Theoretical Study on the Stereoselectivities of

Abstract: Theoretical study of Wittig reactions were performed with density functional theory (DFT) at the B3LYP levels to understand the origin of the different product selectivities for different classes of phosphorus ylides. In case of the reaction using non-stabilized ylide, the transition states for the oxaphosphetane-forming process that the ylide approached to the carbonyl compound were explored and the four kinds of transition states were converged. One of the transition state having lowest activation energy leaded to Z-olefin and the additional investigation of all the reaction process resulted that the ratio of Z-/E-olefin was controlled kinetically. These calculations were consistent with experimental results and the common accepted explanations. However, the reaction using stabilized ylide also proceeded under kinetic control, not under thermodynamic control. The oxaphosphetane intermediate corresponding to the precursor of E-olefin was obtained preferentially via the lowest transition state. Starting from the oxaphosphetane, the activation energy of the forward reaction to olefin was quite lower than that of the reverse reaction to the reactants. These results suggested the possibility that the stereoselectivity of Wittig reaction using stabilized ylide was also under kinetic control