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Direct, aerobic arene olefination

“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.-QScience 2010327, 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.-QJ. 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).

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