Enthalpy of Formation of the Cyclohexadienyl Radical and the C−H Bond Enthalpy of 1,4-Cyclohexadiene: An Experimental and Computational Re-Evaluation

Towards a General Access to 1-Azaspirocyclic Systems via Photoinduced, Reductive Decarboxylative Radical Cyclizations

A convenient and versatile approach to important 1-azaspirocyclic systems relevant to medicinal chemistry and natural products is reported herein. The main strategy relies on a reductive decarboxylative cyclization of redox-active esters which can be rapidly assembled from abundant cyclic azaacids and tailored acceptor sidechains, with a focus on alkyne acceptors enabling the generation of useful exo-alkene moieties. Diastereoconvergent variants were studied and could be achieved either through remote stereocontrol or conformational restriction in bicyclic carbamate substrates. Two sets of metal-free photocatalytic conditions employing inexpensive eosin Y were disclosed and studied experimentally to highlight key mechanistic divergences.

Structure and Electron Configuration of Imidazole-2-carboxaldehyde and Its Excited Triplet: Resonance Raman and Transient Absorption Spectroscopy and DFT Calculation Investigations

Imidazole-2-carboxaldehyde (IC) can be generated in atmospheric waters and absorbs solar radiation in the near UV region to produce its excited triplet state (3IC), which contributes to the formation of a secondary organic aerosol (SOA). The photoreactivity of IC is significantly influenced by its surroundings, such as water and acidic environment, because IC is capable of transforming into gem-diol under above conditions. Meanwhile, the electron configuration of 3IC is critical in elucidating the reaction mechanism of 3IC with other anthropogenic and biogenic volatile organic compounds (VOCs). In this study, steady-state and time-resolved resonance Raman as well as transient absorption spectroscopic experiments were conducted to provide vibrational and kinetic information on IC and 3IC in the presence of water and acid conditions. Using density functional theory (DFT) calculations, the H-bonding at the carbonyl O was confirmed and the hydrated structure of IC and 3IC was determined. 1,4-Cyclohexadiene is a good hydrogen donor, and it has a second-order rate constant of ∼107 M-1 s-1 toward 3IC. The results of CASSCF calculations suggest that the hydrogen abstraction may involve the transition from the ππ* to nπ* triplet state via the surface-crossing point.

Investigation of Methylcyclopentadiene Reactivity: Abstraction Reactions and Methylcyclopentadienyl Radical Unimolecular Decomposition

Understanding the reactivities of methylcyclopentadiene and the methylcyclopentadienyl radical is important in order to improve our comprehension of the chemical kinetics leading to the formation, decomposition, and growth of the first aromatic ring, as it has been shown that five-membered-ring species are important intermediates in the reaction kinetics of aromatic species. In this work, the rate constants of some key H-abstraction reactions from methylcyclopentadiene to produce the methylcyclopentadienyl radical and the formation of fulvene and benzene from the latter are theoretically determined. Rate constants are evaluated using the ab initio transition state theory-based master equation approach, determining structures and Hessians of all stationary points at the ωB97X-D/aug-cc-pVTZ level, energies at the CCSD(T) level extrapolated to the complete basis set limit, RRKM rate constants using conventional and variational transition state theory, and phenomenological rate constants through the solution of the one-dimensional master equation. Variational corrections are determined in both internal and Cartesian coordinates, and it is found that the choice of the coordinate system can impact the accuracy of the calculated rate constants by up to a factor of 4 for H-abstraction reactions and 2 for the unimolecular decomposition of the methylcyclopentadienyl radical. The calculated rate constants are in good agreement with the available literature data. Prompt dissociation of methylcyclopentadienyl radicals accessed following H-abstraction from methylcyclopentadiene was also investigated, and the corresponding rate constants were determined; the results show that prompt dissociation plays a key role under combustion conditions. Finally, lumping of theoretically derived rate constants to account for methylcyclopentadiene ⇄ methylcyclopentadienyl tautomerism allowed the derivation of a simplified set of rate constants suitable to be inserted into kinetic mechanisms.

