Multiorbital Interactions during Acyl Radical Addition Reactions Involving Imines and Electron-Rich Olefins

Photoredox-Catalyzed Stereoselective Radical Reactions to Synthesize Nucleoside Analogues with a C2'-Stereogenic All-Carbon Quaternary Center

The design of novel nucleoside analogues bearing a C2' all-carbon quaternary center is described. The construction of this all-carbon stereogenic center involves the use of photoredox catalysis to initiate an intramolecular attack of a silyl-tethered vinyl functionality on a tertiary radical. Density functional theory calculations were performed to explore the origin of the high syn diastereoselectivity obtained through the preferred 5-exo-trig cyclization mode. The intramolecular vinyl addition also enables the preparation of the complementary configuration of the C2' all-carbon stereocenter when performed after lactonization.

A combined density functional theory and numerical simulation investigation of levels of chirality transfer and regioselectivity for the radical cyclizations of N-methyl-, N-ethyl- and N-isopropyl-substituted ortho-halo-N-acryloylanilides

There is a high chirality transfer ratio when the N-alkyl substituent is larger in volume than isopropyl.

A Density Functional Theory Investigation of the Tandem Radical Cyclization of 1-[2-Yl-3-(2-Methoxyphenyl)-prop-2-enyl]-6-oxo-1,6-dihydropyridine-2-carbonitrile

A density functional theory investigation of the mechanism of the titled reaction has been performed. The results suggest that the compound 1-[2-iodo-3-(2-methoxyphenyl)-prop-2-enyl]-6-oxo-1,6-dihydropyridine-2-carbonitrile would rather be converted into the titled free radical by deiodination than go by way of a Diels–Alder cycloaddition and HI-elimination to access the experimentally observed product. The deiodination of the radical precursor is followed by tandem radical cyclizations and hydrogen-loss oxidations to generate tetracyclic non-radical products. The mechanism of the tandem reaction was determined by an examination of the calculated reaction barriers, attack trajectories, and interaction energies between key orbitals. Furthermore, the H-loss oxidation of the addition intermediates by several H-abstractors were carefully analyzed. The theoretical results are in close agreement with the available experimental evidence. The detailed reaction mechanism and knowledge of such an intramolecular tandem radical cyclization presented in this study not only provide insight into the nature of tandem cyclizations but also serves as a useful guide for future experimental investigations.

An ab initio and DFT study of the autoxidation of THF and THP

Tetrahydropyran (THP) is known to undergo autoxidation much more slowly than tetrahydrofuran (THF). To investigate the difference in reactivity in the autoxidation of these two ethers, ab initio and DFT calculations were carried out. At the BHandHLYP/aug-cc-pVDZ//BHandHLYP/cc-pVDZ level of theory, the energy barrier for hydrogen abstraction from THP is predicted to be 104.1 kJ mol(-1), whereas that for THF is calculated as 94.1 kJ mol(-1). Including solvation effects in the calculations lowers these barriers to 98.0 (THP) and 84.4 kJ mol(-1) (THF); the energy barrier for the process involving THP is smaller by 14 kJ mol(-1) than that for THF. While scanning the potential energy surface for the radical coupling process between the THP (or THF) radical with molecular oxygen, an energy barrier of 11.2 kJ mol(-1) (BHandHLYP/6-311G**) was found for the process involving the THP radical, although no barrier was found for the reaction involving THF. Analysis of the Kohn-Sham singly occupied molecular orbitals (SOMOs) in the hydroperoxy complexes reveals that the SOMO of the THP complex would be blocked by the neighbouring hydrogen atoms in the THP ring. These factors would work together to delay the autoxidation of THP.

Computational comparison of reactions of CS2 with CHX•- (X = F, Cl, and Br): Do F, Cl, and Br substitutions effect differently?

Calculations based on density functional theory (DFT) have been carried out for the gas-phase ion-molecule reactions of CS 2 with CHX •- ( X = F , Cl , and Br ) to investigate the effect of the halogen-substituent on the reaction mechanism. The doublet potential energy surfaces (PESs), involving two striking reaction patterns named by the middle-C attack and the end-S attack in terms of anion attack on the C and S atoms of CS 2, have been explored and characterized in detail. Compared with the results of the end-S attack, reaction with the middle-C attack pattern displays more efficiency on PES. For a given reaction pattern, the reactions of CHCl •- and CHBr •- occur with similar efficiencies and reactivity trends. The CHF •- anion displays remarkably different reactivity, which is traced to its lower electron binding energy and the effect of the electronegative fluorine substituent. This is in good agreement with the experimental observation. According to the property of the product branching ratios in experiment, dynamical effect is used to evaluate the reason that the end-S attack with less energetically favorable obtained in our theoretical studies is comparable to the middle-C attack.

