High Turnover Remote Catalytic Oxygenation of Alkyl Groups:  How Steric Exclusion of Unbound Substrate Contributes to High Molecular Recognition Selectivity

H-bonding mediated molecular recognition between substrate and ligand -COOH groups orients the substrate so that remote, catalyzed oxygenation of an alkyl C-H bond by a Mn-oxo active site can occur with very high (>98%) regio- and stereoselectivity. This paper identifies steric exclusion-exclusion of non H-bonded substrate molecules from the active site-as one requirement for high selectivity, along with the entropic advantage of intramolecularity. If unbound substrate molecules were able to reach the active site, they would react unselectively, degrading the observed selectivity. Both of the faces of the catalyst are blocked by two ligand molecules each with a -COOH group. The acid p-(t)BuC6H4COOH binds to the ligand -COOH recognition site but is not oxidized and merely blocks approach of the substrate therefore acting as an effective inhibitor for ibuprofen oxidation in both free acid and ibuprofen ester form. Dixon plots show that inhibition is competitive for the free acid ibuprofen substrate, no doubt because this substrate can compete with the inhibitor for binding to the recognition site. In contrast, inhibition is uncompetitive for the ibuprofen-ester substrate, consistent with this ester substrate no longer being able to bind to the recognition site. Inhibition can be reversed with MeCOOH, an acid that can competitively bind to the recognition site but, being sterically small, no longer blocks access to the active site.