Flash Photolytic Decarbonylation and Ring-Opening of 2-(N-(Pentafluorophenyl)amino)-3-phenylcyclopropenone. Isomerization of the Resulting Ynamine to a Ketenimine, Hydration of the Ketenimine, and Hydrolysis of the Enamine Produced by Ring-Opening

Flash photolysis of 2-( N -(pentafluorophenyl)amino)-3-phenylcyclopropenone, 4, in aqueous solution was found to produce N -(pentafluorophenyl)phenylethynamine, 3, by the expected photodecarbonylation reaction and also 2-phenyl-3-( N -(pentafluorophenyl)amino)acrylic acid, 5, by an apparently unprecedented photochemical ring-opening process. The ynamine underwent rapid isomerization to N -(pentafluorophenyl)phenylketenimine, 9, by an acid-catalyzed route that involves rate-determining proton transfer to the β-carbon atom of the ynamine and also by a base-catalyzed route involving equilibrium ionization of the N−H bond of the ynamine to give an ynamide ion followed by rate-determining β-carbon protonation of this ion. Saturation of the base catalysis allowed determination of the acidity constant of the ynamine; the result, p Q a = 10.23, makes this amine a remarkably strong nitrogen acid. Hydration of the ketenimine 9 gave N -(pentafluorophenyl)phenylacetamide, 6, as the ultimate product produced by this reaction route, and hydrolysis of the aminoacrylic acid 5 gave pentafluoroaniline, 7, and 2-phenylformylacetic acid, 10, which underwent decarboxylation to phenylacetaldehyde, 8, as the ultimate products of this route.