Borabenzene
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2007-12-19T02:21:01Z
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[[Image:Borabenzenes.png|right|150px|Borabenzene coordinated to pyridine or triphenylphosphine]]A '''borabenzene''' is a [[aromatic|heteroaromatic]] compound that has a [[boron]] [[atom]] instead of the [[carbon]] [[atom]] of a [[benzene]] [[molecule]]. A free borabenzene, which has no donor [[ligand]] on the [[boron]] [[atom]], has not yet been isolated despite its simple structure and the chemical robustness of [[boron]]-[[carbon]] [[chemical bond|bonds]]. The instability of this molecule results from the high [[Lewis acid|Lewis acidity]] of the [[boron]] [[atom]] due to its [[electron]] deficiency. All attempts to create a free borabenzene have resulted in formations of [[complex (chemistry)|complexes]] with [[anion|anionic]] or neutral [[ligand]]s (even a [[nitrogen]] [[molecule]] can coordinate to a borabenzene). However, the complexes with [[anion|anionic]] [[ligand]]s, called '''boratabenzenes''', show rich chemistry including the coordination chemistry as [[anion|anionic]] ''π''-type [[ligand]]s. Borabenzene is also stable with electron donor molecules such as [[pyridine]] or [[triphenylphosphine]].
==Properties as ligands==
Boratabenzenes have a negative [[electric charge|charge]] on their six-membered rings and act as strong ''π''-donating [[ligand]]s like a [[cyclopentadienyl complex|cyclopentadienyl]] [[anion]], which is often used in [[transition metal]] [[complex (chemistry)|complexes]]. In fact, [[metallocene|sandwich or half-sandwich type complexes]] of many [[transition metal]]s have been reported using boratabenzenes, and the [[zirconium]] [[metallocene|sandwich complexes]] exhibit [[catalyst|catalytic activity]] in [[ethylene]] [[polymerization]] similar to [[metallocene|zirconocenes]], which are the [[metallocene|cyclopentadienyl counterparts]].
==Family of boratabenzenes==
Several boracyclic compounds are involved in the family of boratabenzenes: [[naphthalene|1-boratanaphthalene]], [[anthracene|9-borathaanthracene]], [[annulene|boracyclooctatetraene]], and [[biphenyl|2,2’-diboratabiphenyl]]. Among these compounds, [[biphenyl|2,2’-diboratabiphenyl]] is the first [[ligand|bidentate]] [[Lewis acid]] based on a borabenzene framework, so it is regarded as a [[Lewis acid]] analogue of [[2,2’-bipyridine]], a common [[ligand|bidentate ligand]]. This compound forms [[aromatic|polycyclic aromatic hydrocarbon]] analogues containing [[boron]] and [[nitrogen]] [[atom]]s, which show characteristic optical and electrochemical properties due to the [[charge transfer]] between the [[boron]] and [[nitrogen]] [[atom]]s.
==Reactions==
Borabenzene is known to react with electron-deficient [[alkyne]]s in a [[Diels-Alder reaction]] to the [[boron]] pendants of [[barrelene]] called borabarrelenes <ref>''1-Borabarrelene Derivatives via Diels-Alder Additions to Borabenzenes'' Thomas K. Wood, Warren E. Piers, Brian A. Keay, and Masood Parvez [[Org. Lett.]]; '''2006'''; 8(13) pp 2875 - 2878; (Letter) {{DOI|10.1021/ol061201w}}</ref>:
:[[Image:Borabarrelene.png|500px|Borabarrelene synthesis]]
Likewise the benzoborabarrelene is accessible through reaction with [[benzyne]] through 2-(Trimethylsilyl)phenyl triflate and [[caesium fluoride]]:
:[[Image:BenzoborabarreleneSynthesis.png|500px|benzoborabenzene synthesis]]
The boron atom in these barrelene compounds is [[pyramidalization|pyramidalized]] (the sum of the C-B-C angles is 311° not 327°) leading to even greater [[Lewis acid]] strength. This is evident from the unusual small B-N [[bond length]] of 158 [[picometer]] compared to 165pm for ordinary B-N bonds in borane-pyridine adducts.
Another remarkable manifestation of this property is their reluctance to surrender the pyridine ligand (for example in exchange for [[deuterated]] pyridine d<sub>5</sub>-Py) even at 200[[°C]].
In contrast to ordinary [[barrelene]]s, this benzobarrelene is also resistant to [[Ring opening metathesis polymerisation]].
== See also ==
* 6-membered aromatic rings with one carbon replaced by another group: '''borabenzene''', [[benzene]], [[silabenzene]], [[germanabenzene]], [[stannabenzene]], [[pyridine]], [[phosphorine]], [[pyrylium salt]]
==References==
<references/>
[[Category:Heterocyclic compounds]]
[[Category:Boron compounds]]
[[Category:Simple aromatic rings]]
[[ar:بور بنزن]]