Thermodynamic Control of 1,3-Boratropic Shifts of α- and γ-Stannyl-Substituted Allylboranes: Hyperconjugation Outweighs Steric Effects

(E)-delta-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 alpha-stannylallylborane 3a (see abstract figure). An M06-2X density functional analysis shows that the C-Sn to boron sigma-pi hyperconjugation interaction is sufficiently stabilizing to override the steric congestion in 3a.