Study on comparison of reducing ability of three organic hydride compounds

Thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles in acetonitrile

N-heterocycles are important chemical hydrogen-storage materials, and the acceptorless dehydrogenation and hydrogenation of N-heterocycles as organic hydrogen carriers have been widely studied, with the main focus on the catalyst synthesis and design, investigation of the redox mechanisms, and extension of substrate scope. In this work, the Gibbs free energies of the dehydrogenation of pre-aromatic N-heterocycles (YH2) and the hydrogenation of aromatic N-heterocycles (Y), i.e., ΔGH2R(YH2) and ΔGH2A(Y), were derived by constructing thermodynamic cycles using Hess' law. The thermodynamic abilities for the acceptorless dehydrogenation and hydrogenation of 78 pre-aromatic N-heterocycles (YH2) and related 78 aromatic N-heterocycles (Y) were well evaluated and discussed in acetonitrile. Moreover, the applications of the two thermodynamic parameters in identifying pre-aromatic N-heterocycles possessing reversible dehydrogenation and hydrogenation properties and the selection of the pre-aromatic N-heterocyclic hydrogen reductants in catalytic hydrogenation were considered and are discussed in detail. Undoubtedly, this work focuses on two new thermodynamic parameters of pre-aromatic and aromatic N-heterocycles, namely ΔGH2R(YH2) and ΔGH2A(Y), which are important supplements to our previous work to offer precise insights into the chemical hydrogen storage and hydrogenation reactions of pre-aromatic and aromatic N-heterocycles.

Thermodynamic Evaluations of Amines as Hydrides or Two Hydrogen Ions Reductants and Imines as Protons or Two Hydrogen Ions Acceptors, as Well as Their Application in Hydrogenation Reactions

Since the hydrogenation of imines (X) and the dehydrogenation of amines (XH2) generally involve the two hydrogen ions (H- + H+) transfer, the thermodynamic abilities of various amines releasing hydrides or two hydrogen ions as well as various imines accepting protons or two hydrogen ions are important and characteristic physical parameters. In this work, the pKa values of 84 protonated imines (XH+) in acetonitrile were predicted. Combining Gibbs free energy changes of amines releasing hydrides in acetonitrile from our previous work with the pKa(XH+) values, the Gibbs free energy changes of amines releasing two hydrogen ions and imines accepting two hydrogen ions were derived using Hess's law by constructing thermochemical cycles, and the thermodynamic evaluations of amines as hydrides or two hydrogen ions reductants and imines as protons or two hydrogen ions acceptors are well compared and discussed. Eventually, the practical application of thermodynamic data for amines and imines on hydrogenation feasibility, mechanism, and possible elementary steps was shown and discussed in this paper from the point of thermodynamics.

Discovering and Evaluating the Reducing Abilities of Polar Alkanes and Related Family Members as Organic Reductants Using Thermodynamics

In this work, the pK_a values of 69 polar alkanes (YH₂) in acetonitrile were computed using the method developed by Luo and Zhang in 2020, and representative 69 thermodynamic network cards on 22 elementary steps of YH₂ and related polar alkenes (Y) releasing or accepting H₂ were naturally established. Potential electron reductants (YH^-), hydride reductants (YH^-), antioxidants (YH₂ and YH^-), and hydrogen molecule reductants (YH₂) are unexpectedly discovered according to thermodynamic network cards. It is also found that there are great differences between YH₂ and common hydrogen molecule reductants (XH₂), such as Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydro-phenanthridine (PH₂), releasing two hydrogen ions to unsaturated compounds. During the hydrogenation process, XH₂ release hydrides first, then the oxidation state XH⁺ release protons. However, in the case of YH₂, YH₂ release protons first, then YH^- release hydrides. It is the differences on acidic properties of YH₂ and XH₂ that result in the behavioral and thermodynamic differences on YH₂ and XH₂ releasing two hydrogen ions (H^--H⁺). The redox mechanisms and behaviors of Y, YH^-, and YH₂ as electron, hydrogen atom, hydride, and hydrogen molecule donors or acceptors in the chemical reaction are reasonably investigated and discussed in this paper using thermodynamics.

Thermodynamics of the elementary steps of organic hydride chemistry determined in acetonitrile and their applications

This review focuses on the thermodynamics of the elementary step of 421 organic hydrides and unsaturated compounds releasing or accepting hydride or hydrogen determined in acetonitrile as well as their potential applications.

