Secondary Deuterium Kinetic Isotope Effects at the Termini of cis-1,2-Divinylcyclopropane and cis-1,2-Divinylcyclobutane in Their 3,3-Sigmatropic Shifts: Evidence for Different Transition-State Structures

Article abstract of DOI:10.1021/jo00289a050

In an effort to understand the substantially lower activation energy for the 3,3-sigmatropic shift of cis-1,2-divinylcyclopropane relative to that of cis-1,2-divinylcyclobutane, we determined the secondary deuterium kinetic isotope effects (KIEs) at the vinyl termini of both materials.For the cyclopropane derivative, k^H/k^d4 = 1/1.29 (+/- 0.09) at 10 deg C in deuteriochloroform.For the cyclobutane derivative, k^H/k^d4 = 1/1.04 (+/- 0.02) at 77.7 deg C in hexachlorobutadiene solvent.These KIEs indicate that there is much more bonding at the terminal carbons in the transition state for the rearrangement of the cyclopropane analogue and relatively little bonding at the termini in the rearrangement of the cyclobutane material.