--Redox Transformations of the OX063 Radical in Biological Media: Oxidative Decay of Initial Trityl with Further Formation of Structurally-Modified TAM.

Being a low-toxic and hydrophilic representative of TAM, OX063 has shown its suitability for in-vivo and in-cell EPR experiments and design of spin labels. Using 13C labeling, we investigated the course of oxidative degradation of OX063 into quinone-methide (QM) under the influence of superoxide as well as further thiol-promoted reduction of QM into TAM radical, which formally corresponds to substitution of a carboxyl function by a hydroxyl group. We found these transformations being quantitative in model reactions mimicking specific features of biological media and confirmed the presence of these reactions in the blood and liver homogenate of mice in vitro. The emergence of the trityl with the hydroxyl group can be masked by an initial TAM in EPR spectra and may introduce distortions into EPR-derived oximetry data if they have been obtained for objects under hypoxia. 13C labeling allows one to detect its presence, considering its different hyperfine splitting constant on 13C1 (2.04 mT) as compared to OX063 (2.30 mT). The potential involvement of these reactions should be considered when using TAM in spin-labeling of biopolymers intended for subsequent EPR experiments, as well as in the successful application of TAM in experiments in vivo and in cell.

Synthesis of Nitro- and Acetyl Derivatives of 3,7,10-Trioxo-2,4,6,8,9,11-hexaaza[3.3.3]propellane.

Here, we report the study results of the nitration of 3,7,10-trioxo-2,4,6,8,9,11-hexaaza[3.3.3]propellane (THAP) by different nitrating agents such as nitric acid, mixed nitric/sulfuric acids, nitric anhydride, and mixed concentrated nitric acid/acetic anhydride to furnish 3,7,10-trioxo-2-nitro-2,4,6,8,9,11-hexaaza[3.3.3]propellane and 3,7,10-trioxo-2,8-dinitro-2,4,6,8,9,11-hexaaza[3.3.3]propellane, whereas a lactam-lactim rearrangement was found to take place upon vigorous cooling to give 10-hydroxy-2,4,6,8,9,11-hexaazatricyclo[3.3.3.01,5]undec-9-ene-3,7-dione. The two competing reactions, lactam-lactim rearrangement, and nitration were found to take place. The acylation of 3,7,10-trioxo-2,4,6,8,9,11-hexaaza[3.3.3]propellane was examined and the formation conditions of 2,6-di- and 2,6,9-triacetyl-substituted and 3,7,10-trioxo-2,4,6,8,9,11-hexaacetyl-2,4,6,8,9,11-hexaaza[3.3.3]propellane were established. The acetyl derivatives were found to be instable in an acidic medium and to undergo deacylation. The obtained findings correlate well with the quantum-chemical calculations.

Cluster halogenation of adamantane and its derivatives with bromine and iodine monochloride.

Noncatalytic halogenation of adamantane (AdH) with bromine or iodine monochloride was found to proceed according to the cluster mechanism featuring high kinetic order with respect to the halogen and a sharp decrease in the calculated energy barrier when additional halogen molecules are involved in the quantum chemical system. In the reaction with Br2, 1-AdBr formed selectively. This reaction proved to be first order in terms of AdH and approximately seventh order in Br2, and its rate does not depend on the rising concentration of HBr. It was demonstrated that the reaction of AdH with ICl is sixth order in ICl, and at the first stage, 1-AdCl forms. According to kinetic data, this reaction requires 3 equivalents of ICl. The rate of 1-AdCl chlorination leading to the 1,3-di-Cl derivative turned out to be 105 times slower than that of AdH. The halogen exchange reaction of 1-AdBr with ICl yielded 1-AdCl, and this reaction is fast and is first order in ICl. Another halogen exchange reaction, AdCl + Br2 = AdBr + BrCl, proceeded much more slowly, and the equilibrium is strongly shifted to the left (equilibrium constant: 10-6). With an excess of either Br2 or ICl, adamantanol (1-AdOH) was found to enter into a slow (compared to AdH) exchange reaction producing 1-AdBr or 1-AdCl, respectively. In all the studied reactions, ICl is ∼106-fold more active than Br2. According to DFT data, the reactions of AdH with Br2 and ICl have similar rate-limiting stages, where the H atom from AdH and X atom from polarized halogen cluster X2n move toward each other forming an HX molecule and ion pair Ad+X2n-1-.

