“Iron-catalyzed Intermolecular [2π-2π] Cycloaddition” Russell, S. K.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858-8861. DOI: 10.1021/ja202992p
As the cost of precious metals increases dramatically along with concerns over the toxicity of 2nd and 3rd row metals, chemists are increasingly turning to employing earth-abundant metals in catalysis, especially iron.
In their recent contribution in the area of base metal catalysis, the Chirik group at Princeton reports an intermolecular, iron-catalyzed cycloaddition reaction. In addition to building on the intramolecular version of the reaction they had previously reported, the current contribution is also notable for their isolation of a catalytically competent intermediate.
The chemistry starts with their remarkable iron bis(dinitrogen) complex 1 (or a related bridging diiron dinitrogen complex), a formally zero-valent compound with an electronic structure better described as a dianionic bis(imino)pyridine ligand bound to an intermediate spin iron(II) ion.
The redox non-innocence of the supporting ligand enables the iron center to do two electron chemistry (required for oxidative addition and reductive elimination), reactions usually reserved for 2nd and 3rd row transition metals. In Chirik’s system, iron generally stays in the preferred ferrous oxidation state, while the ligand undergoes two electron reactions cycling between a neutral donor and a dianionic form during catalysis.
The bond-making and bond-breaking events still occur at the metal center (as for more traditional organometallic reactions, think Pd(0)/Pd(II) chemistry), the trick is that the accompanying redox changes occur at the ligand.
The bis(dinitrogen) complex 1 can catalyze the intermolecular cycloaddition of 1,3-butadiene with ethylene to form vinylcyclobutane. By introducing a methyl group into the butadiene substrate (isoprene), the 1,4 addition product is formed instead.
When perdeutero ethylene is used, deuterium incorporation occured solely at the 2 and 3 positions of the cyclobutane ring for the butadiene reaction and in the 1 and 2 positions of the diene as well as the methyl group for the isoprene reaction.
To identify potential catalytic intermediates, the Chirik group exposed butadiene complex 2 (reported previously) to ethylene. Interestingly, they isolated the metallocycle 3 resulting from formal insertion of ethylene into the coordinated diene.
Though this complex didn’t thermally reductively eliminate the cyclobutane product, it released vinylcylcobutane upon exposure to more butadiene or CO.
With this information, Chirik proposes the following catalytic cycle: (i) displacement of dinitrogen by butadiene to form 2, (ii) insertion of ethylene to form the metallocycle 3, and (iii) ligand-induced reductive elimination to regenerate the butadiene complex which can undergo subsequent turnovers. The cycle likely diverges for isoprene from the metallocycle to form the olefin hydride complex by β-hydride elimination which provides the 1,4-addition product by reductive elimination.
The future direction of this project would probably be to investigate substrate scope. I’m also interested in whether the iron center maintains the ferrous state during catalysis (with the redox changes occuring solely at the bis(imino)pyridine ligand) as proposed for the intramolecular reaction.