Exploration of Vitamin B-Based Redox-Active Pyridinium Salts Towards the Application in Aqueous Organic Flow Batteries.

Pyridoxal hydrochloride, a vitamin B vitamer, was synthetically converted to a series of diverse redox-active benzoyl pyridinium salts. Cyclic voltammetry studies demonstrated redox reversibility under basic conditions, and two of the most promising salts were subjected to laboratory-scale flow battery tests involving galvanostatic cycling at 10 mM in 0.1 M NaOH.

N-Alkylated Pyridoxal Derivatives as Negative Electrolyte Materials for Aqueous Organic Flow Batteries: Computational Screening.

N-functionalized pyridinium frameworks derived from the three major vitamers of vitamin B6, pyridoxal, pyridoxamine and pyridoxine, have been screened computationally for consideration as negative electrode materials in aqueous organic flow batteries. A molecular database including the structure and the one-electron standard reduction potential of related pyridinium derivatives has been generated using a computational protocol that combines semiempirical and DFT quantum chemical methods. The predicted reduction potentials span a broad range for the investigated pyridinium frameworks, but pyridoxal derivatives, particularly those involving electron withdrawing substituents, have potentials compatible with the electrochemical stability window of aqueous electrolytes.

Unveiling a key catalytic pocket for the ruthenium NHC-catalysed asymmetric heteroarene hydrogenation.

The chiral ruthenium(ii)bis-SINpEt complex is a versatile and powerful catalyst for the hydrogenation of a broad range of heteroarenes. This study aims to provide understanding of the active form of this privileged catalyst as well as the reaction mechanism, and to identify the factors which control enantioselectivity. To this end we used computational methods and NMR spectroscopy to study the hydrogenation of 2-methylbenzofuran promoted by this system.

Two Faces of the Two-Phase Thermodynamic Model.

The quantum harmonic model and the two-phase thermodynamic method (2PT) are widely used to obtain quantum-corrected properties such as isobaric heat capacities or molar entropies. 2PT heat capacities were calculated inconsistently in the literature. For water, the classical heat capacity was also considered, but for organic liquids, it was omitted.

Enantioselective Acetalization by Dynamic Kinetic Resolution for the Synthesis of γ-Alkoxybutenolides by Thiourea/Quaternary Ammonium Salt Catalysts: Application to Strigolactones.

Although acetalization is a fundamental transformation in organic synthesis, intermolecular asymmetric acetalization remains an unsolved problem. In this study, a thiourea-ammonium hybrid catalyst was shown to promote the O-alkylation of enols with a racemic γ-chlorobutenolide through dynamic kinetic resolution to give chiral acetals with good enantioselectivity. The catalyst simultaneously activates both the nucleophile and electrophile in a multifunctional manner.

RuBisCO-Inspired CO Activation and Transformation by an Iridium(I) Complex.

The synthesis of a new iridium(I) complex containing an enamido phosphine anion (dbuP ) and its unique reactivity with CO is reported. The complex binds two equivalents of CO and initiates a highly selective reaction cascade. The reaction leads to the reversible cleavage of CO and the enamido ligand as well.

Stereocontrol in Diphenylprolinol Silyl Ether Catalyzed Michael Additions: Steric Shielding or Curtin-Hammett Scenario?

The enantioselectivity of amine-catalyzed reactions of aldehydes with electrophiles is often explained by simple steric arguments emphasizing the role of the bulky group of the catalyst that prevents the approach of the electrophile from the more hindered side. This standard steric shielding model has recently been challenged by the discovery of stable downstream intermediates, which appear to be involved in the rate-determining step of the catalytic cycle. The alternative model, referred to as the Curtin-Hammett scenario of stereocontrol, assumes that the enantioselectivity is related to the stability and reactivity of downstream intermediates.

Folding Patterns in a Family of Oligoamide Foldamers.

A series of small, unsymmetrical pyridine-2,6-dicarboxylamide oligoamide foldamers with varying lengths and substituents at the end groups were synthetized to study their conformational properties and folding patterns. The @-type folding pattern resembled the oxyanion-hole motifs of enzymes, but several alternative folding patterns could also be characterized. Computational studies revealed several alternative conformers of nearly equal stability.

Mukaiyama-Michael reactions with trans-2,5-diarylpyrrolidine catalysts: enantioselectivity arises from attractive noncovalent interactions, not from steric hindrance.

The scope of the enantioselective Mukaiyama-Michael reactions catalyzed by trans-2,5-diphenylpyrrolidine has been expanded to include both α- and β-substituted enals. However, the rationalization of the observed enantioselectivity is far from obvious since the catalyst is not very sterically hindered. DFT calculations were carried out to rationalize the observed stereoselectivities.

On the mechanism of bifunctional squaramide-catalyzed organocatalytic Michael addition: a protonated catalyst as an oxyanion hole.

