Oxidation of activated methylene groups to ketones catalyzed by new iron phosphinooxazoline complexes and by iron(II) triflate

Abstract

The study describes the first catalytic application of iron phosphinooxazoline complexes and Fe(OTf) 2 in oxidation reactions. New iron phosphinooxazoline (PHOX) complexes of the general formulation [Fe(PHOX) 2] 2+[OTf] 2 ( 3, OTf = CF 3SO 3 −) were synthesized in virtually quantitative yield from anhydrous Fe(OTf) 2 and the corresponding PHOX ligands. The new complexes 3 as well as previously synthesized iron PHOX complexes of the general formula [Fe(Cp)(CO)(PHOX)] +[I] − ( 2) were employed as catalysts in the oxidation of benzylic CH 2 groups to the corresponding ketones. Whereas 3 showed catalytic activity at room temperature (about two equivalents t-BuOOH as the oxidant, 2 mol% catalyst, pyridine solvent, 12 h, 38–93% isolated yields), complexes of 2 were only active at elevated temperatures. The product profile is in accordance with a mechanism previously described as “oxygenated Fenton chemistry”, which is further corroborated by UV–vis investigations. They showed that an iron alkylperoxo species [Fe–OO tBu] might be involved in the catalytic cycle. The reaction follows a pseudo zero order rate law in substrate.