DFT insights into superelectrophilic activation of α,ß-unsaturated nitriles and ketones in superacids.

Superelectrophilic activation of α,ß-unsaturated carbonyl compounds and their isoelectronic analogs, proceeding normally under superacidic conditions, have led to a great variety of beneficial synthetic transformations. However, the essence of such activation is not yet fully recognized, while a number of alternative views on the subject have been discussed at length in the literature. Here, taking the example of virtual reactions of cinnamonitrile and benzalacetone with benzene, their feasible mechanistic variants, including multiple protonation (coordination to AlCl3) of the reactants, were analyzed based on density functional theory (DFT). It is revealed that the most plausible reaction pathways involve the initial N- or O-protonation (coordination to AlCl3) of the activated compounds followed by subsequent protonation on the α-C-atom. Dicationic superelectrophiles thus formed ensure practically barrier-free reactions with benzene in addition to a more favorable energetic profile of their generating, which is in marked contrast to other potential reaction pathways.

Noncatalytic Bromination of Icosahedral Dicarboranes: The Key Role of Anionic Bromine Clusters Facilitating Br Atom Insertion into the B-H σ-Bond.

The mechanism of the noncatalytic bromination of carboranes was studied experimentally and theoretically. We found that the reactions of o- and m-carboranes 1 and 2 with elemental bromine are first order in the substrate but unusually high (approximately fifth) order in bromine. The calculated energy barriers of these reactions decrease sharply as more bromine molecules are added to the quantum-chemical system. A considerable primary deuterium kinetic isotope effect for the bromination of 2 indicates that the rate-limiting stage is B-H bond breakage. According to quantum-chemical reaction path calculations, the bond breakage proceeds after the intrusion of a bromine atom into the B-H σ-bond. The 9-Br and 9-OH substituents in carborane 1 strongly retard the bromination of the corresponding derivatives. The bromination mechanism of 9-OH-1 is complex and includes neutral, deprotonated, and protonated forms of the carborane. The high experimental kinetic reaction order in bromine, together with quantum chemical modeling, points to a specific mechanism of bromination facilitated by anionic bromine clusters which significantly stabilize the transition state.

Unusual temperature-sensitive protonation behaviour of 4-(dimethylamino)pyridine.

Stimuli-responsive and, in particular, temperature-responsive smart materials have recently gained much attention in a variety of applications. On the other hand, 4-(dimethylamino)pyridine (DMAP) and related structures are widely used as nucleophilic catalysts and also as specific parts of rationally designed molecules, where reversible reactions of the pyridinic nitrogen with electrophiles are involved. In our study, we have found an unexpectedly significant impact of temperature on the protonation degree of DMAP derivatives, especially in the case of protonation of the 4-(dimethylamino)-1-(2,3,5,6-tetrafluoropyridin-4-yl)pyridinium cation, derived from the reaction of DMAP with pentafluoropyridine. Thus, when dissolved in the TfOH-SO2ClF-CD2Cl2 acid system at 30 °C, this cation underwent a slight (<7%) protonation on the dimethylamino group, while the temperature decrease to -70 °C resulted in its complete protonation. Notably, such a scale of this phenomenon has never been observed before for other weak nucleophiles, being many times lower at the same change of temperature. The mechanistic aspects of these intriguing results are discussed.

NMR and EPR Study of Homolysis of Diastereomeric Alkoxyamines.

