Perpendicular Effects on Transition-State Structure: Model and Application to Cycloadditions and Sigmatropic Shifts

Effects of structures perpendicular to the reaction coordinate can be described by a model of an upside-down free energy profile whose minimum is constrained to coincide with the barrier height along the reaction coordinate and its “intrinsic barrier” proportional to that of the profile along the reaction coordinate. The model shows the same quality of semiquantitative agreement with experimental results for sigmatropic shifts and cycloadditions as the models based on two-dimensional “potential-energy surfaces”. The model also predicts semiquantitatively the “tightness” of the transition state for hexadiene derivatives, as measured by the secondary kinetic isotope effects.