PHOTOOXYGENATION OF SILYL KETENE ACETALS - DIOXETANES AS PRECURSORS TO ALPHA-SILYLPEROXY ESTERS IN THE SILATROPIC ENE REACTION

Photooxygenation of silyl ketene acetals afforded dioxetanes, which subsequently underwent secondary reactions to give rearrangement products (alpha-silylperoxy esters, major products) and cleavage products (pivalaldehyde, minor product). The kinetics of these reactions were studied by NMR and chemiluminescence. The activation energy of the chemiluminescent cleavage process was 2-3 kcal/mol higher than that of the rearrangement. In the presence of catalytic amounts of CF3COCF3 or CF3COCH3, the (E)-silyl ketene acetals rearranged into their Z isomers. Photooxygenation of the (E)- and (Z)-silyl ketene acetals showed that the [2 + 2] cycloaddition was rigorously diastereoselective. Trapping experiments with acetaldehyde confirmed the intermediacy of 1,4-zwitterions in the rearrangement of the (E)- and (Z)-dioxetanes into alpha-silylperoxy esters, but such intermediates were not detected during the photooxygenation of the silyl ketene acetals; the latter proceeds presumably via perepoxides.