Three alkoxyamines based on imidazoline radicals with a pyridine functional group-potential initiators of nitroxide-mediated, controlled radical polymerization-were synthesized. Electron Paramagnetic Resonance (EPR) measurements reveal biexponential kinetics for the thermolysis for diastereomeric alkoxyamines and monoexponential kinetics for an achiral alkoxyamine. For comparison, the thermolysis of all three alkoxyamines was studied by NMR in the presence of three different scavengers, namely tetramethylpiperidine-N-oxyl (TEMPO), thiophenol (PhSH), and ß-mercaptoethanol (BME), and detailed analysis of products was performed. NMR differentiates between N-inversion, epimerization, and homolysis reactions. The choice of scavenger is crucial for making a reliable and accurate estimate of the true homolysis rate constant.

A Novel Small Molecule Supports the Survival of Cultured Dopamine Neurons and May Restore the Dopaminergic Innervation of the Brain in the MPTP Mouse Model of Parkinson's Disease.

We previously showed that monoterpenoid (1R,2R,6S)-3-methyl-6-(prop-1-en-2-yl)cyclohex-3-ene-1,2-diol 1 alleviates motor manifestations of Parkinson's disease in animal models. In the present study, we designed and synthesized monoepoxides of (1R,2R,6S)-3-methyl-6-(prop-1-en-2-yl)cyclohex-3-ene-1,2-diol 1 and evaluated their biological activity in the MPTP mouse model of Parkinson's disease. We also assessed the ability of these compounds to penetrate the blood-brain barrier (BBB). According to these data, we chose epoxide 4, which potently restored the locomotor activity in MPTP-treated mice and efficiently penetrated the BBB, to further explore its potential mechanism of action. Epoxide 4 was found to robustly promote the survival of cultured dopamine neurons, protect dopamine neurons against toxin-induced degeneration, and trigger the mitogen-activated protein kinase (MAPK) signaling cascade in cells of neuronal origin. Meanwhile, neither the survival-promoting effect nor MAPK activation was observed in non-neuronal cells treated with epoxide 4. In the MPTP mouse model of Parkinson's disease, compound 4 increased the density of dopamine neuron fibers in the striatum, which can highlight its potential to stimulate striatal reinnervation and thus halt disease progression. Taken together, these data indicate that epoxide 4 can be a promising compound for further development, not only as a symptomatic but also as a neuroprotective and neurorestorative drug for Parkinson's disease.

Acid-Catalyzed Versus Thermally Induced C1-C1' Bond Cleavage in 1,1'-Bi-2-naphthol: An Experimental and Theoretical Study.

Experiments show that 1,1'-bi-2-naphthol (BINOL) undergoes facile C1-C1' bond cleavage under action of triflic acid at temperatures above 0 °C to give mainly 2-naphthol along with oligomeric material. CASSCF and MRMP//CASSCF computations have demonstrated unambiguously that this unusual mode of scission of the biaryl bond can occur in the C1,C1'-diprotonated form of BINOL via a mechanism involving homolytic cleavage prompted by the intramolecular electrostatic repulsion. These findings also provide insights into the mechanism of a comparatively easy thermal cleavage of BINOL, implying the intermediacy of its neutral diketo form.

Mechanistic investigation of superelectrophilic activation of 1,1'-bi-2-naphthols in the presence of aluminum halides.

7,7'-Dihydroxy-1,1'-bi-2-naphthol, as a result of superelectrophilic activation in the presence of an excess of aluminum halides, reacts with cyclohexane and benzene to yield 5,6,7,8,5',6',7',8'-octahydro-7,7'-dioxo-bi-2-naphthol and its 5,5'-diphenyl derivative, respectively. In contrast, isomeric 6,6'-dihydroxy-1,1'-bi-2-naphthol does not react at all under the same reaction conditions, while the parent 1,1'-bi-2-naphthol (BINOL) reveals an alternative mode of behavior. The mechanistic aspects of these intriguing results are discussed on the basis of experimental and theoretical (DFT) study of the protonation and complexation properties of the starting BINOLs.

