SECONDARY KINETIC ISOTOPE EFFECTS OF DIASTEREOTOPIC PROTONS IN PERICYCLIC-REACTIONS - A NEW MECHANISTIC PROBE

The transition structures and secondary kinetic isotope effects for representative pericyclic reactions, obtained by Hartree-Fock (RHF/6-31G*) and density functional theory (B-LYP/6-31G*) calculations, are reported. Isotope effects were calculated with the Bigeleisen-Mayer equation and the Bell tunnel correction. Both methods give similar predictions, but the B-LYP/6-31G* results are closer to the experimental data, where available. The cyclobutene electrocyclic ring opening, the hexatriene and octatetraene electrocyclic ring closures, and the reverse reactions show large differences between isotope effects for diastereotopic hydrogens directed inward (IN) or outward (OUT) on the termini of the transition state. Experimental and prior theoretical results are available for the cyclobutene ring opening and the hexatriene cyclization. For the Cope and Claisen [3,3]-sigmatropic shift reactions, the isotope effects of the axial (IN) and equatorial (OUT) hydrogens are predicted to be different. For the Diels-Alder [4 + 2] cycloaddition, there are large differences between isotope effects for the IN and OUT hydrogens at the diene termini and smaller differences for the endo (IN) and exo (OUT) hydrogens of the dienophile. These isotope effect patterns are analyzed in terms of steric and electronic interactions in the transition states for concerted pericyclic reactions. The more sterically crowded IN hydrogens have higher bending force constants than the OUT protons. The OUT often have low bending force constants due to diradicaloid character of some of the transition states. The difference between IN and OUT secondary kinetic isotope effects is a sensitive probe of transition state geometry and therefore of mechanism.