Observation of singlet cycloreversion of thymine oxetanes by direct photolysis

An ultrafast broadband transient absorption spectroscopic study of the direct photolysis of oxetane DMT-BP [which is the oxetane adduct of 1,3-dimethylthymine (DMT) with benzophenone (BP)] is presented. Previous nanosecond time-resolved absorption studies by other researchers observed that direct photolysis of such oxetanes results in a rare, adiabatic photochemical reaction to produce a triplet excited-state carbonyl species. However, the mechanism for this adiabatic photochemical reaction remained unclear for the reaction sequence of the bond scission and the intersystem crossing (ISC) because of the time resolution for the experiments, and this prompted us to further study its mechanism with ultrafast time-resolution. The ultrafast time-resolved spectra presented here indicate that the cycloreversion reaction occurs in a stepwise manner on a singlet excited-state, and then intersystem crossing (ISC) occurs to produce the triplet carbonyl product observed in the previously reported nanosecond time-resolved experiments.     

Density functional investigation on electron-transfer catalysis of cycloreversion of cyclobutane: radical anion mechanism

The mechanism of cycloreversion of cyclobutane radical anion (c-C(4)H(8) (-)) has been investigated at the UB3LYP/6-31++G(d,p) level, and compared with those of neutral c-C(4)H(8) and c-C(4)H(8) (+) radical cation. Although both c-C(4)H(8) (-) and C(2)H(4) are shown to be Rydberg states unstable with respect to electron ejection, the activation barrier for the "rotating" cycloreversion of c-C(4)H(8) (-) (37.3 kcal/mol) is lower by about 25.2 kcal/mol than that of c-C(4)H(8), and even the intervention of tetramethylene radical anion intermediate may reduce the activation barrier for the cycloreversion of c-C(4)H(8) by about 8.4 kcal/mol, mainly due to stronger electron-deficiency of intermediate biradical species than close-shell cyclobutanes. For the cycloreversion for c-C(4)H(8) (-), side isomerization reaction may be efficiently prevented by the low kinetic stability of tetramethylene radical anion intermediate towards dissociation, just different from the radical cation case. Our theoretical results have suggested the possibility of electron-attachment catalysis of the cycloreversion of some electron-deficient substituted cyclobutanes.     

Explicit and implicit solvation of radical ions: the cycloreversion of CPD dimers

The electron transfer catalyzed cycloreversion of cyclobutane pyrimidine dimers is the key step in repair of light-induced DNA lesions catalyzed by the enzyme CPD photolyase. The formation of the CPD radical anion was found to be strongly solvent dependent due to a specific hydrogen bond that stabilizes the valence bound state over the dipole bound state of the additional electron. The effect of solvation on the vertical and adiabatic electron affinity of uracil and uracil dimers as well as on the mechanism of the cycloreversion of the uracil dimer radical anion is explored for three model systems that include explicit solvent molecules at the B3LYP/6-311++G^∗∗/B3LYP/6-31+G^∗ level of theory. The second solvation shell is described using the implicit C-PCM solvation model. These calculations indicate an effectively barrierless mechanism. These results are in agreement with the available experimental data for the reaction energies and isotope effects. It is also shown that a single hydrogen bond donor is a sufficient minimal model for the first solvation shell by adequately describing the stabilization of the valence bound state of the radical anion through hydrogen bonding. The relationship of these model systems with the enzymatic reaction catalyzed by DNA photolyase is also discussed.     

Reductive activation of arenes: XIX. Mechanism and some synthetic applications of the alkylation of phthalodinitrile radical anion

Using cyclopropylmethyl bromide as mechanism-sensitive reagent, it was shown that the reaction of phthalonitrile radical anion with alkyl halides in liquid ammonia involves electron transfer. The effects of the nature of alkyl bromide and counterion in the radical anion salt and reaction conditions on the ratio of 2-alkyl-benzonitrile, 4-alkylphthalonitrile, and 2,5-dialkylbenzonitrile were studied. Phthalodinitrile radical anion was found to undergo dimerization with formation of biphenyl-2,3′,4′-tricarbonitrile. The examined transformations may underlie syntheses of phthalonitriles modified at the 4-position.     

Involvement of triplet excited states and olefin radical cations in electron-transfer cycloreversion of four-membered ring compounds photosensitized by (thia)pyrylium salts

Cycloreversion of 1,2,3,4-tetraphenylcyclobutanes 1a,b and oxetane 2 is achieved using (thia)pyrylium salts as electron-transfer photosensitizers. Radical cation intermediates involved in the electron-transfer process have been detected using laser flash photolysis. The experimental results are consistent with the reaction taking place from the triplet excited state of the sensitizer.     

Reductive openings of acetals: explanation of regioselectivity in borane reductions by mechanistic studies

The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and (11)B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH3 x NMe3 increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3 x THF.     