Elevated reaction order of 1,3,5-tri-tert-butylbenzene bromination as evidence of a clustered polybromide transition state: a combined kinetic and computational study.

The kinetics and mechanism of concurrent bromo-de-protonation and bromo-de-tert-butylation of 1,3,5-tri-tert-butylbenzene at different bromine concentrations were studied experimentally and theoretically. Both reactions have high order in bromine (experimental kinetic orders ∼5 and ∼7, respectively). According to quantum chemical DFT calculations, such high reaction orders are caused by participation of clustered polybromide anions Br2n-1- in transition states. Bromo-de-tert-butylation has a higher order due to its bigger reaction center demanding clusters of extended size. A significant primary deuterium kinetic isotope effect (KIE) for bromo-de-protonation is measured indicating proton removal is rate limiting, as confirmed by computed DFT models. The latter predict a larger value for the KIE than measured and possible explanations for this are discussed.

Radical Anions, Radical-Anion Salts, and Anionic Complexes of 2,1,3-Benzochalcogenadiazoles.

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].- and [2].- , RA [3].- was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1-3 was performed and new thermally stable RA salts [K(THF)]+ [2].- (8) and [K(18-crown-6)]+ [2].- (9) were isolated in addition to known salt [K(THF)]+ [1].- (7). On contact with air, RAs [1].- and [2].- underwent fast decomposition in solution with the formation of anions [ECN]- , which were isolated in the form of salts [K(18-crown-6)]+ [ECN]- (10, E=S; 11, E=Se). In the case of 3, RA [3].- was detected by EPR spectroscopy as the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]+ 2 [3-Te2 ]2- (12) featuring a new anionic complex with coordinate Te-Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]+ 2 [3-Te4 -3]2- (13) containing an anionic complex with two coordinate Te-Te bonds. The structures of 8-13 were confirmed by XRD, and the nature of the Te-Te coordinate bond in [3-Te2 ]2- and [3-Te4 -3]2- was studied by DFT calculations and QTAIM analysis.

A Robust and Cost-Efficient Scheme for Accurate Conformational Energies of Organic Molecules.

Several standard semiempirical methods as well as the MMFF94 force field approximation have been tested in reproducing 8 DLPNO-CCSD(T)/cc-pVTZ level conformational energies and spatial structures for 37 organic molecules representing pharmaceuticals, drugs, catalysts, synthetic precursors, industry-related chemicals (37conf8 database). All contemporary semiempirical methods surpass their standard counterparts resulting in more reliable conformational energies and spatial structures, even though at significantly higher computational costs. However, even these methods show unexpected failures in reproducing energy differences between several conformers of the crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6). Inexpensive force field MMFF94 approximation groups with contemporary semiempirical methods in reproducing the correct order of conformational energies and spatial structures, although the performance in predicting absolute conformational energies compares to standard semiempirical methods. Based on these findings, we suggest a two-step strategy for reliable yet feasible conformational search and sampling in realistic-size flexible organic molecules: i) geometry optimization/preselection of relevant conformers using the MMFF94 force field; ii) single-point energy evaluations using a contemporary semiempirical method. We expect that developed database 37conf8 is going to be useful for development of semiempirical methods.

Superelectrophilic activation of 1-nitronaphthalene in the presence of aluminum chloride. Reactions with benzene and cyclohexane.

1-Nitronaphthalene smoothly reacts with benzene and undergoes selective reduction with cyclohexane in the presence of aluminum chloride to give 2,4,4-triphenyl-3,4-dihydronaphthalen-1(2H)-one oxime and 5,6,7,8-tetrahydro-1-naphthylamine, respectively. The mechanistic aspects of these and related reactions are discussed on the basis of DFT, providing insight into the protonation behavior of 1-nitronaphthalene coordinated to AlCl3.

Protonation Behavior of 1,1'-Bi-2-naphthol and Insights into Its Acid-Catalyzed Atropisomerization.

