No abstract

Chlorination of the N,N′-di(2-pyridyl)malonamide 13a affords 2-chloro-8-methyl-4-(2-(4-picolinyl)imino-4H-pyrido[1,2-a]pyrimidine 17a. Flash vacuum thermolysis of 17a causes efficient ring opening to the valence-tautomeric ketenimine 18a/19a, elimination of HCl, and formation of the bis(4-methyl-2-pyridyl)iminopropadiene, R-N=C=C=C=N-R 20a.

Mesoionic pyrido[1,2-a]pyrimidinium olates 9 undergo rearrangement to the lower-energy pyridopyrimidinones 7 in solution at ordinary temperatures (t 1/2 ≈ 51 min at 75 ЊC), 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.

Matrix photolysis of 3-methoxycarbonyl-1,2,3-triazolo[1,5-a]pyridine (12) affords s-E-2-pyridylketene (4), but flash vacuum thermolysis of 12 gives methoxy(2-pyridyl)ketene (15), predominantly in the s-Zconformation. Matrix photolysis of 15 affords 2-acetylpyridine.