Hole Catalysis of Cycloaddition Reactions: How to Activate and Control Oxidant Upconversion in Radical-Cationic Diels-Alder Reactions

In order to use holes as catalysts, the oxidized product should be able to transfer the hole to a fresh reactant. For that, the hole-catalyzed reaction must increase the oxidation potential along the reaction path, i.e., lead to "hole upconversion." If this thermodynamic requirement is satisfied, a hole injected via one-electron oxidation can persist through multiple propagation cycles and serve as a true catalyst. This work provides guidelines for the rational design of hole-catalyzed Diels-Alder (DA) reactions, the prototypical cycloaddition. After revealing the crucial role of hyperconjugation in the absence of hole upconversion in the parent DA reaction, we show how upconversion can be reactivated by proper substitution. For this purpose, we computationally evaluate the contrasting effects of substituents at the three possible positions in the two reactants. The occurrence and magnitude of hole upconversion depend strongly on the placement and nature of substituents. For example, donors at C1 in 1,3-butadiene shift the reaction to the hole-upconverted regime with an increased oxidation potential of up to 1.0 V. In contrast, hole upconversion in C2-substituted 1,3-butadienes is activated by acceptors with the oxidation potential increase up to 0.54 V. Dienophile substitution results in complex trends because the radical cation can be formed at either the dienophile or the diene. Hole upconversion is always present in the former scenario (up to 0.65 V). Finally, we report interesting stereoelectronic effects that can activate or deactivate upconversion via a conformational change.

Homology Modeling and Molecular Dynamics Simulation Combined with X-ray Solution Scattering Defining Protein Structures of Thromboxane and Prostacyclin Synthases

A combination of molecular dynamics (MD) simulations and X-ray scattering (SAXS) has emerged as the approach of choice for studying protein structures and dynamics in solution. This approach has potential applications for membrane proteins that neither are soluble nor form crystals easily. We explore the water-coupled dynamic structures of thromboxane synthase (TXAS) and prostacyclin synthase (PGIS) from scanning HPLC-SAXS measurements combined with MD ensemble analyses. Both proteins are heme-containing enzymes in the cytochrome P450 family, known as prostaglandin H₂ (PGH₂) isomerase, with counter-functions in regulation of platelet aggregation. Currently, the X-ray crystallographic structures of PGIS are available, but those for TXAS are not. The use of homology modeling of the TXAS structure with ns-μs explicit water solvation MD simulations allows much more accurate estimation of the configuration space with loop motion and origin of the protein behaviors in solution. In contrast to the stability of the conserved PGIS structure in solution, the pronounced TXAS flexibility has been revealed to have unstructured loop regions in connection with the characteristic P450 structural elements. The MD-derived and experimental-solution SAXS results are in excellent agreement. The significant protein internal motions, whole-molecule structures, and potential problems with protein folding, crystallization, and functionality are examined.

Dissociative electron transfer to diphenyl-substituted bicyclic endoperoxides: the effect of molecular structure on the reactivity of distonic radical anions and determination of thermochemical parameters

The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an observed peak potential of −1.4 V at 0.2 V s⁻¹. The major product (90% yield) resulting from the heterogeneous bulk electrolysis of 4 at −1.4 V with a rotating disk glassy carbon electrode is 1,4-diphenyl-cyclopent-2-ene-cis-1,3-diol with a consumption of 1.73 electrons per mole. In contrast, 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1), undergoes a two-electron reduction mechanism in quantitative yield. This difference in product yield between 1 and 4 is suggestive of a radical-anion mechanism, as observed with 1,4-diphenyl-2,3-dioxabicyclo-[2.2.2] octane (2) and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3). Convolution potential sweep voltammetry is used to determine unknown thermochemical parameters of 4, including the O-O bond dissociation energy and the standard reduction potential and a comparison is made to the previously studied bicyclic endoperoxides 1–3 with respect to the effect of molecular structure on the reactivity of distonic radical anions.

Generation of naphthoquinone radical anions by electrospray ionization: solution, gas-phase, and computational chemistry studies

Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q(•-)) and dianion (Q(2-)) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q+15](-) was described as being a reaction between the solvent system and the radical anion, Q(•-). The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.