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

Abstract

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, Finkelsteintype 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/SbCl 5 gave the biscation 9. Analogously, 1-methylimidazole or 2,4,6trimethylpyridine as nucleophiles formed the salts 10a and 12, respectively, and the imidazoliumuracilate 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. X

Synthesis and Characterization of Stable Betainic Pyrimidinaminides.

“…The elemental analyses were performed on a CHNS-932 analyzer (LECO) and in the laboratory for elemental analyses of the Institute of Chemistry and Biochemistry of the University of Greifswald, Germany. In accordance with known hetarenium compounds[17][18][19][20][21][22][23][24][25] and Xray crystallographic results of related systems[18][19][20]23], all substances crystallized with considerable amounts of water.…”

“…In contrast to that, molecules with an odd number of positive and negative charges form a distinct class of compounds, hybrids between mesomeric betaines and heteroarenium salts [17]. In continuation of our previous work on novel mesomeric betaines [18], heteroaromatic tripoles [19][20][21], and betainic nucleobases [22][23][24], we report here the synthesis and characterization of this class of betaines.…”

Anionic N-heterocyclic carbenes derived from sydnone imines such as molsidomine. Trapping reactions with selenium, palladium, and gold

“…Supporting Information) and the imine stretching frequencies of the exocyclic imine bond. In continuation of our interest in the chemistry of N-heterocyclic carbenes, 60 mesomeric betaines 61,62 and the intersection between these two substance classes we aimed at studying the deprotonation of Molsidomine and of another model sydnone imine at C4 for comparison, which resulted in the formation of unique anionic N-heterocyclic carbenes. The anion of Molsidomine 7a can be formulated as anionic abnormal NHC 8a(A) and as anionic normal NHCs 8a(B).…”

Hetarenium salts from pentafluoropyridine. Syntheses, spectroscopic properties, and applications

“…Catalysis of acylations [1][2][3][4], BaylisHillman [5] and Dakin-West reactions [6], silylations [7], and many other syntheses by DMAP is widely applied in organic synthesis. Stabilization of reactive species such as allyl anions [8], allyl radicals [9], uracilates [10,11], pyrimidinium-olates [12][13][14] as well as -aminides [15,16], pyridinium-olates [17] and heteroaromatics with up to ten positive charges within a common -electron system by 4-(dimethylamino)-pyridinium and other hetarenium substituents [18] was reported. The activation of perhalogenated heteroaromatics by hetarenium substituents can be applied for the preparation of otherwise unavailable functionalized heteroaromatics.…”