Novel heterocumulenes: Bisiminopropadienes and linear ketenimines

Flash vacuum thermolysis (FVT) of suitably substituted isoxazol-5(4H)-ones(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 R(2) minus over C-C=plus over N-R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the N-15 and C-13 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-Phenyldicyanoketenimine 32 is also described.