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26.9.1.1.5.1 Variation 1: Oxidative Elimination of Metal Hydride from Enol Derivatives

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261065.

The direct oxidative formation of enones from saturated ketones is promoted by palladi­um(II) salts, but is a poor-yielding process and suffers from regiochemical ambiguities.[‌88‌] Prior formation of an enol derivative overcomes this problem and allows regiospecific enone synthesis (Scheme 23). Silyl enol ethers undergo facile oxidation by catalytic palladium(II) salts in the presence of allyl carbonates as co-oxidant to yield the corresponding enone 95.[‌89‌] The reaction is driven by the formation of an enolate bound to a π-allylpalladium complex, which leads to oxidation through hydride transfer to palladium. The use of acetonitrile or benzonitrile as solvent is found to be crucial in avoiding or minimizing competing allyl transfer to the enolate. Under similar conditions, palladium complexes also mediate oxidation of enol acetates to enones (e.g., 96 to 97) provided that a tin alkoxide is added to the reaction mixture.[‌90‌] This promotes formation of reactive stannyl enol ethers in situ. Allyl enol carbonates can also be used to form enones directly, although the latter reactions are complicated by competing allyl transfer.[‌91‌]

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