Effect of electron-donating substitution on the triplet state reactivities of 1-nitronaphthalene

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), often found in polluted air, are carcinogenic and mutagenic. The nitro group increases the spin-orbit coupling and results in the lowest excited triplet (T₁) on the picosecond time scale with a high yield. The electron-donating substituents have a significant influence on the photophysics and photochemistry of nitro-PAHs. We used transient absorption spectroscopy and kinetic analysis to investigate the reactivities of the T₁ state 1-methoxy-4-nitronaphthalene (³MeO-NN) and 1-methyl-4-nitronaphthalene (³Me-NN). The results show that the methoxy and methyl substitutions have a minor effect on their hydrogen abstraction and electron accepting abilities. The main distinction is their reaction rates towards protons. The second order rate constant of ³MeO-NN towards protons is three orders of magnitude greater than that of ³Me-NN, indicating that ³MeO-NN has a stronger hydrogen bond accepting ability. The kinetic analysis reveals that the dimer of 2,2,2-trifluoroethanol participates in the reaction with ³MeO-NN. These results suggest that the formation of the hydrogen-bonded complex is responsible for the unusually short lifetime of ³MeO-NN in methanol solution and the lack of hydrogen abstraction radicals during the decay of ³MeO-NN in methanol.

Reduced Molecular Flavins as Single-Electron Reductants after Photoexcitation

Flavoenzymes mediate a multitude of chemical reactions and are catalytically active both in different oxidation states and in covalent adducts with reagents. The transfer of such reactivity to the organic laboratory using simplified molecular flavins is highly desirable, and such applications in (photo)oxidation reactions are already established. However, molecular flavins have not been used for the reduction of organic substrates yet, although this activity is known and well-studied for DNA photolyase enzymes. We report a catalytic method using reduced molecular flavins as photoreductants and γ-terpinene as a sacrificial reductant. Additionally, we present our design for air-stable, reduced flavin catalysts, which is based on a conformational bias strategy and circumvents the otherwise rapid reduction of O₂ from air. Using our catalytic strategy, we were able to replace superstoichiometric amounts of the rare-earth reductant SmI₂ in a 5-exo-trig cyclization of substituted barbituric acid derivatives. Such flavin-catalyzed reductions are anticipated to be beneficial for other transformations as well and their straightforward synthesis indicates future use in stereo- as well as site-selective transformations.

Defluorodearomatization: A Photocatalytic Birch-Like Reduction That Enables C-C Bond Formation and Provides Access to Unnatural Cannabinoids

Within the framework of discovery chemistry, polyfluorination remains a synthetic challenge despite its ability to provide useful characteristics, such as a reduction in the number of hydrogen bond donors and metabolic stability. Coupling a reversal of this methodology with photocatalysis has been demonstrated to allow the rapid synthesis of previously difficult or impossible targets by starting with fluorines everywhere and selectively removing or functionalizing them. Herein, we demonstrate a novel method to synthesize 1,4-cyclohexadienes through a dearomative photocatalytic C-C coupling reaction. This allows for access to materials that are orthogonal to the selectivity of the Birch reaction and are more functional-group-tolerant. The reaction also allows the efficient synthesis of polyfluorinated cannabinoids. While the yields are modest, the access to the new chemical space provided by the reaction is unprecedented by any means. The trifluorinated analog of THC, 1-deoxy-1,2,4-trifluoro-THC, is synthesized, demonstrating the importance of discovery chemistry and the ability to explore otherwise unknown structure-activity relationships.

Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

The high-valent nickel(iii) complex Ni(pyalk)2 + (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ∼91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.

Phenolic Hydrogen Transfer by Molecular Oxygen and Hydroperoxyl Radicals. Insights into the Mechanism of the Anthraquinone Process

Hydrogen atom transfer (HAT) by ³O₂ and HO₂^• from arenols (ArOH), aryloxyls (ArO^•), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With ³O₂, excellent linear correlations have been found between the activation enthalpy and the overall reaction enthalpy. Different pathways have been discerned for HATs involving OH or CH moieties. The results for ArOH + HO₂^• → ArO^• + H₂O₂ neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH₂Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO₂^• occurs with a high rate constant of ≥6 × 10⁸ M^–1 s^–1 (toluene). The second propagation step pertains to a diffusion-controlled HAT by ³O₂ from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH₂Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward ³O₂/HO₂^• is much lower. Combination of the computed free energies and Abrahams’ HB donating (α₂^H) and accepting (β₂^H) parameters has afforded an α₂^H(HO₂^•) of 0.86 and an α₂^H(H₂O₂) of 0.50.

