Organocatalytic Hydrogen Evolution Reaction by Diazaphospholenes.

The electrochemical hydrogen evolution reaction (HER) is currently recognized as a prospective way to obtain clean energy. The electrocatalysts used currently are dominantly based on transition metals. In this work, we have demonstrated a diazaphospholene (N-heterocyclic phosphine (NHP))-type small molecular organocatalyst that can catalyze the HER with a maximum current density of 130 mA·cm-2, an overpotential of 354 mV, and a faradaic efficiency of 90%. Mechanistic studies verify a Heyrovsky-type process with NHP, whereas its hydricity and aromaticity favor hydrogen release and catalyst regeneration.

Hydroxylamine-Mediated C(sp2)-H Trifluoromethylation of Terminal Alkenes.

Introduction of the trifluoromethyl (CF3) group into organic compounds has garnered substantial interest because of its significant role in pharmaceuticals and agrochemicals. Here, we report a hydroxylamine-mediated radical process for C(sp2)-H trifluoromethylation of terminal alkenes. The reaction shows good reactivity, impressive E/Z selectivity (up to >20 : 1), and broad functional group compatibility. Expansion of this approach to perfluoroalkylation and late-stage trifluoromethylation of bioactive molecules demonstrates its promising application potential. Mechanistic studies suggest that the reaction follows a radical addition and subsequent elimination pathway.

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications.

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

Deoxygenation of Phosphine Oxides by PIII/PV═O Redox Catalysis via Successive Isodesmic Reactions.

Deoxygenation of phosphine oxides is of great significance to synthesis of phosphorus ligands and relevant catalysts, as well as to the sustainability of phosphorus chemistry. However, the thermodynamic inertness of P═O bonds poses a severe challenge to their reduction. Previous approaches in this regard rely primarily on a type of P═O bond activation with either Lewis/Brønsted acids or stoichiometric halogenating reagents under harsh conditions. Here, we wish to report a novel catalytic strategy for facile and efficient deoxygenation of phosphine oxides via successive isodesmic reactions, whose thermodynamic driving force for breaking the strong P═O bond was compensated by a synchronous formation of another P═O bond. The reaction was enabled by PIII/P═O redox sequences with the cyclic organophosphorus catalyst and terminal reductant PhSiH3. This catalytic reaction avoids the use of the stoichiometric activator as in other cases and features a broad substrate scope, excellent reactivities, and mild reaction conditions. Preliminary thermodynamic and mechanistic investigations disclosed a dual synergistic role of the catalyst.

Revisiting the Electrochemistry of TEMPOH Analogues in Acetonitrile.

Hydroxylamines, represented by 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH), are widely involved as active species in various chemical and electrochemical oxidations. The electrochemical behavior of TEMPOH is crucial to understanding the mechanisms of TEMPO-mediated redox sequences. However, compared to abundant studies on TEMPOH electrochemistry in aqueous solutions, the sole value of its oxidation potential Eox(TEMPOH) in organic solutions was reported to be 0.7 V (vs Fc in acetonitrile), seemingly conflicting with experimentally observed facile oxidation of TEMPOH. Herein, the electrochemistry of TEMPOH derivatives in acetonitrile was revisited, featuring much smaller oxidation potentials (about 0 V) than literature ones. Acid/base effects and kinetic studies lent credibility to these new values. Such a 0.7 V energy discrepancy impelled us to review the thermodynamic properties and oxidation mechanisms of TEMPOH deduced from the old value.

Catalytic Vilsmeier-Haack Reactions for C1-Deuterated Formylation of Indoles.

The Vilsmeier-Haack reaction is a powerful tool to introduce formyl groups into electron-rich arenes, but its wide application is significantly restricted by stoichiometric employment of caustic POCl3. Herein, we reported a catalytic version of the Vilsmeier-Haack reaction enabled by a P(III)/P(V)═O cycle. This catalytic reaction provides a facile and efficient route for the direct construction of C1-deuterated indol-3-carboxaldehyde under mild conditions with stoichiometric DMF-d7 as the deuterium source. The products feature a remarkably higher deuteration level (>99%) than previously reported ones and are not contaminated by the likely unselective deuteration at other sites. The present transformation can also be used to transfer other carbonyl groups. Further downstream derivatizations of these deuterated products manifested their potential applications in the synthesis of deuterated bioactive molecules. Mechanistic insight was disclosed from studies of kinetics and intermediates.

Synthetic applications of NHPs: from the hydride pathway to a radical mechanism.

The unique heterocyclic skeletons of N-heterocyclic phosphines (NHPs) endow them with excellent hydridic reactivity, which has enabled NHPs to be applied in a great array of catalytic hydrogenations of unsaturated substrates in the past few decades. Recently, applications of NHPs in radical reductions, especially in a catalytic fashion, have emerged as a promising forefront area. This new reaction pattern, distinctive from but complementary to the well-established hydride pathway, can greatly expand the reaction scope to σ-bond scission. Herein, we briefly summarized some representative examples of synthetic applications of NHPs in both hydridic and radical reductions with an emphasis on their radical reactivity, including the structural and property studies of NHP radicals and their precursors as well as their applications in radical processes.

