Borata-Alkene Species as Nucleophilic Reservoir

alpha-Monoboryl anions show remarkable stability due to the valence deficiency of the adjacent three-coordinate boron center and can be expressed as its resonance form, the borata-alkene systems [R2B=CH2](-). The diversity of synthetic approaches as well as the properties of the C=B bond are disclosed in this review, dealing with both electronic and structural properties of the boryl moieties involved. Full characterization in solid state by X-ray diffraction demonstrated the short distances between B and C, as a consequence of the boron ylide character in the boron-stabilized carbanions. This review includes a collection of C-B length distances as well as B-11 NMR data that can be useful for diagnostic evidence of the partial boron-carbon double-bond character. Natural bond orbital (NBO) analysis on DFT computed structures also supports and justifies the borata-alkene character. The reactivity of the C=B bond acting as nucleophilic synthon is unveiled through extensive electrophilic trapping examples. The homologation protocols with diborylmethane, via single carbon chain extension, involves a facile introduction of the C(sp(3))-B bonds, which can be subsequently transformed into functionalized target products, containing C-O, C-N or C-C bonds.

Automated summary

The stability of alpha-Monoboryl anions and their resonance form borata-alkene systems are discussed in this review, along with the synthetic approaches and properties of the C=B bond. The characterization by X-ray diffraction and NMR data provide evidence for the partial boron-carbon double bond character, while reactivity studies demonstrate the nucleophilic potential of the C=B bond in electrophilic trapping examples. Homologation protocols with diborylmethane offer a facile method for introducing C(sp(3))-B bonds for subsequent transformations into functionalized products.