Reactions of coordinated cyclic polyolefins. A kinetic study of the pericyclic [3,3]-sigmahaptotropic rearrangement

The reaction of tricarbonyl(tropone)iron with tetracyanoethylene (TCNE) was reinvestigated. Two primary cycloadducts, the 3 + 2 and 4 + 2 isomers are formed in a 96/4 ratio. The second order rate constant for the reaction is k 4.4 × 10^-2 M^-1 s^-1 (at 24°C, in acetone) with a free activation energy of ΔG^# 19.2 kcal mol^-1 The 3 + 2 cycloadduct undergoes a facile sigmahaptotropic rearrangement to the formal 5 + 2 adduct with rate constants of k 3.8 × 10^-10 s^-1 (24°C, acetone) and k 5.1 × 10^-4 s^-1 (24°C, methanol), and with activation parameters ΔH^# 20 kcal mol^-1 and ΔS^# -7 e.u. (acetone). The kinetic results suggest that both the cycloaddition and the rearrangment are concerted, nonsynchronous, one-step reactions, involving a slightly polar transition state. Frontier molecular orbital considerations imply that the 3 + 2 and 4 + 2 cycloadditions are symmetry allowed reactions. The structural reorganization which takes place in the [3,3]-sigmahaptotropic rearrangement suggests that the metal migration proceeds by way of a Berry pseudorotation, and the simultaneous 1,3-sigmatropic shift occurs with configuration retention at the migrating carbon. A detailed molecular orbital analysis of the rearrangement is given.