Zum Einfluß von Substituenten auf σ-π-Wechselwirkung: Winkelverzerrungen in C-Borylmethylenboranen / Effect of Substituents on σ-π-Interaction: Distortion of Angles in C-Borylmethyleneboranes

Abstract

C-Borylmethyleneboranes with one or two O-Aryl substituents in the boryl group, 3 a and 4b, are obtained by cleavage of B–C single bonds in the presence of a B–C double bond. The duryl(methyl)borylmethyleneborane 7 is formed by thermal rearrangement of the vinylborane 5a. 4b has an almost normal B–C– B angle (118.3(7)°) and the most strongly deshielded dicoordinated boron atom (δ11B = 80) observed so far for methyleneboranes. This indicates weak C–B hyperconjugation in 4b. The distorted B–C– B angles in 3a (108.8(5)°) and 7 (98.1(5)°) and the shielded dicoordinated boron atom of 7 (δ 11 B = 63) in spite of its positivation by π-π delocalization demonstrate an increase of C–B hyperconjugation upon decrease of electronegativity of substituents in the boryl group.