Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42− as a Proton Acceptor at pH 8

The complex Cu^II(Py₃P) ( 1 ) is an electrocatalyst for water oxidation to dioxygen in H₂PO₄^?/HPO₄^2? buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single‐site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom–proton transfer in a rate‐limiting O? O bond‐forming step with HPO₄^2? as the acceptor base or by concerted electron–proton transfer with electron transfer to the electrode and proton transfer to the HPO₄^2? base.     

1H NMR studies of intramolecular hydrogen bonding in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of seven N-(pyridyl)amides of 6-methylpicolinic acid N-oxide in chloroform were obtained. The influence on the chemical shifts of the N? H protons of temperature, concentration and the CH₃ substituent in the pyridine ring was studied. The N? H protons were found to be shifted to low fields (～14 ppm) owing to the formation of strong intramolecular hydrogen bonding. The influence of the pyridine ring on the chemical shift of the N? H proton is comparable with the inductive effect of the p-nitrophenyl group. The hindered rotation around the N-pyridyl bond of N-(α-pyridyl)amides of 6-methylpicolinic acid in solution is discussed.     

NH3‐Driven Benzene C−H Activation with O2 that Opens a New Way for Selective Phenol Synthesis

Catalytic benzene C?H activation toward selective phenol synthesis with O₂ remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three‐steps cumene process in liquid phase, which is energy‐intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas‐phase single‐path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O₂ under coexisting NH₃ by the new inter‐ligand concerted mechanism. Among alkali and alkaline‐earth metal ions and transition and precious metal ions, single Cs⁺ and Rb⁺ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas‐phase single‐path reaction process. The single Cs⁺ and Rb⁺ sites that possess neither significant Lewis acidic?basic property nor redox property, cannot activate benzene, O₂, and NH₃, respectively, whereas when they coadsorbed together, the reaction of the inter‐coadsorbates on the single alkali‐metal ion site proceeds concertedly (the inter‐ligand concerted mechanism), bringing about the benzene C?H activation toward phenol synthesis. The NH₃‐driven benzene C?H activation with O₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH₃ on Pt₆ metallic cluster/β and Ni₄O₄ oxide cluster/β. The NH₃‐driven selective oxidation mechanism observed with the Cs⁺/β and Rb⁺/β differs from the traditional redox catalysis (Mars‐van Krevelen) mechanism, simple Langmuir‐Hinshelwood mechanism, and acid?base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.     

Supported palladium catalysis using a heteroleptic 2-methylthiomethylpyridine-N,S-donor motif for Mizoroki-Heck and Suzuki-Miyaura coupling, including continuous organic monolith in capillary microscale flow-through mode

Flow-through catalysis utilising (2-methylthiomethylpyridine)palladium(II) chloride species covalently attached to a macroporous continuous organic polymer monolith synthesised within fused silica capillaries of internal diameter 250 μm is described, together with related studies of ground bulk monolith compared with supported catalysis on Merrifield and Wang beads and homogeneous catalysis under identical conditions to bulk supported catalysis. The monolith substrate, poly(chloromethylstyrene-co-divinylbenzene), has a backbone directly related to Merrifield and Wang resins. The homogeneous precatalyst PdCl₂(L²) (L²=4-(4-benzyloxyphenyl)-2-methylthiomethylpyridine) contains the benzyloxyphenyl group on its periphery as a model for the spacer between the ‘PdCl₂(N∼S)’ centre and the polymer substituent of the resins and monolith. Suzuki-Miyaura and Mizoroki-Heck catalysis exhibit anticipated trends in reactivity with variation of aryl halide reagents for each system, and show that supported catalysis on beads and monolith gives higher yields than for homogeneous catalysis. The synthesis of 2-methylthiomethylpyridines is presented, together with crystal structures of 4-bromo-2-bromomethylpyridine hydrobromide, 4-(4-hydroxyphenyl)-2-methylthiomethylpyridine (L¹), PdCl₂(L¹) and PdCl₂(L²). Hydrogen bonding occurs in 4-bromo-2-bromomethylpyridine hydrobromide as N-H?Br interactions, in 4-(4-hydroxyphenyl)-2-methylthiomethylpyridine as O-H?N to form chains, and in PdCl₂(L¹) as O-H?Cl interactions leading to adjacent π-stacked chains oriented in an antiparallel fashion.     

