O. Wiest et al., SECONDARY KINETIC ISOTOPE EFFECTS OF DIASTEREOTOPIC PROTONS IN PERICYCLIC-REACTIONS - A NEW MECHANISTIC PROBE, Journal of the American Chemical Society, 117(33), 1995, pp. 8594-8599
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.