Kinetic energy release distributions from dissociative photoionization of weakly bound trimers at 14-27 eV

The formation of the intriguing ions C4H6O+, C6H6Cl+, and C6H6O+, by dissociative ionization of heterotrimers of butadiene/sulfur dioxide, benzene/hydrogen chloride and benzene/oxygen by 14-27 eV photons, illustrates the possibility that VUV irradiation of clusters comprised of three or more molecules could provide a route to make ions containing bonds not previously accessible. Kinetic energy release distributions were measured in an attempt to understand the formation of these ions and why clusters larger than dimers are needed. Standard theory was applied to find whether more complicated theoretical treatments are needed to understand the data. It was found that all of the above ions were most likely produced by essentially the same mechanism: excitation of one moiety, transfer of its excitation energy to the moiety that dissociates, followed by slow decay of the remaining excited ion into the unexcited moiety as the "solvent" plus the ion with the new bond. The very low reaction probabilities to produce these ions, combined with very low target densities in the presence of many orders of magnitude higher densities of other molecules, precluded the usual imaging techniques. However, we found that the retarding-potential method can give useful data. Also, at present laser photon energies higher than 15 eV provide significantly smaller average intensities than are needed.

Why metal-oxos react with dihydroanthracene and cyclohexadiene at comparable rates, despite having different C-H bond strengths. A computational study

1,4-Cyclohexadiene (CHD) and 9,10-dihydroanthracene (DHA) are two substrates used to probe the steric requirements of metal-oxo oxidants in H-atom-transfer (HAT) reactions, based on the assumption that they have comparable C-H bond dissociation enthalpies (BDEs). We use computations to demonstrate that the BDE of DHA is ∼3.5 kcal mol(-1) larger than that of CHD and that their often comparable reactivity is based on a competing interplay of bond strengths and favorable van der Waals interactions.

Nonmonotonic Temperature Dependence of the Pressure-Dependent Reaction Rate Constant and Kinetic Isotope Effect of Hydrogen Radical Reaction with Benzene Calculated by Variational Transition-State Theory

The reaction between H and benzene is a prototype for reactions of radicals with aromatic hydrocarbons. Here we report calculations of the reaction rate constants and the branching ratios of the two channels of the reaction (H addition and H abstraction) over a wide temperature and pressure range. Our calculations, obtained with an accurate potential energy surface, are based on variational transition-state theory for the high-pressure limit of the addition reaction and for the abstraction reaction and on system-specific quantum Rice-Ramsperger-Kassel theory calibrated by variational transition-state theory for pressure effects on the addition reaction. The latter is a very convenient way to include variational effects, corner-cutting tunneling, and anharmonicity in falloff calculations. Our results are in very good agreement with the limited experimental data and show the importance of including pressure effects in the temperature interval where the mechanism changes from addition to abstraction. We found a negative temperature effect of the total reaction rate constants at 1 atm pressure in the temperature region where experimental data are missing and accurate theoretical data were previously missing as well. We also calculated the H + C₆H₆/C₆D₆ and D + C₆H₆/C₆D₆ kinetic isotope effects, and we compared our H + C₆H₆ results to previous theoretical data for H + toluene. We report a very novel nonmonotonic dependence of the kinetic isotope effect on temperature. A particularly striking effect is the prediction of a negative temperature dependence of the total rate constant over 300-500 K wide temperature ranges, depending on the pressure but generally in the range from 600 to 1700 K, which includes the temperature range of ignition in gasoline engines, which is important because aromatics are important components of common fuels.

Ph(i-PrO)SiH2: An Exceptional Reductant for Metal-Catalyzed Hydrogen Atom Transfers

We report the discovery of an outstanding reductant for metal-catalyzed radical hydrofunctionalization reactions. Observations of unexpected silane solvolysis distributions in the HAT-initiated hydrogenation of alkenes reveal that phenylsilane is not the kinetically preferred reductant in many of these transformations. Instead, isopropoxy(phenyl)silane forms under the reaction conditions, suggesting that alcohols function as important silane ligands to promote the formation of metal hydrides. Study of its reactivity showed that isopropoxy(phenyl)silane is an exceptionally efficient stoichiometric reductant, and it is now possible to significantly decrease catalyst loadings, lower reaction temperatures, broaden functional group tolerance, and use diverse, aprotic solvents in iron- and manganese-catalyzed hydrofunctionalizations. As representative examples, we have improved the yields and rates of alkene reduction, hydration, hydroamination, and conjugate addition. Discovery of this broadly applicable, chemoselective, and solvent-versatile reagent should allow an easier interface with existing radical reactions. Finally, isotope-labeling experiments rule out the alternative hypothesis of hydrogen atom transfer from a redox-active β-diketonate ligand in the HAT step. Instead, initial HAT from a metal hydride to directly generate a carbon-centered radical appears to be the most reasonable hypothesis.