A joint experimental-theoretical study of a bifunctional squaramide-amine-catalyzed Michael addition reaction between 1,3-dioxo nucleophiles and nitrostyrene has been undertaken to gain insight into the nature of bifunctional organocatalytic activation. For this highly stereoselective reaction, three previously proposed mechanistic scenarios for the critical CC bond-formation step were examined. Accordingly, the formation of the major stereoisomeric products is most plausible by one of the bifunctional pathways that involve electrophile activation by the protonated amine group of the catalyst.

Reactivity models of hydrogen activation by frustrated Lewis pairs: synergistic electron transfers or polarization by electric field?

Two alternative qualitative reactivity models have recently been proposed to interpret the facile heterolytic cleavage of H2 by frustrated Lewis pairs (FLPs). Both models assume that the reaction takes place via reactive intermediates with preorganized acid/base partners; however, they differ in the mode of action of the active centers. In the electron transfer (ET) model, the hydrogen activation is associated with synergistic electron donation processes with the simultaneous involvement of active centers and the bridging hydrogen, showing similarity to transition-metal-based and other H2-activating systems.

Copper(II)-binding ability of stereoisomeric cis- and trans-2-aminocyclohexanecarboxylic acid-L-phenylalanine dipeptides. A combined CW/pulsed EPR and DFT study.

With the aim of an improved understanding of the metal-complexation properties of alicyclic β-amino acid stereoisomers, and their peptides, the complex equilibria and modes of coordination with copper(II) of L-phenylalanine (F) derivatives of cis/trans-2-aminocyclohexanecarboxylic acid (c/tACHC), i.e. the dipeptides F-c/tACHC and c/tACHC-F, were investigated by a combination of CW and pulsed EPR methods.

Stereoelectronic requirements for optimal hydrogen-bond-catalyzed enolization.

Protein crystallographic analysis of the active sites of enolizing enzymes and structural analysis of hydrogen-bonded carbonyl compounds in small molecule crystal structures, complemented by quantum chemical calculations on related model enolization reactions, suggest a new stereoelectronic model that accounts for the observed out-of-plane orientation of hydrogen-bond donors (HBDs) in the oxyanion holes of enolizing enzymes. The computational results reveal that the lone-pair directionality of HBDs characteristic for hydrogen-bonded carbonyls is reduced upon enolization, and the enolate displays almost no directional preference for hydrogen bonding. Positioning the HBDs perpendicular to the carbonyl plane induces strain in the catalyst-substrate complex, which is released upon enolization, resulting in more favorable kinetics and thermodynamics than the in-plane arrangement of HBDs.

Rationalizing the reactivity of frustrated Lewis pairs: thermodynamics of H(2) activation and the role of acid-base properties.

The acid-base strengths of recently reported frustrated Lewis pairs and their relation with the thermodynamic feasibility of heterolytic hydrogen splitting reactions are analyzed in terms of quantum chemical calculations. Reaction free energies of hydrogenation processes are computed, and an energy partitioning scheme is introduced, which involves quantitative measures of the acidity and basicity of the reacting Lewis centers. Additional terms are also included that account for possible dative bond formation between the active sites and for stabilizing electrostatic interactions occurring in the product species.

On the mechanism of B(C6F5)3-catalyzed direct hydrogenation of imines: inherent and thermally induced frustration.

The reaction mechanism for the transition metal free direct hydrogenation of bulky imines catalyzed by the Lewis acid B(C6F5)3 is investigated in detail by quantum chemical calculations. A recently introduced mechanistic model of heterolytic hydrogen splitting that is based on noncovalent association of bulky Lewis acid-base pairs is shown to account for the reactivity of imine-borane as well as amine-borane systems. Possible catalytic cycles are examined, and the results provide solid support for the imine reduction pathway proposed from experimental observations.

Concerted attack of frustrated Lewis acid-base pairs on olefinic double bonds: a theoretical study.

A computational approach reveals cooperative action of the preorganized acidic and basic centers of the frustrated P(t-Bu)(3)/B(C(6)F(5))(3) Lewis pair on olefinic bonds as the key to the observed regioselective addition reaction.

Computing reliable energetics for conjugate addition reactions.

The performance of various density functionals along with second-order perturbation treatments has been tested for a set of conjugate addition reactions relevant to stereoselective organocatalysis. It is shown that B3LYP predictions seriously underestimate the reaction energies, whereas two newly designed functionals (M05-2X and M06-2X) and the SCS-MP2 method provide very accurate data. These new methods represent promising alternative approaches in future mechanistic studies.

Theoretical studies on the bifunctionality of chiral thiourea-based organocatalysts: competing routes to C-C bond formation.

The mechanism of enantioselective Michael addition of acetylacetone to a nitroolefin catalyzed by a thiourea-based chiral bifunctional organocatalyst is investigated using density functional theory calculations. A systematic conformational analysis is presented for the catalyst, and it is shown that both substrates coordinate preferentially via bidentate hydrogen bonds. The deprotonation of the enol form of acetylacetone by the amine of the catalyst is found to occur easily, leading to an ion pair characterized by multiple H-bonds involving the thiourea unit as well.