Isomerization of bicyclo[5.1.0]-2,4-octadieneiron tricarbonyl to bicyclo[4.2.0]-2,4-octadieneiron tricarbonyl

It has been shown that bicyclo[5.1.0]-2,4-octadieneiron tricarbonyl undergoes thermal isomerization to bicyclo[4.2.0]-2,4-octadieneiron tricarbonyl. The first-order rate constant for this process at 120.6° was determined to be 1.8 X 10^?5 s^?1 which corresponds to ΔF³ = 31.8 kcal/mol. Gas chromatographic results coupled with a kinetic analysis very strongly suggest that 1,3,5-cyclooctatrieneiron tricarbonyl is an intermediate in this isomerization. Two plausible mechanisms are offered to explain this rearrangement.