The behavior of 1,1'-bi-2-naphthol (BINOL) in variety of (super)acid media has been studied by NMR. The results are combined with the theoretical (DFT) study of the role of mono- and diprotonated forms of BINOL in the acid-catalyzed atropisomerization of this compound. It is demonstrated that the process of enantiomeric configuration exchange proceeds mainly via internal rotation around the C1(sp3)-C1'(sp3) bond in intermediates such as C1-monoprotonated keto or C1,C1'-diprotonated forms of BINOL, depending on the acidity level.

Noncatalytic bromination of benzene: A combined computational and experimental study.

The noncatalytic bromination of benzene is shown experimentally to require high 5-14 M concentrations of bromine to proceed at ambient temperatures to form predominantly bromobenzene, along with detectable (<2%) amounts of addition products such as tetra and hexabromocyclohexanes. The kinetic order in bromine at these high concentrations is 4.8 ± 0.06 at 298 K and 5.6 ± 0.11 at 273 K with a small measured inverse deuterium isotope effect using D6 -benzene of 0.97 ± 0.03 at 298 K. These results are rationalized using computed transition states models at the B3LYP+D3/6-311++G(2d,2p) level with an essential continuum solvent field for benzene applied. The model with the lowest predicted activation free energies agrees with the high experimental kinetic order in bromine and involves formation of an ionic, concerted, and asynchronous transition state with a Br8 cluster resembling the structure of the known Br9 (-). This cluster plays three roles; as a Br(+) donor, as a proton base, and as a stabilizing arm forming weak interactions with two adjacent benzene CH hydrogens, these aspects together combining to overcome the lack of reactivity of benzene induced by its aromaticity. The computed inverse kinetic isotope effect of 0.95 agrees with experiment, and arises because C-Br bond formation is essentially complete, whereas C-H cleavage has not yet commenced. The computed free energy barriers for the reaction with 4Br2 and 5Br2 for a standard state of 14.3 M in bromine are reasonable for an ambient temperature reaction, unlike previously reported theoretical models involving only one or two bromines.

Migration of methylethynyl group in a long-lived carbocation.

Degenerate 1,2-shift of methylethynyl group in long-lived 9,10-dimethyl-9-methylethynyl-phenanthrenium ion has been detected by NMR. This is the first example of ethynyl migration in a long-lived carbocation.

Interaction of acetonitrile with trifluoromethanesulfonic acid: unexpected formation of a wide variety of structures.

Interaction of acetonitrile with trifluoromethanesulfonic acid has been studied by multinuclear NMR and ESI-MS. It has been found that the interaction results in formation of a great variety of different cations and neutral compounds which is controlled by the ratio of CH(3)CN to TfOH. In the presence of an excess of the acid (molar ratio 1 : 8-14) diprotonated N-acetylacetamidine 1 is formed as the major product, which eventually transforms into protonated acetamidine 3 and acetic acid 4. At molar ratio of (1 : 1-2) diprotonated 2,4-dimethyl-6-methylidene-3H-1,3,5-triazine 12, tautomer of the diprotonated trimethyl-s-triazine 11, becomes the main product at an early stage of the reaction and diprotonated 1-(dimethyl-1,3,5-triazin-2-yl)prop-1-en-2-ol 15 at a later stage. In the case of a large excess of acetonitrile (4-20 : 1) trication 17 is formed as a result of the interaction between 11 and 12 along with some oligomers [(CH(3)CN)(3)](n) (n = 4-12).

Triarylmethanols bearing bulky aryl groups and the NOESY/EXSY experimental observation of two-ring-flip mechanism for helicity reversal of molecular propellers.

Triarylmethanols - the direct precursors of persistent trityl radicals - are racemic mixtures of chiral three-bladed molecular propellers. Depending on bulkiness of aryl groups they exhibit various liabilities to interconversion, the half- life time of room temperature racemization varying in a range between 8.4 hours and 1.32 years. NOESY/EXSY experiment performed on two representative models strongly supports the two-ring flip mechanism for the configurational interchange.