Photoinduced superoxide radical anion and singlet oxygen generation in the presence of novel selenadiazoloquinolones (an EPR Study)

Novel 7‐substituted 6‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐h]quinoline ( SeQ(1–6) ) and 8‐substituted 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f ]quinoline derivatives ( SeQN(1–5) ) with R₇, R₈ = H, COOC₂H₅, COOCH₃, COOH, COCH₃ or CN were synthesized and their spectral characteristics were obtained by UV/Vis spectroscopy. Ultraviolet A photoexcitation of the selenadiazoloquinolones in dimethylsulfoxide or acetonitrile resulted in the formation of paramagnetic species coupled with molecular oxygen activation generating the superoxide radical anion or singlet oxygen, evidenced by electron paramagnetic resonance spectroscopy. The cytotoxic/photocytotoxic impact of selenadiazoloquinolones on murine and human cancer cell lines was demonstrated using the derivative SeQ5 (with R₇ = COCH₃).     

Interaction of singlet oxygen and superoxide radical anion with guanine and formation of its mutagenic modification 8‐oxoguanine

Formation of 8‐oxoguanine (8OG) from guanine in biological systems is known to cause lethal mutation and cancer. It has been suggested earlier, on the basis of experimental studies, that the oxygen molecule in its lowest singlet excited state (¹O₂) plays an important role in the formation of 8OG. In order to understand the possible mechanisms in this context, B3LYP/6‐31+G* and MP2/6‐31+G* calculations were carried out on the structures and stabilities of different molecules and complexes involved in the formation of 8OG. All the molecules, complexes, and transition states studied in the present report were solvated in aqueous media. Guanine has been found to make a strong complex with ¹O₂ with the latter species located above the imidazole ring plane, and the complex of guanine with ³O₂ is much weaker than that with ¹O₂. Transition state calculations were carried out to study formation of 7,8‐dihydro,8‐hydroxyguanine (8OHG) and 2‐oxo‐imidazole. It has been shown that 8OG can be formed in two different ways: (i) due to interaction of the radical cation of guanine with O where 8OHG complexed with ¹O₂ would occur as an intermediate, and (ii) due to interaction of guanine with ¹O₂ leading to the formation of guanine hydroperoxide that would react with a water molecule in the presence of two ¹O₂ molecules serving as a source of energy to overcome the barrier. It is shown that because the interaction strengths of ³O₂ and ¹O₂ with other molecules, e.g., guanine, are very different, a crossing of their potential energy surfaces takes place in both gas phase and aqueous media, as a result of which the lifetime of ¹O₂ is strongly decreased. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Reactivity and regioselectivity of the pyrrole,furan, and thiophene radical cations in polymerization reactions

Within the framework of the delocalization model, based on the perturbation theory, the rate constants of the reactions of the pyrrole, furan, and thiophene radical cations with the initial compounds were compared. It was estabished that attack of a cation in the 2-position of the heterocycle is more probable in comparison with the 3-position. The results of a comparison of the reactivity of the radical cations and the corresponding molecules in the triplet state in these reactions are discussed in connection with the experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 483–487, September–December, 1992.     

Origins of regioselectivity in radical arylation of aniline: A computational study

The radical arylation of the para‐substituted anilines under three different conditions (A: arylhydrazines as the radical precursors and MnO₂ as the oxidant in acetonitrile; B: arylhydrazine hydrochlorides as the radical precursors and O₂ as the oxidant in aqueous sodium hydroxide solution; C: arenediazonium salts as the radical precursors and TiCl₃ as the reductant in aqueous hydrochloric acid solution) has been theoretically studied and the origins of the ortho/meta regioselectivity have been explored. The arylation process is suggested to contain three steps: radical generation, radical addition, and rearomatization. Calculations show that the arylation of the neutral anilines is kinetically controlled under conditions A and B, and the regioselectivity is determined by the radical addition. As a directing group, ? NH₂ plays an important role in these cases with the assistance of Mn(OH)₂ (the reduced product of MnO₂ under condition A) and Na⁺ (condition B). As for the arylation of the protonated anilines under condition C, the regioselectivity is affected by the substituents in the para‐position of anilines. Electron‐donating groups support meta‐addition and the selectivity is decided by the radical addition. Conversely, electron‐withdrawing groups favor ortho‐addition, and in this situation the arylation process is thermodynamically controlled and the regioselectivity is determined by the radical addition and rearomatization. © 2015 Wiley Periodicals, Inc.     

Geometrical preferences of the crotyl anion,radical and cation

ab initio MO calculations have been performed on the cis and trans isomers of the crotyl cation, free radical and anion in each of two orientations of the Me rotor about the allylic framework. In agreement with available experimental data, both the crotyl cation and free radical prefer trans skeletal geometries. On the other hand, the cis isomer of the crotyl anion is found to be more stable than the trans, the same preference as has been noted for alkali metal allyl organometallics in solution, but opposite to that recently reported for the free (gas phase) anion. The Me groups are predicted to eclipse the partial double bond for the trans isomers of all three systems and for the cis cation. These results are rationalized with the aid of perturbation MO theory.     