Diazaphospholene-Catalyzed Hydrodefluorination of Polyfluoroarenes with Phenylsilane via Concerted Nucleophilic Aromatic Substitution.

The metal-free catalytic C-F bond activation of polyfluoroarenes was achieved with diazaphospholene as the catalyst and phenylsilane as the terminal reductant. Density functional theory calculations suggested a concerted nucleophilic aromatic substitution mechanism.

Recent progress in reactivity study and synthetic application of N-heterocyclic phosphorus hydrides.

N-heterocyclic phosphines (NHPs) have recently emerged as a new group of promising catalysts for metal-free reductions, owing to their unique hydridic reactivity. The excellent hydricity of NHPs, which rivals or even exceeds those of many metal-based hydrides, is the result of hyperconjugative interactions between the lone-pair electrons on N atoms and the adjacent σ*(P-H) orbital. Compared with the conventional protic reactivity of phosphines, this umpolung P-H reactivity leads to hydridic selectivity in NHP-mediated reductions. This reactivity has therefore found many applications in the catalytic reduction of polar unsaturated bonds and in the hydroboration of pyridines. This review summarizes recent progress in studies of the reactivity and synthetic applications of these phosphorus-based hydrides, with the aim of providing practical information to enable exploitation of their synthetically useful chemistry.

Chemoselective catalytic hydrodefluorination of trifluoromethylalkenes towards mono-/gem-di-fluoroalkenes under metal-free conditions.

Fluorine-containing moieties show significant effects in improving the properties of functional molecules. Consequently, efficient methods for installing them into target compounds are in great demand, especially those enabled by metal-free catalysis. Here we show a diazaphospholene-catalyzed hydrodefluorination of trifluoromethylalkenes to chemoselectively construct gem-difluoroalkenes and terminal monofluoroalkenes by simple adjustment of the reactant stoichiometry. This metal-free hydrodefluorination features mild reaction conditions, good group compatibility, and almost quantitative yields for both product types. Stoichiometric experiments indicated a stepwise mechanism: hydridic addition to fluoroalkenes and subsequent ß-F elimination from hydrophosphination intermediates. Density functional theory calculations disclosed the origin of chemoselectivity, regioselectivity and stereoselectivity, suggesting an electron-donating effect of the alkene-terminal fluorine atom.

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation.

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔGhet (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔGhet (Pd-X) scales span a range of 2.6-9.0 kcal mol-1 for ΔGhet (Pd-O) values and of 14.5-19.5 kcal mol-1 for ΔGhet (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔGhet (Pd-X) and pKa (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Holistic Prediction of the pKa in Diverse Solvents Based on a Machine-Learning Approach.

While many approaches to predict aqueous pKa values exist, the fast and accurate prediction of non-aqueous pKa values is still challenging. Based on the iBonD experimental pKa database (39 solvents), a holistic pKa prediction model was established using machine learning. Structural and physical-organic-parameter-based descriptors (SPOC) were introduced to represent the electronic and structural features of the molecules. The models trained with a neural network or the XGBoost algorithm showed the best prediction performance with a low MAE value of 0.87 pKa units. The approach allows a comprehensive mapping of all possible pKa correlations between different solvents and it was validated by predicting the aqueous pKa and micro-pKa of pharmaceutical molecules and pKa values of organocatalysts in DMSO and MeCN with high accuracy. An online prediction platform was constructed based on the current model, which can provide pKa prediction for different types of X-H acidity in the most commonly used solvents.

Predicting Absolute Rate Constants for Huisgen Reactions of Unsaturated Iminium Ions with Diazoalkanes.

The kinetics and stereochemistry of the reactions of iminium ions derived from cinnamaldehydes and MacMillan's imidazolidinones with diphenyldiazomethane and aryldiazomethanes were investigated experimentally and with DFT calculations. The reactions of diphenyldiazomethane with iminium ions derived from MacMillan's second-generation catalysts gave 3-aryl-2,2-diphenylcyclopropanecarbaldehydes with yields >90 % and enantiomeric ratios of ≥90:10. Predominantly 2:1 products were obtained from the corresponding reactions with monoaryldiazomethanes. The measured rate constants are in good agreement with the rate constants derived from the one-center nucleophilicity parameters N and sN of diazomethanes and the one-center electrophilicity parameters E of iminium ions as well as with quantum chemically calculated activation energies.

Toward Rational Understandings of α-C-H Functionalization: Energetic Studies of Representative Tertiary Amines.

Functionalization of α-C-H bonds of tertiary amines to build various α-C-X bonds has become a mainstream in synthetic chemistry nowadays. However, due to lack of fundamental knowledge on α-C-H bond strength as an energetic guideline, rational exploration of new synthetic methodologies remains a far-reaching anticipation. Herein, we report a unique hydricity-based approach to establish the first integrated energetic scale covering both the homolytic and heterolytic energies of α-C-H bonds for 45 representative tertiary amines and their radical cations. As showcased from the studies on tetrahydroisoquinolines (THIQs) by virtue of their thermodynamic criteria, the feasibility and mechanisms of THIQ oxidation were deduced, which, indeed, were found to correspond well with experimental observations. This integrated scale provides a good example to relate bond energetics with mechanisms and thermodynamic reactivity of amine α-C-H functionalization and hence, may be referenced for analyzing similar structure-property problems for various substrates.

Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: thermodynamics, kinetics, and synthetic applications.

Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (E red) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, being -1.94 and -2.39 V (vs. Fc+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P-H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.

Counterintuitive solvation effect of ionic-liquid/DMSO solvents on acidic C-H dissociation and insight into respective solvation.

How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL (the "apparent" or "measured" dielectric constant ε of the IL is around 10-15) into a strongly polar molecular solvent (e.g., ε of DMSO: 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard. Toward this, we, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic-molecular binary systems on self-dissociation of C-H acid phenylmalononitrile PhCH(CN)2 via pK a determination. As disclosed, in this category of binary media, (1) no linear correspondence exists between pK a and molar fractions of the respective solvent components; (2) only ∼1-2 mol% of weakly polar ILs in strongly polar DMSO make C-H bonds even more dissociative than in neat DMSO; (3) a small fraction of DMSO in ILs (<10 mol%) can dramatically ease acidic C-H-dissociation; and (4) while the DMSO fraction further increases, its acidifying effect becomes much attenuated. These findings, though maybe counterintuitive, have been rationalized on the basis of the precise pK a measurement of this work in relation to the respective roles of each solvent component in solvation.

Exploiting the radical reactivity of diazaphosphinanes in hydrodehalogenations and cascade cyclizations.

The remarkable reducibility of diazaphosphinanes has been extensively applied in various hydrogenations, based on and yet limited by their well-known hydridic reactivity. Here we exploited their unprecedented radical reactivity to implement hydrodehalogenations and cascade cyclizations originally inaccessible by hydride transfer. These reactions feature a broad substrate scope, high efficiency and simplicity of manipulation. Mechanistic studies suggested a radical chain process in which a phosphinyl radical is generated in a catalytic cycle via hydrogen-atom transfer from diazaphosphinanes. The radical reactivity of diazaphosphinanes disclosed here differs from their well-established hydridic reactivity, and hence, opens a new avenue for diazaphosphinane applications in organic syntheses.

Diazaphosphinyl radical-catalyzed deoxygenation of α-carboxy ketones: a new protocol for chemo-selective C-O bond scission via mechanism regulation.

C-O bond cleavage is often a key process in defunctionalization of organic compounds as well as in degradation of natural polymers. However, it seldom occurs regioselectively for different types of C-O bonds under metal-free mild conditions. Here we report a facile chemo-selective cleavage of the α-C-O bonds in α-carboxy ketones by commercially available pinacolborane under the catalysis of diazaphosphinane based on a mechanism switch strategy. This new reaction features high efficiency, low cost and good group-tolerance, and is also amenable to catalytic deprotection of desyl-protected carboxylic acids and amino acids. Mechanistic studies indicated an electron-transfer-initiated radical process, underlining two crucial steps: (1) the initiator azodiisobutyronitrile switches originally hydridic reduction to kinetically more accessible electron reduction; and (2) the catalytic phosphorus species upconverts weakly reducing pinacolborane into strongly reducing diazaphosphinane.

Understanding the role of thermodynamics in catalytic imine reductions.

The past decade has witnessed a booming growth of research activity in catalytic imine reduction, due to the ongoing motivation to find more efficient conversions of the rather unreactive C[double bond, length as m-dash]N bonds to the desired C-N segments found in many pharmaceuticals. While several timely reviews have well documented various C[double bond, length as m-dash]N reduction methodologies with respect to the type of catalyst (acid, base, or transition metal), a detailed discussion of the core role of thermodynamic driving forces in governing these catalyses is still lacking, however. Hence, this tutorial review describes some of the most practical considerations for adjusting reduction thermodynamics by choosing appropriate catalytic strategies, in order to make the target reduction energetically feasible. The combined use of relevant thermodynamic parameters of the substrate imines, hydrogen donors, and catalysts in realizing such a goal is demonstrated on the basis of the energetics of the possible elementary paths. Experimental observations from the literature that are in line with the present energetics-based analyses are exemplified.

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications.

Nucleophilicity parameters (N, sN ) of a group of representative diazaphospholenium hydrides were derived by kinetic investigations of their hydride transfer to a series of reference electrophiles with known electrophilicity (E) values, using the Mayr equation log k2 =sN (N+E). The N scale covers over ten N units, ranging from the most reactive hydride donor (N=25.5) to the least of the scale (N=13.5). This discloses the highest N value ever quantified in terms of Mayr's nucleophilicity scales reported for neutral transition-metal-free hydride donors and implies an exceptional reactivity of this reagent. Even the least reactive hydride donor of this series is still a better hydride donor than those of many other nucleophiles such as the C-H, B-H, Si-H and transition-metal M-H hydride donors. Structure-reactivity analysis reveals that the outstanding hydricity of 2-H-1,3,2-diazaphospholene benefits from the unsaturated skeleton.