Thermal cis → trans and metal shift isomerizations in acyclic electron-deficient (triene)tricarbonyliron derivatives

The synthesis of cis/trans-tricarbonyl[(4,5,6,7-η)-2,4,6-heptatriene]iron derivatives containing electron-withdrawing groups (COOR, COPh, CN) is reported. At 120°C in toluene the isomeric esters and ketones undergo cis → trans isomerization and metal shift tautomerization to give an equilibrium mixture of isomers containing predominantly the trans shift isomers tricarbonyl[(2,3,4,5-η)-2,4,6-heptatriene]iron ester and ketone, respectively. The thermal cis → trans isomerization is faster than metal tautomerization. Tetracyanoethylene reacts readily in polar solvents only with the trans shift isomers, to give the corresponding 2 + 2 cycloaddition products.