Thermodynamic control of 1,3-boratropic shifts of α- and γ-stannyl-substituted allylboranes: hyperconjugation outweighs steric effects.

(E)-δ-Stannyl homoallylic alcohols are prepared by an allene hydroboration-aldehyde allylboration sequence ( Chen , M. et al. J. Am. Chem. Soc. 2010 , 132, 7881 ). Key to this reaction sequence is that the kinetic allene hydroboration product, 2a, is less stable than and isomerizes to the more sterically congested α-stannylallylborane 3a (see abstract figure). An M06-2X density functional analysis shows that the C-Sn to boron σ-π hyperconjugation interaction is sufficiently stabilizing to override the steric congestion in 3a.