A Mechanistic Study on Benzylic Alcohols Oxidation with PPh4HSO5 Catalyzed by Manganese(III) Porphyrins in Homogeneous Solution

The oxidation of variously ring-substituted 1-phenylethanols with Ph4PHSO5 catalysed by Mn(TMP)Cl and Mn(TDCPP)Cl in the presence of 4-tert.-butylpyridine was studied in 1,2-dichloroethane homogeneous solution. The process leads only to C-H bond cleavage products, namely acetophenones. The oxidation rates are independent of the substrate concentration and, when Mn(TMP)Cl is the catalyst, even of the substrate nature. By increasing the concentration of 4-tert.-butylpyridine, which acts as an axial ligand of the catalyst, a bell shaped curve for the rate constants trend is observed. Hammett plots obtained by changing the substituents on the phenyl ring of the benzylic alcohol give different rho values depending on the technique employed for rate constants determination, i.e. individual or competitive experiment. The observations reported above, together with a KIE of 2.5 in 1-D-1-phenylethanol oxidation measured by competitive experiment, are rationalised on the basis of a mechanistic scheme in which the oxo-manganese derivative is formed in the rate determining step of the catalytic process. Furthermore, it is suggested that alcohol dehydrogenation proceeds through a hydride abstraction involving an alcohol-oxo-porphyrinato complex.