Variable transition-state structure in the cope rearrangement as deduced from secondary deuterium kinetic isotope effects

Kinetic isotope effects (KIE's) were determined in several acrylic systems using gas chromatography for analysis. Reaction of diphenyl-1,5-hexadiene was monitored by HR-220 1H NMR spectroscopy. The normal KIE at C3 and C4 in nearly unperturbed systems was found to be about 1/2 to 2/3 that of the inverse KIE at C1 and C6, while in 2-phenyl-1,5-hexadiene, the normal KIE at C3 and C4 was approximately 1/3 that of the inverse KIE at C1 and C6. Also, the normal KIE at C3 and C4 of 2,5-hexadiene was about 1/8 of the inverse KIE at C1 and C6. The transition-state structures deduced by changes in the relative KIE's agree with those predicted by the authors from a study of a More O'Ferrall-Jacks diagram