Photochemical oxacarbene formation

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

Only epimerization was observed on photolysis of 2-methoxynorbomane, in contrast with the Norrish II eliminations known to occur with other a-alkoxycyclo-alkanones. The suppression of enal formation by a suitable substitution pattern and molecular geometry is responsible for the observed photochemical ring expansion of the camphor derivative (432), in methanol containing sodium bicarbonate, to the mixture of acetals (433). The reaction involves oxacarbene formation the ketene-derived product is also formed, but in a minor reaction pathway. A second short paper considers the effects of structure and substitution pattern upon ketene versus oxacarbene formation in the photolysis of bridged 2-norbomanones. ... 

The photochemical formation of ring-expanded acetals from ketones in alcoholic solvents was originally suggested by Yates and Kilmurry (9,38) to proceed by way of oxacarbenes (e.g., Scheme II) (39). The inter-... 

The photochemical behavior of cyclobutanone (IS) contrasts sharply with that of other ketones. Cyclobutanone undergoes a cleavage also from the (n, r ) state, with subsequent fragmentation to ketene and olefin, decarbonylation to cyclopropane or cyclization to oxacarbene (16), whose concerted formation has also been proposed on the basis of stereochemical observations (Stohrer et al., 1974). In contrast, cyclohexanone cleaves exclusively from the triplet state and undergoes disproportionation reactions. The photochemical activity of cyclobutanone persists even at low temperatures (77 K) where cyclohexanone is photostable. 

As frequently noted before, the formation of the silicon-oxygen bond occurs here also with virtually complete retention of configuration based on studies with (+ )-l-naphthylphenylmethylbenzoylsilane. Similar photochemical formation of oxacarbenes have recently been reported by several workers 65-68). The role of base and acid in leading to these apparently unrelated pathways is not yet understood. Qualitative rate studies indicate that conversion of the mixed acetal to the alkoxysilane and acetal under comparable acid conditions is much slower than their direct photochemical formation so that the mixed acetal does not appear to be an intermediate in the reactions containing acid. 

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