Selective molecular oxygen cleavage of alkynes catalyzed by Ce(IV)

Abstract

The selective molecular oxygen oxidation of alkynes is catalyzed in homogeneous systems (CH 3CN and H 2O) by soluble salts of Ce(IV); e.g., ceric ammonium nitrate CAN [1]. The rate enhancements observed with catalytic amounts of cerium are often dramatic compared to the background autoxidation of alkynes. For example, aryl alkynes, such as phenylacetylene, exhibit no autoxidation at 115 °C and 70 bar O 2 pressure in CH 3CN, but with a sub/cat ratio = 20 under similar conditions CAN catalyzes the reaction such that complete substrate conversion is achieved in < 10 min. The cerium-catalyzed autoxidation of alkynes gives with very high selectivity ( > 90%) the cleavage products of the alkyne under mild conditions (70–100 °C and 100psig O 2 pressure) with very high rates (T.N. > 100min −1) in many cases. Mechanistic studies (including kinetic, gas uptake, solvent labeling and autocatalysis experiments) reveal that a novel oxygen-driven Ce(IV)/Ce(III) redox cycle gives rise to catalysis of the hydrocarbon oxidation. Cerium(IV) oxidizes an alkyne-dioxygen adduct generating a dioxetene radical cation which, it is proposed, reoxidizes Ce(III) to Ce(IV). The dioxetene so produced rearranges to an α-diketone intermediate whose presence was demonstrated by GC/mass spectroscopy and whose competence as an intermediate was confirmed in separate addition experiments.