Reactions of coordinated cyclic polyolefins. Substituent effects on the 1,3-haptotropic rearrangement of tricarbonyl (η⁴-heptafulvene) iron complexes

The kinetics of the fluxional 1,3-haptotropic rearrangement of two 8-substituted tricarbonyl(η⁴-heptafulvene)iron complexes have been studied by dynamic NMR methods. The first order rate constants for the anti-syn interconversion of (8-diphenylmethylheptafulvene)Fe(CO)₃ (acetone-d₆, -1°C) are k_as = 6.77 × 10^-5 s^-1 and k_sa = 8.47 × 10^-5 s^-1, with free activation energies ΔG^# of 21.06 and 20.94 kcal mol^-1, respectively. The rate constants for (8-methoxyheptafulvene)Fe(CO)₃ (acetone-d₆, 25°C) are k_as = 0.26 s^-1 and k_sa = 0.13 s^-1, with activation barriers ΔG^# of 18.24 and 18.65 kcal mol^-1, respectively. Analysis of the substituent effects on the metal shift by Hammett and Taft linear free-energy correlations reveal the dominance of steric over electronic effects, indicating the preference for an η² way-point mechanism.