A computational study on Lewis acid-catalyzed diastereoselective acyclic radical allylation reactions with unusual selectivity dependence on temperature and epimer precursor

In stereoselective radical reactions, it is accepted that the configuration of the radical precursor has no impact on the levels of stereoinduction, as a prochiral radical intermediate is planar, with two identical faces, independently of its origin. However, Sibi and Rheault (J. Am. Chem. Soc. 2000, 122, 8873-8879) remarkably obtained different selectivities in the trapping of radicals originated from two epimeric bromides, catalyzed by chelating Lewis acids. The selectivity rationalization was made on the basis of different conformational properties of each epimer. However, in this paper we show that the two epimers have similar conformational properties, which implies that the literature proposal is unable to explain the experimental results. We propose an alternative mechanism, in which the final selectivity is dependent on different reaction rates for radical formation from each epimer. By introducing a different perspective of the reaction mechanism, our model also allows the rationalization of different chemical yields obtained from each epimer, a result not rationalized by the previous model. Adaptation to other radical systems, under different reaction conditions, is also possible.

Gas-phase reaction of ClO−with CHnCl4-n(n= 0, 1, 2, 3) and CX3H (X = F, Cl and Br): Substituent effect from a comparative study

Substituent effects on reactivity are studied using the hybrid B3LYP and BHandHLYP methods of density functional theory with the aug-cc-pVDZ basis set. The chosen testing models includes two very representative reactions in chemical research, the bimolecular nucleophilic substitution (SN2) reaction and the deprotonation reaction, in which the former is represented by ClO−+ CHnCl4-n(n = 0, 1, 2, 3), and the latter is based on reactions of ClO−with CX3H (X = F, Cl, and Br). Our theoretical findings suggest that a heavier substituent X in substrate results in a higher activation energy, a slower SN2 reaction, but a faster deprotonation reaction. Those are well confirmed by some presented results from bond orders, second-order perturbative energy E(2), and activation strain model analysis. Moreover, we have further explored the reactivity difference derived from substituent effects in term of the relationships of reactive barrier with the charges transferred and the leaving-bond distance in TSs, respectively, especially the TSs in SN2 reactions. Again, the rate constants at 298–1000 K are also evaluated for the SN2 reactions presented through the transition state theory.

Imino exchange reaction in a dearomatization strategy: synthesis of N-acyl diarylamines and phenothiazines from two anilines

A dearomatization strategy of anilines involving an imino exchange as the key step was developed to prepare N-acyl-diarylamines and phenothiazines.

Diastereoselective hydrogen-transfer reactions: an experimental and DFT study

Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.

DFT study on the reactions of ClO⁻/BrO⁻ with RCl (R = CH₃, C₂H₅, and C₃H₇) in gas phase

Gas-phase reactions of ClO(-)/BrO(-) with RCl (R = CH₃, C₂H₅, and C₃H₇) have been investigated in detail using the popular DFT functional BHandHLYP/aug-cc-pVDZ level of theory. As a result, our findings strongly suggest that the type of reaction is firstly initiated by a typical SN2 fashion. Subsequently, two competitive substitution steps, named as SN2-induced substitution and SN2-induced elimination, respectively, would proceed before the initial SN2 product ion-dipole complex separates, in which the former exhibits less reactivity than the latter. Those are consistent with relevant experimental results. Moreover, we have also explored reactivity difference for the title reactions in term of some factors derived from methyl group, p-π electronic conjugation, ionization energy (IE), as well as molecular orbital (MO) analysis.

Rate Coefficients for Intramolecular Homolytic Substitution of Oxyacyl Radicals at Sulfur

It is predicted on the basis of ab initio and density functional calculations that intramolecular homolytic substitution of oxyacyl radicals at the sulfur atom in ω-alkylthio-substituted radicals do not involve hypervalent intermediates. With tert-butyl as the leaving radical, free energy barriers ΔG‡ (G3(MP2)-RAD) for these reactions range from 45.8 kJ mol–1 for the formation of the five-membered cyclic thiocarbonate (8) to 56.7 kJ mol–1 for the formation of the six-membered thiocarbonate (9). Rate coefficients in the order of 104–106 s–1 and 101–104 s–1 for the formation of 8 and 9, respectively, at 353.15 K in the gas phase are predicted at the G3(MP2)-RAD level of theory.

