Secondary Deuterium Kinetic Isotope Effects in Phospha-Michael Addition of Tertiary Phosphines to Electron-Deficient Alkenes

摘要

For the reaction of PPh3 with isotopomeric acrylonitriles in acetic acid solvent, replacement of H by D on the position α to the CN group unexpectedly produced normal secondary kinetic isotope effects (KIEs). These effects were interpreted on the basis of the known vibrational-rotational coupling between two lowest vibrational modes of acrylonitrile, the in-plane and out-of-plane C–C≡N bendings. For isotopomeric acrylonitriles this coupling leads to the difference in zero-point energies (ZPEs) for the in-plane C–C≡N bending, which defines the reaction coordinate during rate-determining protonation of zwitterionic intermediate at heteroatom by acetic acid. Observation of these effects became possible due to dynamic behavior of the zwitterion, whose li10.1002/slct.201700321fetime appears to be comparable with the time of the in-plane bending. The untypical origin of secondary KIEs observed for acrylonitrile was evidenced by the absence of such effect for 2-d-acrylamide. Steric isotope effects for the reaction of d_(15)-PPh3 with a range of electron-deficient alkenes revealed a significant increase in steric strain in transition state for nucleophilic attack step, when a substituent is introduced to the b-position of the alkene.