Stereoselectivity of Sigmatropic Homodienyl [1,5]-Hydrogen Migrations

Abstract

Interactions affecting the transition state conformational preference of substituents (R¹) at the migration origin have been extensively examined for a range of diversely substituted alkenylcyclopropane substrates undergoing homodienyl [1,5]-hydrogen migration. A chair cyclohexane -like transition state model (11) best rationalizes the stereoselectivity observed in these cases. Thus, the 1,2-double-bond (E)/(Z) ratio for the rearrangement products is in reasonable agreement with prediction based on ΔG° values for R¹ equatorial/axial interconversion in substituted cyclohexanes (at least for cases where R³ = H). For systems incorporating a bulky R³ substituent, a marked increase in stereoselectivity for the (E)-olefin product is observed. In agreement with the proposed transition state model (11), bulky R² or R⁴ substituents have negligible effect on the conformational preference of R¹.