Ketenes and mesoions. Interconversion of mesoionic pyridopyrimidinium olates and pyridopyrimidinones. (2-Pyridyl)iminopropadienone. Part 2†1

Abstract

Mesoionic pyrido[1,2-a]pyrimidinium olates 9 undergo rearrangement to the lower-energy pyridopyrimidinones 7 in solution at ordinary temperatures (t1/2 ≈ 51 min at 75 °C), formally via the higher-energy ketene valence isomers 11. These ketenes are not directly detectable, and DFT calculations at the B3LYP/6-31G* level indicate that the rearrangement may be concerted via the ketenoid transition state 11TS, although the ketene conformer 11M is locally stable. FVT of the pyridopyrimidinones 7 is a method of synthesis of (2-pyridyl)iminopropadienone 4, a reaction thought to proceed via ring opening to the same ketenes 11 followed by elimination of the 2-(methylamino)pyridine 8. Recombination of 8 and 4 leads to mesoions 9 together with minor amounts of the isomers 10.