The RuO₄-Catalyzed Ketohydroxylation. Part 1. Development, Scope, and Limitation

A new straightforward oxidation of C,C-double bonds to unsymmetrical α-hydroxy ketones using catalytic amounts of RuCl₃ and stoichiometric amounts of Oxone under buffered conditions has been developed, a reaction for which we coined the expression “ketohydroxylation”. The transformation allows the direct formation of α-hydroxy ketones (acyloins) from olefins without intermediate formation of syn-diols. The present paper will provide detailed information starting with the underlying concept and the subsequent development of the reaction. The effect of base, solvent stoichiometry, and temperature will be discussed resulting in an improved mechanistic model that might help to explain the influence of different reaction parameters on reactivity and selectivity in RuO₄-catalyzed oxidations of C,C-double bonds. Furthermore, an improved workup procedure allows the recovery of the ruthenium catalyst by precipitation while simplifying the overall product purification. The second part of the paper focuses on exploration of scope and limitation. A variety of substituted olefins are oxidized to α-hydroxy ketones in good to excellent regioselectivities and yield. Cyclic substrates proved to be problematic to oxidize; however, a careful analysis of temperature effects resulted in the development of a successful protocol for the ketohydroxylation of cyclic substrates by simply decreasing the reaction temperature.