Regioselective Oxidation of Nonactivated Alkyl C–H Groups Using Highly Structured Non-Heme Iron Catalysts
2013-02-15T00:00:00Z (GMT) by
Selective oxidation of alkyl C–H groups constitutes one of the highest challenges in organic synthesis. In this work, we show that mononuclear iron coordination complexes Λ-[Fe(CF3SO3)2((S,S,R)-MCPP)] (Λ-1P), Δ-[Fe(CF3SO3)2((R,R,R)-MCPP)] (Δ-1P), Λ-[Fe(CF3SO3)2((S,S,R)-BPBPP)] (Λ-2P), and Δ-[Fe(CF3SO3)2((R,R,R)-BPBPP)] (Δ-2P) catalyze the fast, efficient, and selective oxidation of nonactivated alkyl C–H groups employing H2O2 as terminal oxidant. These complexes are based on tetradentate N-based ligands and contain iron centers embedded in highly structured coordination sites defined by two bulky 4,5-pinenopyridine donor ligands, a chiral diamine ligand backbone, and chirality at the metal (Λ or Δ). X-ray diffraction analysis shows that in Λ-1P and Λ-2P the pinene rings create cavity-like structures that isolate the iron site. The efficiency and regioselectivity in catalytic C–H oxidation reactions of these structurally rich complexes has been compared with those of Λ-[Fe(CF3SO3)2((S,S)-MCP)] (Λ-1), Λ-[Fe(CF3SO3)2((S,S)-BPBP)] (Λ-2), Δ-[Fe(CF3SO3)2((R,R)-BPBP)] (Δ-2), Λ-[Fe(CH3CN)2((S,S)-BPBP)](SbF6)2 (Λ-2SbF6), and Δ-[Fe(CH3CN)2((R,R)-BPBP)](SbF6)2 (Δ-2SbF6), which lack the steric bulk introduced by the pinene rings. Cavity-containing complexes Λ-1P and Λ-2P exhibit enhanced activity in comparison with Δ-1P, Δ-2P, Λ-1, Λ-2, and Λ-2SbF6. The regioselectivity exhibited by catalysts Λ-1P, Λ-2P, Δ-1P, and Δ-2P in the C–H oxidation of simple organic molecules can be predicted on the basis of the innate properties of the distinct C–H groups of the substrate. However, in specific complex organic molecules where oxidation of multiple C–H sites is competitive, the highly elaborate structure of the catalysts allows modulation of C–H regioselectivity between the oxidation of tertiary and secondary C–H groups and also among multiple methylene sites, providing oxidation products in synthetically valuable yields. These selectivities complement those accomplished with structurally simpler oxidants, including non-heme iron catalysts Λ-2 and Λ-2SbF6.