Novel Heterocumulenes: Bisiminopropadienes and Linear Ketenimines

Flash vacuum thermolysis (FVT) of suitably substituted isoxazol‐5(4H)‐ones 7–9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2, the novel bisiminopropadienes RNCCCNR (5), and the C‐cyanoketenimines 14, all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear CCN‐C backbone in ketenimines 2g and 2h is revealed by their exceptional spectroscopic properties as well as an X‐ray crystal structure of 2g, and confirmed by density functional calculations (B3LYP/6–31 G*); these compounds are best described as resonance hybrids of ketenimines and isonitrile ylides R2C‐CN‐R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the 15N and 13C isotopomers as well as theoretical calculations, tert‐Butyl‐substituted isoxazolones 7e and 7f, and 8i form the expected ketenimines 2, which then undergo elimination of isobutene and CO2 to generate C‐cyanoketenimines 14 and 14i. N‐Phenyl‐dicyanoketenimine 32 is also described.