Borabenzene anion, aromaticity

Boron. Earlier papers reviewed the properties of the borabenzene anion and discussed its potential aromaticity.130 182 A few examples have been shown in Schemes 31 and 53. The borabenzene anion (borinate) 141, with R = phenyl or methyl, gives complexes with transition metals such as Co (142) and Mn (143) (Scheme 61).183 Aza- and polyaza-... 

There are a number of heteroatom six-membered aromatic rings which are analogous to benzene in that they can donate six electrons to a metal. These include phosphabenzene. borabenzene anion, bomzire. and arsabenzene shown in Fig. I5.42.uv... 

The classic aromatic systems are planar molecules. Solid-state structural data have been obtained only for heavily substituted arsenic derivatives and for the borabenzene anion in a transition metal complex. The following data were obtained for these systems (1, 171, 303) ... 

The data obtained for arsenin seem to support the premise of aromatic character however, the evidence is more tenuous for borabenzene anion. Future synthetic studies may produce a neutral, uncomplexed borabenzene derivative useful for structural analysis. This will be necessary before the presence or absence of aromaticity can be established. 

Neutral borabenzene-PMe3, generated through the route described in Scheme 2, reacts with a variety of anionic nucleophiles to furnish 5-substituted borataben-zenes (Scheme 7).15 This approach provides efficient access to boratabenzenes that bear a range of boron substituents (H, C, N, O, P) with diverse electronic and steric properties.16 Mechanistic studies establish that this novel aromatic substitution process follows an addition-elimination pathway. 

Boratabenzenes represent a second important class of unsaturated boracycles. " While the dicoordinate borabenzene itself has not yet been reported, many Lewis base adducts, boratabenzene salts, and metal complexes are known. Boratabenzenes are six r-electron aromatic anions that resemble cyclopentadienides. For example, (V, A-diisopropyl-1-amino-boracyclohexadiene shows an acidity in DMSO which is similar to that of cyclopentadiene. The chemistry of boratabenzene ligands dates back to work by Herberich and Ashe in the early 1970s, which has been reviewed. However, many new achievements have been reported over the last decade and will be discussed in the following sections. 

Deuterolysis of the boron derivative with acetic acid-di gives m-1,2-pentadiene which has incorporated three deuterium atoms. Thus, chemical evidence appears to support a borabenzene structure. If delocalization of the negative charge generated at the position a to the Si atom into the d orbitals on the silicon occurs, an aromatic system may result. The spectral properties of this system are discussed in a later section. Formation of germacyclopentadiene anion may also involve formation of a bw-electron system. Protonation of the anion generates the starting material. 

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