“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.
“Ligand-Enabled Reactivity and Selectivity in a Synthetically Versatile Aryl C–H Olefination” Wang, D.-H.; Engle, K. M.; Shi, B.-F. and Yu, J.-Q. Science 2010, 327, 315-319. DOI: 10.1126/science.1182512
“Highly Convergent Total Synthesis of (+)-Lithospermic Acid via a Late-Stage Intermolecular C–H Olefination” Wang, D.-H. and Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 5767-5769. DOI: 10.1021/ja2010225
The Mizoroki-Heck reaction is a widely-used method for cross-coupling of the C-X bond of aryl halides/pseudo-halides with the C-H bond of an olefin – the reaction is so popular that Richard Heck won a share of the 2010 Chemistry Nobel for palladium cross-coupling. An equivalent of H-X is produced as a byproduct, which is typically neutralized by the addition of stoichiometric base.
A direct coupling of two C-H bonds is an interesting and potentially green alternative to this reaction, since it could simplify the synthesis of the halide coupling partner (shorter synthesis = less waste) and improve the reaction’s atom economy. In this sort of reaction a directing group is typically needed so the functionalization occurs at only one of the many aryl C-H positions. Additionally, an oxidant is needed in the catalytic cycle because the metal catalyst, typically Pd(II), is reduced when it mediates the C-C coupling (this sort of mechanism has been proposed). Typically silver or copper salts are used in superstoichiometric amounts for this purpose. From a Green Chemistry perspective this is only a marginal improvement over the original Heck reaction – using several equivalents of a transition metal oxidant isn’t a real improvement over generating an equivalent of neutralized acid (like triethylammonium chloride).