FVP of triazole 13 produces the isolable ketenimine 11 together with indole 15. 11 undergoes reversible interconversion with imidoylketene 10 above 380 °C. The latter cyclizes to quinolone 12. Meldrum's acid derivative 16 produces the same ketene 10 above 200 °C, and the latter isomerizes to ketenimine 11 at 200 °C by a 1,3-shift of a MeO group. A competing elimination of MeOH from 16 produces (phenylimino)propadienone 20.

Imidoylketenes 11 and oxoketenimines 12 are generated by flash vacuum thermolysis of Meldrum's acid derivatives 9, pyrrolediones 17 and 18, and triazole 19 and are observed by IR spectroscopy. Ketenimine-3-carboxylic acid esters 12a are isolable at room temperature. Ketenes 11 and ketenimines 12 undergo rapid interconversion in the gas phase, and the ketenes cyclize to 4-quinolones 13. When using an amine leaving group in Meldrum's acid derivatives 9c, the major reaction products are aryliminopropadienones, ArN=C=C=C=O (15). The latter react with 1 equiv of nucleophile to produce ketenimines 12 and with 2 equiv to afford malonic acid imide derivatives 16. N-Arylketenimine-C-carboxamides 12c cyclize to quinolones 13c via the transient amidinoketenes 11c at temperatures of 25-40 degrees C. This implies rapid interconversion of ketenes and ketenimines by a 1,3-shift of the dimethylamino group, even at room temperature. This interconversion explains previously poorly understood outcomes of the ynamine-isocyanate reaction. The solvent dependence of the tautomerism of 4-quinolones/4-quinolinols is discussed. Rotational barriers of NMe(2) groups in amidoketenimines 12c and malonioc amides and amidines 16 (24) are reported.

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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.

Flash vacuum thermolysis (FVT) of triazoles 6a-c generates alpha-oxoketenimines 10, the ester 10a being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a. Ketenes 9 and ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups under mild FVT conditions (ca. 350-400 degrees C). Both 9 and 10 are directly observable by IR spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca. 400 degrees C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal mol(-)(1) (161 kJ mol(-)(1)) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal mol(-)(1).