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

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 = CF3SO3−) 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 CH2 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–OOtBu] might be involved in the catalytic cycle. The reaction follows a pseudo zero order rate law in substrate.

Graphical abstractNovel iron phosphinooxazoline complexes were synthesized and their catalytic activity in the oxidation of benzylic methylene groups was investigated. At a catalyst load from 2 to 5%, isolated yields ranged from 38 to 93%. The reaction presumably proceeds through a Fenton type mechanism and is pseudo zero order in substrate.Figure optionsDownload full-size imageDownload as PowerPoint slide