Borazine compared with benzene

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, N3B3H6, is isoelectronic and isostructural with benzene. See Section 14.4. The NB bond distance, 144 pm, is intermediate between the bond distances in the planar and orthogonal forms of aminoborane. The bonding may be described in terms of molecular orbitals similar to those of benzene each N or B atom may be regarded as sp hybridised and to form three 2c, 2ea-bonds to its nearest neighbors. Six valence electrons occupy the three r-bonding orbitals formed by combination of the six 2p AOs as indicated in the Fig. 16.9. Compare with the corresponding orbitals in benzene. Fig. 14.6. 

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. 

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... 

The inorganic benzene borazine has been shown to undergo electrophilic substitution in a manner very similar to benzene itself. Does the electrophile attack at or N Use orbital interaction theory to predict the site of attack. Compare this with the prediction on the basis of an SHMO calculation and experimental findings (Chia-varino, B. Crestoni, M. E. Di Marzio, A. Fomarini, S. Rosi, M., J. Am. Chem. Soc., 1999,121, 11204-11210). 

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