Spectroscopy of free-base N-confused tetraphenylporphyrin radical anion and radical cation

The radical anions and radical cations of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin have been studied using a combination of cyclic voltammetry, steady state absorption spectroscopy, and computational chemistry. N-Confused porphyrins (NCPs), alternatively called 2-aza-21-carba-5,10,15,20-tetraphenylporphyrins or inverted porphyrins, are of great interest for their potential as building blocks in assemblies designed for artificial photosynthesis, and understanding the absorption spectra of the corresponding radical ions is paramount to future studies in multicomponent arrays where electron-transfer reactions are involved. NCP 1e was shown to oxidize at a potential of E(ox) 0.65 V vs Fc(+)|Fc in DMF and reduce at E(red) -1.42 V, while the corresponding values for 1i in toluene were E(ox) 0.60 V and E(red) -1.64 V. The geometries of these radical ions were computed at the B3LYP/6-31+G(d)//B3LYP/6-31G(d) level in the gas phase and in solution using the polarizable continuum model (PCM). From these structures and that of H(2)TPP and its corresponding radical ions, the computed redox potentials for 1e and 1i were calculated using the Born-Haber cycle. While the computed reduction potentials and electron affinities were in excellent agreement with the experimental reduction potentials, the calculated oxidation potentials displayed a somewhat less ideal relationship with experiment. The absorption spectra of the four radical ions were also measured experimentally, with radical cations 1e(?+) and 1i(?+) displaying significant changes in the Soret and Q-band regions as well as new low energy absorption bands in the near-IR region. The changes in the absorption spectra of radical anions 1e(?-) and 1i(?-) were not as dramatic, with the changes occurring only in the Soret and Q-band regions. These results were favorably modeled using time-dependent density functional calculations at the TD-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level. These results were also compared to the existing data of free base tetraphenylporphyrin and free base tetraphenylchlorin.     

Theoretical elucidation of kinetic and thermodynamic control of radical addition regioselectivity

The cyclizations of two structurally similar 2-oxo-5-hexenyl-type radicals have been investigated by ab initio and density functional (UB3LYP/6-31+G**//UHF/6-31G* and UB3LYP/6-31G*//UB3LYP/6-31G*) calculations. The origin of apparently contradictory reports of 6-endo and 5-exo cyclizations is determined. Kinetic control favors 6-endo cyclization, while thermodynamic control gives 5-exo cyclization, and the observation of different products from different research groups arises from the difference in experimental conditions used by the two groups. The outcome of a new cyclization reaction was predicted by using these theoretical techniques. Kinetic control is predicted to yield exclusively the products of 6-endo cyclization, while thermodynamic control would lead to an approximately equal mixture of one 6-endo and one 5-exo cyclized product. Experimental studies revealed that the reaction yields only the products of 6-endo cyclization through kinetic control.     

Reaction of singlet oxygen with sulfide: A similarity of singlet oxygenation and coupling reaction of cation radical and superoxide ion

Photosensitized oxygenation of 5H,7H-dibenzo[b,g][1,5]dithiocin afforded the sulfoxides. The reaction of its cation radical derived from oxidation by nitrosyl tetrafluoroborate with superoxide ion gave the same products.     

Generation, isolation, and reactivity of a kinetically stabilized diphosphene anion radical

The stable lithium diphosphene anion radical, [Li(dme)₃]⁺[TbtPPTbt]⁻ (dme: 1,2-dimethoxyethane, Tbt: 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), was readily synthesized by the one-electron reduction of the corresponding neutral diphosphene (TbtPPTbt). The molecular structure of the diphosphene anion radical was discussed in detail on the basis of its ESR, UV-Vis and Raman spectra, and theoretical calculations. The diphosphene anion radical was found to undergo ready chalcogenation reactions using elemental sulfur and selenium to afford the corresponding thiadiphosphirane and selenadiphosphirane, respectively.     

Metal-controlled reactivity of a pincer-type, sigma-coordinated naphthyl radical anion

Reduction of the new, naphthalene-based pincer complex [(C10H5(CH2PiPr2)2)Rh(eta1-N2)] with potassium metal gave the corresponding sigma-coordinated naphthyl radical anion complex, with a ring-centered radical and no change in the formal metal oxidation state. This paramagnetic complex can be reoxidized to the diamagnetic one with [Cp2Fe][BF4], the structural integrity being retained. Unexpectedly, treatment of a THF solution of the reduced complex with water leads to the immediate evolution of dihydrogen, with reoxidation to the starting complex. This is in striking contrast with the well-known reactivity of the naphthide radical anion, which undergoes ring protonation by water to form 1,4-dihydronaphthalene and naphthalene in a 1:1 ratio.