Photodimerizations of hydroxy- and benzoylated 4-azachalcones and quantum chemical investigation of the reactions

Photodimerization reactions of compounds 4-6 gave four new cyclobutane-containing compounds (7-9) with full control over the stereochemistry at the four stereogenic centers. These new cyclobutane-containing compounds had beta-truxinic (7a), delta-truxinic (7b and 9), and epsilon-truxillic (8) structures. However, o-, m-, and p-hydroxy 4-azachalcones (1-3) did not give photochemical cyclization products under any conditions (in solvent or in their solid or molten states). Experimental data suggested the possibility of frontier orbital control over stereochemical behavior, so some theoretical calculations were performed. Full geometrical optimization of compounds 1-9 was performed via DFT B3LYP/6-31(+)G**, and their electronic structures were also investigated. The geometries of the singlet and triplet states were initially optimized by density functional theory (DFT) and the configuration interaction singles (CIS) B3LYP/3-21(+)G** level. An additional calculation was performed for the triplet state using the ground-state geometry. The possible photochemical dimerization products of compounds 7-9 (a-g) and the intrinsic reaction coordinates (IRCs) of the reactions of compounds 4-6 were calculated theoretically by the DFT/3-21(+)G** method. The configurations (reactant, transition state, product, and reaction pathway) corresponding to the stationary points (minima or saddle points) were determined. The intrinsic reaction coordinates were followed to verify the energy profiles that connect each TS to the appropriate local minimum. The dimeric products expected from the calculations coincided with the dimers produced experimentally.

Ab Initio Studies of Carbonyl Radical Additions to Hydrazone Systems

Ab initio and DFT calculations reveal that intermolecular radical additions of both acyl and oxyacyl radials to hydrazones occur through SOMO → π*hydrazone, πhydrazone → SOMO and LPN → SOMO interactions between the radical and the hydrazone π-system. Both acetyl and methoxycarbonyl radicals show preference for addition to the carbon end of the carbon–nitrogen π-bond. At the highest level of theory used in this study (G2//MP2(full)/6-31G*), energy barriers of 11.2 and 22.6 kJ mol–1 are calculated for acetyl radical addition to the carbon and nitrogen-ends of N-aminomethanimine respectively. The analogous energy barriers for the methoxycarbonyl radical are 4.9 and 25.7 kJ mol–1 at the same level of theory.

Radical acetylation of 2'-deoxyguanosine and L-histidine coupled to the reaction of diacetyl with peroxynitrite in aerated medium

Diacetyl, like other alpha-dicarbonyl compounds, is reportedly cytotoxic and genotoxic. A food and cigarette contaminant, it is related with alcohol hepatotoxicity and lung disease. Peroxynitrite is a potent oxidant formed in vivo by the diffusion-controlled reaction of the superoxide radical anion with nitric oxide, which is able to form adducts with carbon dioxide and carbonyl compounds. Here, we investigate the nucleophilic addition of peroxynitrite to diacetyl forming acetyl radicals, whose reaction with molecular oxygen leads to acetate. Peroxynitrite is shown to react with diacetyl in phosphate buffer (bell-shaped pH profile with maximum at 7.2) at a very high rate constant ( k 2 = 1.0 x 10 (4) M (-1) s (-1)) when compared with monocarbonyl substrates ( k 2 < 10 (3) M (-1) s (-1)). Phosphate ions (100-500 mM) do not affect the rate of spontaneous peroxynitrite decay, but the H 2PO 4 (-) anion catalyzes the nucleophilic addition of the peroxynitrite anion to diacetyl. The intermediacy of acetyl radicals is suggested by a three-line spectrum ( a N = a H = 0.83 mT) obtained by EPR spin trapping of the reaction mixture with 2-methyl-2-nitrosopropane. The peroxynitrite reaction is accompanied by concentration-dependent oxygen uptake. Stoichiometric amounts of acetate from millimolar amounts of peroxynitrite and diacetyl were obtained under nonlimiting conditions of dissolved oxygen. In the presence of either l-histidine or 2'-deoxyguanosine, the peroxynitrite/diacetyl system afforded the corresponding acetylated molecules identified by HPLC-MS ( n ). These studies provide evidence that the peroxynitrite/diacetyl reaction yields acetyl radicals and raise the hypothesis that protein and DNA nonenzymatic acetylation may occur in cells and be implicated in aging and metabolic disorders in which oxygen and nitrogen reactive species are putatively involved.