Valenzisomerisierung von cis ‐Dienonen, III Lichtinduzierte Umlagerung von 5‐Phenyl‐penta‐2,4‐dienaldehyden [1]

Irradiation of 5‐phenyl‐2,4‐pentadienaldehydes 1 and 2 leads to a rapid cis , trans ‐isomerization of the 4,5‐double bond followed by formation of unsaturated ketenes 7 and 12 , respectively, through a [1.5]‐hydrogen shift. In the dark, ketenes 7 and 12 revert at room temperature to the 5‐phenyl‐2,4‐pentadienaldehydes 1 und 2. Activation parameters and solvent dependence have been determined for this remarkably facile [1.5]‐sigmatropic rearrangement. The ketenes 7 and 12 are stable below − 20° in inert solvents. With alcohols and amines mixtures of α,β‐ and β,γ‐unsaturated acid derivatives are formed. Upon extended irradiation of 7 and 12 in aprotic solvents olefins 9 and 13 are formed through decarbonylation.