Cyanoketene and Iminopropadienones

Cyanoketene (8) is generated in high yields on flash vacuum thermolysis (FVT) of suitably substituted Meldrum's acid derivatives (5-[(alkylamino)(methylthio or alkylamino)methylene]-2,2-dimethyl-1,3-dioxane-4,6-diones) (3e−j), and also on FVT of cyanoacetic acid derivatives 9e,f,g,j,k,m. The major reaction pathway from 3 proceeds via ketenimines 6 and (alkylimino)propadienones 7, the latter undergoing a retro-ene reaction to 8. A minor pathway is via imidoylketenes 4e,h and oxoketenimines 5e,h, which undergo retro-ene reactions to 9. All intermediates were characterized by Ar matrix FTIR and tandem mass spectrometry (collisional activation MS). Trapping of 4, 5, and 8 with nucleophiles is also reported. The preference of 1,3-X shifts over 1,5-H shifts in imidoylketenes 12 (X = SMe or NMe2) is corroborated by the calculated activation barriers. Neat cyanoketene is highly reactive, reacting at or below 80 K, and this is attributed to the availability of a low-lying ketene LUMO. The IR spectrum of cyanoketene (Ar, 14 K) is dominated by two absorptions at 2163 (s; CCO) and 2239 (w; CN) cm-1 in excellent agreement with density functional (B3-LYP/6-31G*) and ab initio (QCISD/6-31G*) calculations.