Heterocyclic Betaines. Aza Analogues of Sesquifulvalene. 1. Structural Studies of 1-Alkyl-4-azolylidene-1,4-dihydropyridines and Azolium Azolate Inner Salts

The aza analogs of sesquifulvalene may adopt various structures, and of these several 1-alkyl-4-azolylidene-1, 4-dihydropyridines 8A ↔ 8B have been prepared by deprotonation of their corresponding 1-alkyl-4-azolylpyridinium salts. These novel structures 8 could show a spectrum of properties ranging from those of ethylenes to betaines. Semiempirical (MNDO//MNDO), experimental dipole moment values (ca. 9.05 D), 1H and 13C NMR data, and single-crystal X-ray diffraction analysis of compound 16 are consistent with the betaine character of these compounds. The electronic and molecular structure of azolium azolate inner salts 10 has been investigated. Theoretical calculations (MNDO//MNDO), experimental dipole moments (9.18 to 11.33 D), 1H and 18C NMR spectra, EIMS, and single-crystal X-ray diffraction analysis of compound 35 are consistent with the highly dipolar structure of this type of mesomeric betaines. © 1991, American Chemical Society. All rights reserved.