Elucidating the mechanism of the Ley-Griffith (TPAP) alcohol oxidation.

The Ley-Griffith reaction is utilized extensively in the selective oxidation of alcohols to aldehydes or ketones. The central catalyst is commercially available tetra--propylammonium perruthenate (TPAP, -PrN[RuO]) which is used in combination with the co-oxidant -methylmorpholine -oxide (NMO). Although this reaction has been employed for more than 30 years, the mechanism remains unknown. Herein we report a comprehensive study of the oxidation of diphenylmethanol using the Ley-Griffith reagents to show that the rate determining step involves a single alcohol molecule, which is oxidised by a single perruthenate anion; NMO does not appear in rate law. A key finding of this study is that when pure -PrN[RuO] is employed in anhydrous solvent, alcohol oxidation initially proceeds very slowly. After this induction period, water produced by alcohol oxidation leads to partial formation of insoluble RuO, which dramatically accelerates catalysis a heterogeneous process. This is particularly relevant in a synthetic context where catalyst degradation is usually problematic. In this case a small amount of -PrN[RuO] must decompose to RuO to facilitate catalysis.