Mechanism for iron-catalyzed alkene isomerization in solution

Here we report nano- through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO)(4)(eta(2)-1-hexene), in neat 1-hexene solution. UV photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO)(3)(eta(2)-1-hexene), in a singlet spin state. This CO-loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO)(3)(eta(3)-C6H11), via an internal C-H bond-cleavage reaction in 5-25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO-loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers Fe(CO)(3)(eta(2)-1-hexene)(eta(2)-2-hexene) and Fe(CO)(3)(eta(2)- 1-hexene)(2). Because of the thermodynamic stability of Fe(CO)(3)(eta(2)-1-hexene)(eta(2)-2-hexene) over Fe(CO)(3)(eta(2)- 1-hexene)(2) (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.