Reactions of coordinated cyclic polyolefins. The kinetics and stereochemistry of the pericyclic [2,2]-sigmahaptotropic rearrangement

The [2,2]-sigmahaptotropic rearrangement of the [2 + 2] adducts of (cycloheptatriene)Fe(CO)₃ with aryl ketenes to the corresponding σ,π-allylic [3 + 2] adducts, in which a σ-bonded carbon and a Fe(CO)₃ fragment exchange bonding sites antarafacially across a σ carboncarbon bond, has been shown to proceed in a single kinetic step. The first order rate constants for the rearrangement are k 6.3 × 10^-6 s^-1 (methanol, 23°C) and k 3.4 × 10^-6 s^-1 (acetone, 51°C), with activation parameters ΔH^# 21 kcal mol^-1 and ΔS^# - 12 e.u. (methanol). Rearrangement without ring opening occurred also in trifluoroethanol. The ratio between the rate constants in methanol and acetone at 51°C is moderate, k_rel 40, indicating a partial charge separation in the transition state without formation of a fully developed zwitterionic intermediate. When the migrating carbon is unsymmetrically substituted, rearrangement occurs stereospecifically with retention of configuration. On the basis of these results a mechanism for the [2,2]-sigmahaptotropic rearrangement which involves a 2a + (2s + 2a)_mo (mo = metal-organic) thermally allowed pericyclic reaction is proposed.