Abstract
π-Facial diastereoselection has been rationalized in terms of the antiperiplanar hyperconjugative stabilization effect (the AP effect) in the transition state. Reported herein is unequivocal evidence against these conventional models using adamantyl carbocations and carbenes.By quantitative evaluation of the AP effects expressed by the magnitude of AP bond elongation as well as bond population, it was strongly suggested that the AP effect are attenuated toward the transition state and that they operates against observed facial stereo-selection. Namely, the AP bonds are shortened at the transition state. The AP effect should be regarded as a mere internal energy relaxation mechanism that operates against the direction of bond formation process.Therefore the ground-state cations and carbenes were analyzed by ab initio calculation. In every substituted cation and carbene calculated, two conformers (i.e. the E- and the Z-isomers) were obtained. Good correlation was shown between relative energies and experimentally observed stereoselectivities. Moreover, their structural analyses revealed that steric effects around reaction center are entirely consistent with experimental diastereoselectivities. The good linear correlation strongly suggests that steric effects and the equilibrium of the epimeric cations and carbenes (E and Z conformers) are important factors of diastereoselectivity in the 2-adamantyl cations and carbenes.
