Borabenzene

Boraanthracene — see Dibenzo[6,e]borin 9-Boraanthracene anions nomenclature, 1, 631 Borabenzene anions nomenclature, 1, 631... 

Cobaltocene is partially oxidized and in part undergoes insertion of a borylene group, RB. The borinato ligands derive from the unknown borabenzene ( 6.5.3.4). Some porphyrinatoindium and thallium complexes ( 6.5.2.2) can also be synthesized via oxidative addition reactions (TPP)InCl is added oxidatively to Co2(CO)g and Mn2(CO) to give (TPP)In—Co(CO)4 and (TPP)ln — Mn(CO)j, respectively, and (oep)InCl is added to CojfCOg to yield (oep)ln— 0(00)4. 

Comparable reactions between cyclic divinyl boranes and metal carbonyls lead to borinato (borabenzene) complexes... 

As described ( 6.5.3.1) l-phenylbora-2,5-cyclohexadiene (cyclic divinylborane) reacts with FeCCOj under irradiation to the Fe(CO)3 complex. The dinuclear borabenzene complex [(C5H5BPh)Fe(CO>2]2 is found as a by-product. It is to be compared with the well-known [Tj -CpFe(CO)2]2. CjHjBPh reacts spontaneously with Co2(CO)u to form three products. Above 60°C (C5H5BPh)Co(CO)2 is the only product . At 30°C yellow isomers III and IV having partially hydrogenated borabenzene ligands form above 40°C III isomerizes to IV. 

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). 

The two faces of the borabenzene ring of this borabenzene-oxazoline adduct are inequivalent (diastereotopic), and complexation to Cr(CO)3 occurs on the less hindered face with high diastereoselectivity (Scheme 3). This work provided the first description of an enantiopure borabenzene and of an enantiopure planar-chiral Lewis acid complex. 

Attempts to synthesize free borabenzene through an isomerization/retro-ene sequence12 or to stabilize it through incorporation of a bulky /-butyl substituent in the 2-position have been unsuccessful (Scheme 6).13... 

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. 

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. 

The condensation of pyridine and trimethylphosphine complexes of borabenzene with dimethyl acetylenedicar-boxylate leads to the corresponding 1-borabarrelene complexes 146 and 147 (Scheme 58). 

The discovery of 1 (1), in 1970, opened a new and fascinating chapter of organometallic chemistry. This cation was the first compound derived from the hypothetical borabenzene 2 and the first complex of a classical boron-carbon ligand. Since then approximately 100 borabenzene derivatives, mainly complexes of 3d metals, have been characterized. Other unsaturated boron-carbon systems have been shown to act as ligands to metals (2). This development has also strongly stimulated the challenging quest for the simple species 2-5. 

Several earlier reviews have treated borabenzene metal complexes in various contexts and in varying depth (2-5). This article places emphasis on recent results and perspectives while still being comprehensive. 

The nomenclature of boron compounds involves some intricacies. IUPAC rules allow the terms borabenzene or borinine for 2 the older name borin has become obsolete with the recent revision of the extended Hantzsch-Widman system (6). Anions 4 are termed boratabenzene ions an alternative would be borininate instead of the earlier borinate (7). 

Borabenzene itself (2) has remained elusive. A qualitative contention that 2 should be a highly reactive species that can be stabilized by Lewis bases (7) has been reinstated by semiempirical and ab initio calculations (24,25). Moreover, the calculation predicts a singlet ground state (2a) and a singlet (2a)-triplet (2b) separation of approximately 1 eV (25). 

In mass spectra of borabenzene metal complexes, ions of type [M(C5H5B)]+ are commonly observed [Co(CsH5B)]+ decays via loss of an uncharged C5H5B fragment as evidenced by the observation of a metastable ion peak (7). 

Recently the pyridine adduct of borabenzene 5 has been synthesized as a yellow crystalline solid (26). Its structural characterization by X-ray methods provides the first structural data of an uncomplexed borabenzene... 

All borabenzene-metal complexes investigated structurally so far show very similar patterns for the ligand geometry (Table I) and for the metal-ligand bonding (Table II) only the cobalt complex 6 deserves separate consideration (see below). 

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