Borabenzenes ligand adducts

Herberich s 1970 report of the synthesis of a cobalt-bound boratabenzene1 and Ashe s 1971 description of the synthesis of lithium 1-phenylboratabenzene2 marked the starting point of the study of borabenzenes, a family of molecules that includes neutral borabenzene-ligand adducts as well as anionic boratabenzenes (Scheme 1). 

Reaction of the same neutral borabenzene-ligand adduct, C5H5B-PMe3, with a transition, rather than an alkali, metal alkyl or amide can furnish r 6-boratabenzene complexes in a single step (Scheme 8).17 This efficient transformation presumably proceeds through initial ir-coordination of CsHsB-PMes to the transition metal, followedby an intramolecular substitution reaction. In contrast to other approaches to the synthesis of T 6-boratabenzene complexes, this synthetic route does not have a parallel in if-cyclopentadienyl chemistry. 

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. 

Fu recently developed a convenient general access to borabenzene adducts and boratabenzene ligands that is based... 

First examples of chiral boratabenzene complexes, which are of interest owing to potential applications as enantioselective catalysts, have recently been reported. The planar chiral chromium tricarbonyl complex (137) was obtained by adduct formation between borabenzene and an (S) -valinol-derived oxazoline ligand. The Cr(CO)3 fragment stereoselectively complexes with this borabenzene adduct. A different approach has been chosen by Herberich, who developed the chiral boratabenzene ligand (138), which is derived from a-pinene. ... 

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