White–Chen catalyst

The White–Chen catalyst is an Iron-based coordination complex named after Professor M. Christina White and her graduate student Mark S. Chen. The catalyst is used along with hydrogen peroxide and acetic acid additive to oxidize aliphatic sp3 C-H bonds in organic synthesis. The catalyst is the first to allow for preparative and predictable aliphatic C–H oxidations over a broad range of organic substrates. Oxidations with the catalyst have proven to be remarkably predictable based on sterics, electronics, and stereoelectronics allowing for aliphatic C–H bonds to be thought of as a functional group in the streamlining of organic synthesis. In the case where an electron withdrawing group (EWG) is present in the substrate the highly electrophilic catalyst will oxidize the more electron-rich C–H bond that is most remote from the EWG. In the case above the C–H bond shaded yellow is further from the electron withdrawing group and therefore has a higher electron density than the one not shaded yellow. The yellow shaded C–H bond is therefore the primary site for oxidation by the catalyst. Example of Electronic Selectivity The reaction selectivity is highly influenced by electronics due to the highly electron withdrawing ester group present in the substrate. For that reason the reaction proceeds with oxidation at the tertiary C–H bond most remote from the ester with reaction yields of greater than 50% and selectivities of >99:1. Electronically guided site-selectivity is also observed for secondary sites, affording yields of mono-oxidized products of 50% or greater. In the case where a bulky group, denoted at the right by BG, is present near multiple aliphatic C-H groups that are electronically equivalent, the bulky White–Chen catalyst will target the less sterically hindered C-H bond. In the case at the right there is a large bulky group in close proximity to one of two aliphatic C-H bonds. The C-H bond shaded in yellow is further from the bulky group, is less sterically hindered, and will therefore be the site of oxidation by the catalyst in this case.