Organoboron Chemistry - Boryllithium

Nucleophilic anionic boryl compounds have long been elusive but a 2006 study described a boryllithium compound which reacts as a nucleophile :

This is remarkable because in other period 2 elements lithium salts are common e.g. lithium fluoride, lithium hydroxide lithium amide and methyllithium. Reaction of base with a borohydride R₂BH does not result in deprotonation to the boryl anion R₂B- but to formation of the boryl anion R₂B-H(base)+ because only this reaction path gives a complete octet. Instead the boryl compound is prepared by reductive heterolysis of a boron-bromide bond by lithium metal. The new boryl lithium compound is very similar to and isoelectronic with N-heterocyclic carbenes. It is designed to benefit from aromatic stabilization (6-electron system counting the nitrogen lone pairs and an empty boron p-orbital, see structure A) and from kinetic stabilization from the bulky 2,6-diisopropylphenyl groups. X-ray diffraction confirms sp2 hybridization at boron and its nucleophilic addition reaction with benzaldehyde gives further proof of the proposed structure.