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

Abstract

AbstractFor 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 lifetime 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 d15‐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 β‐position of the alkene.