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

The transition structures and secondary kinetic isotope effects for re presentative pericyclic reactions, obtained by Hartree-Fock (RHF/6-31G ) and density functional theory (B-LYP/6-31G*) calculations, are repo rted. Isotope effects were calculated with the Bigeleisen-Mayer equati on and the Bell tunnel correction. Both methods give similar predictio ns, but the B-LYP/6-31G results are closer to the experimental data, where available. The cyclobutene electrocyclic ring opening, the hexat riene and octatetraene electrocyclic ring closures, and the reverse re actions show large differences between isotope effects for diastereoto pic hydrogens directed inward (IN) or outward (OUT) on the termini of the transition state. Experimental and prior theoretical results are a vailable for the cyclobutene ring opening and the hexatriene cyclizati on. For the Cope and Claisen [3,3]-sigmatropic shift reactions, the is otope effects of the axial (IN) and equatorial (OUT) hydrogens are pre dicted to be different. For the Diels-Alder [4 + 2] cycloaddition, the re are large differences between isotope effects for the IN and OUT hy drogens at the diene termini and smaller differences for the endo (IN) and exo (OUT) hydrogens of the dienophile. These isotope effect patte rns are analyzed in terms of steric and electronic interactions in the transition states for concerted pericyclic reactions. The more steric ally crowded IN hydrogens have higher bending force constants than the OUT protons. The OUT often have low bending force constants due to di radicaloid character of some of the transition states. The difference between IN and OUT secondary kinetic isotope effects is a sensitive pr obe of transition state geometry and therefore of mechanism.