Borabenzene is a hypothetical organoboron compound with the formula C5H5B. Relative to the related but highly stable benzene molecule, borabenzene has two fewer electrons; by Hückel's rule, it would be anti-aromatic.
Adducts of borabenzene with Lewis bases are aromatic and indirectly isolable. A particularly large family are derived from the boratabenzene anion, [C5H5BH]−.
Synthesis Since borabenzene is unavailable, 4silyl-1methoxyboracyclohexadiene or similar boracyclohexadienes (derived from dibutylstannacyclohexadiene, itself from double hydrostannylation of 1,4-pentadiyne) are used as a borabenzene equivalent, e.g.: + → + MeOSiMe3 Possible Lewis bases for the reaction (besides pyridine) include tertiary amines and phosphines, phosphole anions, isonitriles and cyanide anion, bis(IMes)platinum, and (under heat) certain metal carbonyls.
Alternatively, piperyleneboramides and -borate esters cyclize in base (e.g., lithium diethylamide).
Boratabenzene ligands form through (reversible) ring-expansion-oxidation of certain 19-electron cyclopentadienide complexes, and can then be released from the metal through either ligand exchange with cyanide or alkali metal reduction.
In rare cases, dihydroborepins rearrange and deprotonate to borabenzenes; likewise hydride reduction of a pentagonal pyramidal boracation also rearranges to a borabenzene.
As of 2019, there were no known syntheses of uncomplexed borabenzene. Boracyclohexadiene flash vacuum pyrolysis only proceeded under a nitrogen atmosphere to give a borabenzene-dinitrogen complex.
Neutral adducts
Adducts of the form react with many nucleophiles, exchanging out the L moiety in a nucleophilic aromatic substitution addition-elimination. These reactions are generally catalyzed by minute quantities of Lewis acid; thus for example the pyridine adduct is tightly bound in free pyridine: no exchange is observed even at elevated temperatures.
The pyridine adduct is structurally related to biphenyl. It is a yellow whereas biphenyl is colorless, indicating distinct electronic structures.
Adducts with amines generally have a B-N π bond; adducts with phosphines do not. Nevertheless, both behave like a diene in Diels-Alder reactions:
Boratabenzenes
Boratabenzene, sometimes called borinate, is the heteroaromatic anion with the formula [C5H5BH]−. Derivatives of boratabenzene often replace the adduced hydride with a (formal) organyl or alkoxide anion. They have some use as spectator ligands in olefin polymerization catalysts.
Sandwich or half-sandwich type complexes with many transition metals have been reported; indeed, the first known instance of a borabenzene adduct was Cyclopentadienylcobalt 1-phenylboratabenzene, derived from cobaltocene and phenylboron dichloride.
Facially coordinated, these species are 6-electron ligands, weaker donors and stronger acceptors than cyclopentadiene. The metal is typically shifted away from boron in the ring, and it appears that any exocyclic π bond to B delocalizes into a metal-B antibond. Indeed, in ZrCl2(C5H5B−NiPr2)2, the Zr is approximately η5-coordinated (the -OEt analogue returns to η6).
The boron-coordinated hydrogen in the parent ligand ((CH)5BH−) is rather hydridic, and the B−H bond can perform hydroboration.
When coordinated to a metal center, boratabenzenes can exchange boron substituents through nucleophilic substitution. In electrophilic aromatic substitution reactions, carbons 2 and 6 (ortho to boron) react first, with carbon 4 (resp. para) reacting later.
The 1-methyl and 1-phenyl borinates can form some of the few organo-thallium(I) compounds.
See also 6-membered aromatic rings with one carbon replaced by another element: silabenzene, germabenzene, stannabenzene, pyridine, phosphorine, arsabenzene, stibabenzene, bismabenzene, pyrylium, thiopyrylium, selenopyrylium, telluropyrylium Borazine Hexachloroborazine
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