Competitive Desulfonylative Reduction and Oxidation of α-Sulfonylketones Promoted by Photoinduced Electron Transfer with 2-Hydroxyaryl-1,3-dimethylbenzimidazolines under Air

Desulfonylation reactions of α-sulfonylketones promoted by photoinduced electron transfer with 2-hydroxyarylbenzimidazolines (BIH-ArOH) were investigated. Under aerobic conditions, photoexcited 2-hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylketones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examination of the effects of solvents, radical trapping reagents, substituents of sulfonylketones, and a variety of hydroxyaryl- and aryl-benzimidazolines (BIH-ArOH and BIH-Ar) suggest that the oxidation products are produced by dissociation of α-ketoalkyl radicals from the initially formed solvent-caged radical ion pairs followed by reaction with molecular oxygen. In addition, the observations indicate that the reduction products are generated by proton or hydrogen atom transfer in solvent-caged radical ion pairs derived from benzimidazolines and sulfonylketones. The results also suggest that arylsulfinate anions arising by carbon-sulfur bond cleavage of sulfonylketone radical anions act as reductants in the oxidation pathway to convert initially formed α-hydroperoxyketones to α-hydroxyketones. Finally, density functional theory calculations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.

Sterically Regulated α-Oxygenation of α-Bromocarbonyl Compounds Promoted Using 2-Aryl-1,3-dimethylbenzimidazolines and Air

A debrominative oxygenation protocol has been developed for the conversion of α-bromo-α,α-dialkyl-substituted carbonyl compounds to their corresponding α-hydroxy analogues. For example, stirring a solution of α-bromoisobutyrophenone and 2-aryl-1,3-dimethylbenzimidazoline (BIH-Ar) at room temperature under an air atmosphere leads to the efficient formation of α-hydroperoxyisobutyrophenone, which can be converted to α-hydroxyisobutyrophenone using Me₂S reduction. In contrast, reaction of α-bromoacetophenone under the same conditions produces the α-hydrogenated product acetophenone. α-Keto-alkyl and benzimidazolyl radicals (BI^•-Ar), generated via dissociative electron transfer from BIH-Ar to α-bromoketone substrates, serve as key intermediates in the oxidation and reduction processes. The dramatic switch from hydrogenation to oxygenation is attributed to a steric effect of α-alkyl substituents, which causes hydrogen atom abstraction from sterically crowded BIH-Ar to α-keto-alkyl radicals to be slow and enable preferential reaction with molecular oxygen. Generation of the α-keto-alkyl radical and BI^•-Ar intermediates in these process and their sterically governed hydrogen atom transfer reactions are supported by results arising from DFT calculations. Moreover, an electron spin resonance study showed that visible light irradiation of phenyl benzimidazoline (BIH-Ph) in the presence of molecular oxygen produces the benzimidazolyl radical (BI^•-Ph). The addition of thiophenol into the reaction of α-bromoisobutyrophenone and BIH-Ph predominantly produced α-phenylthiolated isobutyrophenone even if a high concentration of molecular oxygen exists. Furthermore, the developed protocol was applied to other α-bromo-α,α-dialkylated carbonyl compounds.

The α-alkylation of ketones with alcohols in pure water catalyzed by a water-soluble Cp*Ir complex bearing a functional ligand

A water-soluble dinuclear Cp*Ir complex bearing 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine as a bridging ligand was found to be a highly effective catalyst for the α-alkylation of ketones with alcohols in water.

Benzimidazolium Naphthoxide Betaine Is a Visible Light Promoted Organic Photoredox Catalyst

Benzimidazolium naphthoxide (^-ONap-BI⁺) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of ^-ONap-BI⁺ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of ^-ONap-BI⁺ was proposed.

Metal-free, one-pot, sequential protocol for transforming α,β-epoxy ketones to β-hydroxy ketones and α-methylene ketones

A new sequential, one-pot protocol for transforming 1,3-disubstituted 2,3-epoxy ketones to β-hydroxy ketones and α-methylene ketones has been developed. Reaction of epoxy ketones with boron trifluoride etherate (BF3·OEt2) generates the cationic intermediates by regioselective epoxide ring opening and an acyl shift. Then, a treatment of these cations with 2-aryl-1,3-dimethylbenzimidazolines (DMBIH) results in formation of 1,2-disubstituted 3-hydroxy ketones. DMBIH serves as a hydride donor in the second step of this process. Finally, the β-hydroxy ketones can be converted to 1,2-disubstituted 2-methylene ketones by treatment with methanesulfonic acid or a combination of methanesulfonyl chloride and triethylamine. Importantly, the sequential steps involved in formation of the α-methylene ketone products can be carried out in one pot.

Quinine catalysed asymmetric Michael additions in a sustainable solvent

Diethyl carbonate is shown to be a suitable, sustainable solvent in which to carry out quinine catalysed asymmetric Michael additions of malononitriles to enones.