Synchronous or asynchronous? An 'experimental' transition state from a direct comparison of experimental and theoretical kinetic isotope effects for a Diels-Alder reaction

The first direct comparison of high-level transition structure/kinetic isotope effect (KIE) calculations with high-precision experimental KIEs in a Diels-Alder reaction is presented. Experimental KIEs are consistent with a moderately asynchronous transition state closely resembling the transition structure for the endo pathway. Findings provide the most direct knowledge of synchronicity of symmetrical Diels-Alder reactions.