Group 13 systems boron-nitrogen

Most attention has been devoted to an exploration of the chemistry of the borazaromatics. In these compounds, two neighboring carbon atoms of an aromatic system are replaced by the isoelectronic boron-nitrogen groups as illustrated by the 2,1-borazaronaphthalene, 7 2>3>. Their great chemical stability is a distinct feature of heteroaromatic materials and several reviews of their chemistry have been published 4 6>. 

The concept of isosterism between the atom groupings B-N and C-C, when introduced more than 60 years ago, implied the expectation that a relatively extensive boron-nitrogen system of compounds, analogue to hydrocarbons, should be forthcoming. 

The BN unit is isoelectronic with C2 and many boron-nitrogen analogues of carbon systems exist. However useful this analogy is structurally, a BN group does not mimic a CC unit chemically, and reasons for this difference can be understood by considering the electronegativity values x (B) = 2.0, x (C) = 2.6 and x (N) = 3.0. 

The formation of a system containing two boron and three nitrogen atoms in a five membered ring cannot be expected to proceed in high yield from a spontaneous reaction. Indeed, preparations of this ring system utilize the most versatile reaction type applicable to B—N compounds the transamination. Transamination usually proceeds by replacement of organo-substituted amino groups by less volatile amines i8 -i8 ) jn an equilibrium reaction. 

Our group has devoted two articles for the study of systems where the chiral subunits are bonded to a boron or to a metallic atom (Scheme 3.28). In both cases, the weakness of the bonds formed can be easily broken. In the first case, a series of bisdiphenylborates and the effect of the fluoro substitution on the relative stability of the homo vs. heterochiral complexes have been studied by means of DFT calculations (B3LYP/6-31G ) [29]. In addition, the corresponding isoelectronic structures, in which the boron has been substituted by a carbon or a positively charged nitrogen, have been considered. In all the cases, the homo and heterochiral complexes present D2 and S4 symmetries, respectively. 

This volume follows the organizational patterns initiated in Volumes X and XI. Syntheses are grouped according to interest area, using this criterion, we have placed in Vol. XII a chapter on Metal Complexes of Molecular Nitrogen and one on Significant Solids. More traditional chapter headings such as Nonmetal Systems and Boron Compounds also fit appropriately under the criterion of interest areas. 

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