Mesoionic pyridopyrimidinylium and pyridooxazinylium olates and non-mesoionic pyridopyrimidinones. Structures in the solid state, solution, and matrices †

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Abstract
X-Ray crystal structures, 13C NMR spectra and theoretical calculations (B3LYP/6-31G*) are reported for the mesoionic (zwitterionic) pyridopyrimidinylium- and pyridooxazinyliumolates 2a, 3a and 5a,b as well as the enol ether 11b and the enamine 11c. The 1-NH compounds like 1a, 2a and 3a exist in the mesoionic form in the crystal and in solution, but the OH tautomers such as 1b and 2b dominate in the gas phase as revealed by the Ar matrix IR spectra in conjunction with DFT calculations. All data indicate that the mesoionic compounds can be regarded as intramolecular pyridine–ketene zwitterions (cf.16 → 17) with a high degree of positive charge on the pyridinium nitrogen, a long pyridinium N–CO bond (ca. 1.44–1.49 Å), and normal CO double bonds (ca. 1.22 Å). All mesoionic compounds exhibit a pronounced tilting of the “olate” CO groups (the CO groups formally derived from a ketene) towards the pyridinium nitrogen, giving NCO angles of 110–118°. Calculations reveal a hydrogen bond with 6-CH, analogous to what is found in ketene–pyridine zwitterions and the C3O2–pyridine complex. The 2-OH tautomers of type 1b, 2b, and 11 also show a high degree of zwitterionic character as indicated by the canonical structures 11 ⇆ 12.

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