Manganese Catalysts for C−H Activation: An Experimental/Theoretical Study Identifies the Stereoelectronic Factor That Controls the Switch between Hydroxylation and Desaturation Pathways

• Published in: Journal of the American Chemical Society Vol. 132; no. 22; pp. 7605 - 7616
• Main Authors: Hull, Jonathan F, Balcells, David, Sauer, Effiette L. O, Raynaud, Christophe, Brudvig, Gary W, Crabtree, Robert H, Eisenstein, Odile
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 09-06-2010
• Subjects: Catalysis; Chemical Sciences; Hydroxylation; Inorganic chemistry; Manganese - chemistry; Models, Molecular; or physical chemistry; Oxidation-Reduction; Theoretical and; Thermodynamics
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja908744w

We describe competitive C−H bond activation chemistry of two types, desaturation and hydroxylation, using synthetic manganese catalysts with several substrates. 9,10-Dihydrophenanthrene (DHP) gives the highest desaturation activity, the final products being phenanthrene (P1) and phenanthrene 9,10-oxide (P3), the latter being thought to arise from epoxidation of some of the phenanthrene. The hydroxylase pathway also occurs as suggested by the presence of the dione product, phenanthrene-9,10-dione (P2), thought to arise from further oxidation of hydroxylation intermediate 9-hydroxy-9,10-dihydrophenanthrene. The experimental work together with the density functional theory (DFT) calculations shows that the postulated Mn oxo active species, [Mn(O)(tpp)(Cl)] (tpp = tetraphenylporphyrin), can promote the oxidation of dihydrophenanthrene by either desaturation or hydroxylation pathways. The calculations show that these two competing reactions have a common initial step, radical H abstraction from one of the DHP sp3 C−H bonds. The resulting Mn hydroxo intermediate is capable of promoting not only OH rebound (hydroxylation) but also a second H abstraction adjacent to the first (desaturation). Like the active MnVO species, this MnIV−OH species also has radical character on oxygen and can thus give H abstraction. Both steps have very low and therefore very similar energy barriers, leading to a product mixture. Since the radical character of the catalyst is located on the oxygen p orbital perpendicular to the MnIV−OH plane, the orientation of the organic radical with respect to this plane determines which reaction, desaturation or hydroxylation, will occur. Stereoelectronic factors such as the rotational orientation of the OH group in the enzyme active site are thus likely to constitute the switch between hydroxylase and desaturase behavior.