Novel Heterocumulene (RN=C=C=C=X) and Ketene Rearrangements

The novel iminopropadienones RN=C=C=C=O (8) and the corresponding thione 21 have been synthesized by FVP of isoxazole or Meldrum's acid derivatives. Bisimhes 16 and the unusual, linear ketenimine 18 were generated in related reactions. In the Meldrum's acid series, a competing fragmentation leads to imidoylketenes (R"=CR-CR"=C=O) (28). a-Oxoketenes (MR-CR'=C=O) (22), imidoylketenes (28), and vinylketenes (39c) undergo reversible interconversion with a-oxoketenes (23), oxoketenimines (29), and acylallenes (~OC), respectively, via a thermal 1,3-shifi of the group R. This is favored by electron-donating substituents R by means of interaction with the ketene LUMO.