Quantum tunneling observed without its characteristic large kinetic isotope effects

Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (≳ 100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.

Anthrone and related hydroxyarenes: tautomerization and hydrogen bonding

The keto-enolization of hydroxyl-substituted naphthols and 9-anthrols has been investigated by means of CBS-QB3 calculations. An excellent agreement between experiment and theory is found for the energetics for the anthrone (5) ⇌ anthrol (6) equilibrium, with an enthalpy of tautomerization, Δ(t)H, of 3.8 kcal mol(-1). In contrast, 1-naphthol is the preferred tautomer with a Δ(t)H = -9.0 kcal mol(-1). Substitution of the hydrogens at the adjacent carbons by hydroxyl groups leads to the formation of strong intramolecular hydrogen bonds within a six-membered ring in the enones and the enols. Due to the difference in the intramolecular hydrogen bond enthalpy, Δ(HB)H(intra), the equilibrium shifts further to the enone. Thus, for 1,8-dihydroxy-anthrone (anthralin, dithranol) Δ(t)H increases to 12.7 kcal mol(-1) with an enol/enone ratio of 10(-10). The solvent effect on the 5 ⇌ 6 equilibrium has been quantified by considering the formation of intermolecular hydrogen bond(s), leading to an acidity parameter α₂(H) for anthrol of 0.42. It is shown that the hydrogen bond donating ability of bulk methanol is greatly attenuated through the formation of cyclic oligomers. The benzylic and phenolic bond dissociation enthalpies for anthrone up to anthralin suggest some antioxidant potency but the precise (radical) mechanism of action remains uncertain.

Spectroscopic and thermochemical properties of the c-C6H7 radical: a high-level theoretical study

The electronic ground state (X(2)B(1)) of the cyclohexadienyl radical (c-C(6)H(7)) has been studied by explicitly correlated coupled cluster theory at the RCCSD(T)-F12x (x = a, b) level, partly in combination with the double-hybrid density functional method B2PLYP. An accurate equilibrium structure has been established and the ground-state rotational constants are predicted to be A(0) = 5347.3 MHz, B(0) = 5249.7 MHz, and C(0) = 2692.5 MHz. The calculated vibrational wavenumbers agree well with the recent p-H(2) matrix IR data [M. Bahou, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 136, 154304 (2012)] and several predictions have been made. A low value of 6.803 ± 0.005 eV is predicted for the adiabatic ionization energy of c-C(6)H(7). Owing to a moderately large change in the equilibrium structure upon ionization, the first band of the photoelectron spectrum is dominated by the adiabatic peak (100%) and only the peaks corresponding to excitation of the two lowest totally symmetric vibrations (ν(12) and ν(11)) by one vibrational quantum have relative intensities of more than 15%. The C(6)H(6)-H dissociation energy is calculated to be D(0) = 85.7 kJ mol(-1), with an estimated error of ~2 kJ mol(-1).

Ligand effects on hydrogen atom transfer from hydrocarbons to three-coordinate iron imides

A new β-diketiminate ligand with 2,4,6-tri(phenyl)phenyl N-substituents provides protective bulk around the metal without exposing any weak C-H bonds. This ligand improves the stability of reactive iron(III) imido complexes with Fe═NAd and Fe═NMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates Fe═N-C bending, a longer Fe═N bond, and greater access to the metal as potential reasons for the increase in C-H bond activation rates.

Radical stability and its role in synthesis and catalysis

The thermodynamic stability of carbon-centered radicals may be defined in quantitative terms using the hydrogen transfer reaction shown in . The stability values obtained in this way for substituted systems may be understood as the stabilizing or destabilizing influence of substituents on the neighboring radical center. This approach can be easily adapted to oxygen- or sulfur-centered radicals as expressed in eqn (b). [Formula: see text] The stability values obtained in this way do not only serve as a quantitative basis for the discussion of substituent effects, but also allow for quantitative estimates of reaction energies for hydrogen transfer reactions. These occur as key steps in a multitude of synthetically useful radical-chain processes in apolar solution, in enzyme-mediated non-chain processes in biological systems, and in the oxidative degradation of a variety of biomolecules. The review will highlight the usefulness of radical stability values for the rationalization of successful (and not so successful) synthetic radical reactions as well as the potential design of new radical reactions.