Aromaticity borazine

An excellent and detailed account of boron—nitrogen compounds and their similarities with carbon is given in N.N. Greenwood and A. Earnshaw, The Chemistry of the Elements, Pergamon Press, Oxford, 1997 pp. 234—240.The question of whether borazine displays aromatic characteristics is a matter of some debate in the literature. A.K. Phukan, E.D.Jemmis, Is Borazine Aromatic Inorganic Chemistry, 40, 3615—3618,2001. 

So, is borazine aromatic Our results show that the electronic system of borazine is not as homogeneous as that of benzene. The difference of electronegativity between... 

There are similar analogues to other aliphatic hydrocarbons, for example HjN BHj, which is isoelectronic with ethene, and a most interesting compound called borazine, B3N3H6, which possesses physical properties remarkably like those of the aromatic analogue ... 

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 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... 

Prior to 1995 no such inorganic compound, save perhaps for borazine, could boast of aromaticity. The utilization of m-terphenyl sterically demanding ligands in organogallium chemistry would change this forever. 

Figure 46 The planar structures of the boron ring systems, borazine (71) and boroxine (72), with nonexistent aromaticities and of triphosphatriborin (73) with an appreciable degree of aromaticity. (Adapted from ref. 95.)...

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... 

Therefore, the borazine molecule is considered to be aromatic. Borazine has many properties that are similar to those of benzene, although it is more reactive as a result of the B-N bonds being somewhat polar rather than purely covalent. 

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... 

Calculated values of the HSE (see Section II,A,5) (88JA4204) indicate that aromatic character is decreased with an increase in the difference between electronegativities of the neighboring atoms. For example, for the series benzene (22), s-triazine (38), borazine (50), and boroxine (51) the ratio between their 4-31G calculated HSE values is roughly 3 2 1 0... 

A detailed comparison of spectral data concluded that borazine has a delocalized 7T-electron system like that of benzene (112) other workers, however, have concluded from spin-coupled calculations that borazine has litde aromatic stabilization as compared to benzene (113). 

It has been shown that aromatic rings of B-triaryl-A/-triaryl substituted rings are orientated perpendicular to the plane of the borazine ring (114,115). If the aryl rings are substituted it is possible to obtain mixtures of cis and trans isomers (atropisomerism) in which the aryl substituents are on the same or opposite sides of the plane of the borazine ring, respectively (115). 

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