituted borazines to those of similarly substituted benzene is constant. This similarity in physical properties led to a labeling of borazine as "inorganic benzene." This is a misnomer because tbe chemical properties of borazine and benzene are quite different Both compounds have aromatic rr clouds of electron density with potential for delocalization over all of the ring atoms. Due to the difference m electronegativity between boron and nitrogen, the cloud in borazine is "lumpy" because more electron... 

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... 

Considerations of magnetic susceptibilities and H n.m.r. data suggest that B-tribromo-, B-trifluoro-, and B-trialkoxy-borazines can be considered as aromatic compounds.296 Their aromatic characters, however, are in the sequence B-tribromo- >B-trialkoxy-> B-trifluoro-derivatives. Comparison of spin-spin coupling constants through 5, 6, or 7 bonds between protons in benzene derivatives and borazines indicates that there is a relatively weak transmission of spin density via the presumed 7r-electron system of borazine.297... 

These criteria have been applied in an assessment of the aromaticity of some borazines and boroxines,151 and to provide the following order of decreasing aromaticity pyridine 1-oxide, pyridine, benzene158 and... 

The absorption spectrum of borazine vapour in the 2015—1800 A region consists of a single 77-w, dipole-forbidden, electronic transition 4 Faraday-effect studies on 26 substituted borazines show that these compounds possess a much greater magnetic rotation than corresponding acyclic compounds. This is analogous to the situation with aromatic hydrocarbons, and can be attributed to electron delocalization around the ring. 

Borazine is a colourless liquid (mp 215 K, bp 328 K) with an aromatic odour and physical properties that resemble those of benzene. The B N distances in the planar B3N3 ring are equal (144 pm) and close to those in the layered form of BN (Table 13.2). This is consistent with a degree of delocalization of the N lone pairs arotmd the ring as represented in 13.28. Structure 13.27 gives one resonance form of borazine, analogous to a Kekule structure for benzene. Despite the... 

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