Novel Heterocumulenes: Bisiminopropadienes and Linear Ketenimines

Flash vacuum thermolysis (FVT) of suitably substituted isoxazol‐5(4 H )‐ones 7–9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2 , the novel bisiminopropadienes RNCCCNR ( 5 ), and the C ‐cyanoketenimines 14 , all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear CCN‐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 R 2 C‐CN‐R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the 15 N and 13 C 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 CO 2 to generate C ‐cyanoketenimines 14 and 14i . N ‐Phenyl‐dicyanoketenimine 32 is also described.