Amino-imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system

Excited-state proton transfer and phototautomerism in nucleobase and nucleoside analogs: a mini-review

Intermolecular excited-state proton transfer (ESPT) has been observed in several fluorescent nucleobase and/or nucleoside analogs. In the present work, some new examples of ESPT in this class of compounds are presented together with a brief recapitulation of the previously published data. The nucleobases, nucleosides, and their analogs contain many basic and acidic centers and therefore their ESPT behavior may be complex. To interpret the complex data, it is usually necessary to determine the microscopic pK* values for each (or most) of the possible ESPT centers. Typical approach to solve this problem is by analysis of the alkyl derivatives, in which the possibility of the ESPT is reduced. Of particular interest are examples of "phototautomerization via the cation," observed in several systems, which in the neutral media do not undergo ESPT. Protonation of the molecule in the ground state facilitates the two-step phototautomerism in several systems, including formycin A and 2-amino-8-azadenine. Fluorescence of the nucleobase and nucleoside analogs undergoing ESPT is usually solvent-, isotope-, and buffer-ion sensitive, and in some systems the ESPT can be promoted by environmental factors, e.g., the presence of buffer ions. This sensitivity to the microenvironment parameters makes the ESPT systems potentially useful for biological applications.

Geometric and energetic consequences of prototropy for adenine and its structural models – a review

Prototropy for adenine and its convenient models causes parallel changes of geometric (HOMED) and energetic (ΔE) parameters for neutral tautomers.

Mechanistic models for LAH reductions of acetonitrile and malononitrile. Aggregation effects of Li+ and AlH3 on imide-enamide equilibria

The results are reported of an ab initio study of the addition of LiAlH(4) to acetonitrile and malononitrile at the MP2(full)/6-311+G* level considering the effects of electron correlation at higher levels up to QCISD(T)/6-311++G(2df,2pd) and including ether solvation. All imide (RCH(2)CH═N(-)) and enamide (RCH(-)CH═NH ↔ RCH═CHN(-)H) adducts feature strong interactions between the organic anion and both Li(+) and AlH(3). The relative stabilities of the tautomeric LAH adducts are compared to the tautomer preference energies of the LiH adducts and of the hydride adducts of the nitriles. Alane affinities were determined for the lithium ion pairs formed by LiH addition to the nitriles. The results show that alane binding greatly affects the imide-enamide equilibria and that alane complexation might even provide a thermodynamic preference for the imide intermediate. While lithium enamides of malononitrile are much more stable than lithium imides, alane binding dramatically reduces the enamide preference so that both tautomers are present at equilibrium. Implications are discussed regarding to the propensity for multiple hydride reductions and with regard to the mechanism of reductive nitrile dimerization. A detailed mechanism is proposed for the formation of 2-aminonicotinonitrile (2ANN) in the LAH reduction of malononitrile.

DFT studies on one-electron oxidation and one-electron reduction for 2- and 4-aminopyridines

Quantum-chemical calculations {DFT(B3LYP)/6-311+G(d,p)} were performed for all possible tautomers (aromatic and nonaromatic) of neutral 2- and 4-aminopyridines and their oxidized and reduced forms. One-electron oxidation has no important effect on the tautomeric preference for 2-aminopyridine. The amine tautomer is favored. However, oxidation increases the stability of the imine NH tautomer, and its contribution in the tautomeric mixture cannot be neglected. In the case of 4-aminopyridine, one-electron oxidation increases the stability of both the amine and imine NH tautomers. Consequently, they possess very close energies. As major tautomers, they dictate the composition of the tautomeric mixture. The CH tautomers may be considered as very rare forms for both neutral and oxidized aminopyridines. A reverse situation takes place for the reduced forms of aminopyridines. One-electron reduction favors the C3 atom for the labile proton for both aminopyridines. This may partially explain the origin of the CH tautomers for the anionic states of nucleobases containing the exo NH₂ group.

Fragmentation of diamide derivatives of 3,4-ethylenedioxythiophene

The sequential product ion (MS(n)) fragmentation of four symmetric diamide derivatives of 3,4-ethylenedioxythiophene were characterized using ion trap mass spectrometry with electrospray ionization and their fragmentation patterns were studied. The experimental data consists of mass spectra obtained by tandem mass spectrometry, and calculations were obtained by the M06-2X/6-31 G (d,p) method. Investigated compounds represent building blocks in synthesis of compounds used in different areas of chemistry and industry such as in medicinal chemistry, as potential anticancer and anticonvulsant agents, in organic chemistry as linkers for solid-phase synthesis, and in the synthesis of a variety of materials in polymer chemistry. We present herein the investigation of the fragmentation pathway of protonated diamide derivatives of 3,4-ethylenedioxythiophene that involves the identification of fragments, influence of proton transfer on direction of fragmentation and mechanisms of reactions by which the fragmentation process occurs. Data obtained from product ion spectra of these protonated compounds and density functional theory (DFT) calculations indicate that the fragmentation process takes place via four main reactions: amido-iminol proton transfer, reverse cycloaddition, cleavage of the amide bond, and isocyanic acid elimination. The 3,4-ethylenedioxythiophene-2,5-dicarboxamide was observed as an intermediate in the fragmentation of its alkyl derivatives. To our knowledge, this work brings the first correct description of the mechanism of elimination of isocyanic acid.