| Popis: |
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. |
