Charged and Betainic Nucleobases. On Syntheses and Properties of First Mesomeric Uracilylbetaines, Uracilates, and Novel Uracilium Salts

• Published in: Journal of organic chemistry 1997-06, Vol.62 (12), p.3910-3918
• Main Authors: Schmidt, Andreas, Kindermann, Markus Karl
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo961890x

Nucleophilic heteroaromatic substitution of DMAP and 1-methylimidazole on the 6-chlorouracil 1 gave the uracilyl hetarenium salts 2 and 4, respectively. Depending on the nucleophile, the substitution could be catalyzed by interception of the leaving group either with sodium tetraphenylborate or with antimony pentachloride in a non-coordinating solvent. Catalysis with other Lewis acids failed due to the formation of stable coordination compounds 3. Surprisingly, Finkelstein-type reaction conditions provide an efficient approach to the imidazolium iodide 4. Depending on the reaction conditions, treatment of the 6-chlorouracil 5 with DMAP resulted in the formation of the pyridinium uracilate 6, the uracilyl pyridinium chloride 7a, and the novel pyridinium-uracilate 8 which is a cross-conjugated mesomeric betaine (CCMB). Alkylation of the mesomeric betaine 8 with 1,2-dichloroethane/SbCl5 gave the biscation 9. Analogously, 1-methylimidazole or 2,4,6-trimethylpyridine as nucleophiles formed the salts 10a and 12, respectively, and the imidazolium-uracilate 11. This is the first representative of a cross-conjugated heterocyclic mesomeric betaine isoconjugate with odd nonalternant hydrocarbon anions. PM3 calculations were performed on the cation 2 and on the mesomeric betaine 8. Whereas the cation 2 was found to have an essentially perpendicular torsional angle Φ around the CN+ bond linking the uracil and the pyridinium units, the mesomeric betaine 8 adopts a nearly planar conformation, thus allowing stabilizing π interactions. Accordingly, upon rotation about the CN+ bond, the expected maxima were found at essentially perpendicular conformations. Due to the balance between three effects which act in opposite directions, (i) stabilizing p overlap, (ii) N(8)/2-H attractive interaction, and (iii) steric repulsion between 6-H and 12-H, two additional small minima were surprisingly found at Φ = 36.6° and 219.3°. The charge separation in the ground state of the mesomeric betaine 11, and the ground-state ion-pair complexes of the hetarenium iodides 2d, 4, 7c, and 10b were confirmed by the effect of negative solvatochromism.