Early Main Group Metal Catalysis: How Important is the Metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph₂N^?][Me₄N⁺] and [Ph₃C^?][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (C?O and C?N) in the hydroamination of Ar? N?C?O and R? N?C?N? R (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated C?C bonds in H₂C?CHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.     

POCl3 promoted metal-free synthesis of tertiary amides by coupling of carboxylic acids and N,N-disubstituted formamides

Herein we report a robust and synthetically useful catalyst-free amination methodology by the coupling of carboxylic acids and N-substituted formamides using POCl₃ as a promoter. Versatile amides with a wide array of substituent groups were prepared within only 1?h in good to excellent yields. And even multi-substituted aromatic carboxylic acids could give the desired products with satisfactory results.     

The Mechanism of the Acid-catalysed Hydrolysis of 1-Aryl-2,2,2-trifluorodiazoethanes

The rates of the acid-catalysed hydrolysis of a series of 1-aryl-2,2,2-trifluorodiazoethanes la-d have been measured in dioxan/water/HClO₄. The results are well correlated with the Hammett equation when σ_p substituent constants are used (?_H = ?1.74 and ?_D = ?1.75). Kinetic solvent isotope effects, about 2.1, and general acid catalysis indicate that proton transfer is rate-determining (A-S_E2 mechanism). Rate measurements have also been made at pressures up to 1400 atm. The evaluated activation volumes, about ?20 cm³/mole, indicate that at least one water molecule is bound in the transition state of protonation. Rate measurements at low water concentrations indicate that no apparent change in mechanism has occured.     

Mechanistic studies on the hydrolysis of a dimeric alkylzinc bis(2-pyridylmethyl)amide

Bis(2-pyridylmethyl)amine 7 reacts with selected dialkylzinc compounds to give dimeric alkylzinc bis(2-pyridylmethyl)amides 8. Regardless of the steric bulk of the alkyl substituent, dimers with central Zn₂N₂ rings are formed. Compounds 8 undergo spontaneous hydrolysis reactions upon exposure to air/moisture which can be partially controlled if the alkyl substituent R is bulky enough [R = CH(SiMe₃)₂]. A dimeric compound 9 containing both zinc-alkyl substituents and a μ-OH functionality results. In the course of this reaction, an amide instead of the expected RH is eliminated. Extensive DFT calculations show that the facile formation of a three-centered Zn[μ-(HO?H?NHR)]Zn functionality is a crucial step. Further evidence for the importance of Zn[μ-(X?H?Y)]Zn intermediates (X, Y = O and now N) in the general mechanism of hydrolysis catalyzed by binuclear zinc compounds is thus provided.     

Heterogeneous Water Oxidation: Surface Activity versus Amorphization Activation in Cobalt Phosphate Catalysts

Is water oxidation catalyzed at the surface or within the bulk volume of solid oxide materials? This question is addressed for cobalt phosphate catalysts deposited on inert electrodes, namely crystallites of pakhomovskyite (Co₃(PO₄)₂?8 H₂O, Pak) and phosphate‐containing Co oxide (CoCat). X‐ray spectroscopy reveals that oxidizing potentials transform the crystalline Pak slowly (5–8 h) but completely into the amorphous CoCat. Electrochemical analysis supports high‐TOF surface activity in Pak, whereas its amorphization results in dominating volume activity of the thereby formed CoCat material. In the directly electrodeposited CoCat, volume catalysis prevails, but not at very low levels of the amorphous material, implying high‐TOF catalysis at surface sites. A complete picture of heterogeneous water oxidation requires insight in catalysis at the electrolyte‐exposed “outer surface”, within a hydrated, amorphous volume phase, and modes and kinetics of restructuring upon operation.     

Novel Ortho effects detected in the fast atom bombardment mass spectra of substituted bisaryl phosphate antagonists of platelet-activating factor

A series of substituted bisaryl phosphate compounds, (R¹CH₂)⁺ ArOP = O(O^?)(OArR²R₃), was analyzed and characterized by fast atom bombardment mass spectrometry. Abundant fragment ions were observed and correlated with the proposed structures. From fragmentation pattersn, ‘ortho effect’ reactions were demonstrated to have occurred when the phosphoryl oxygen reacted with the (CH₂R¹)⁺ and C?O(OCH₃) substituents in the ortho position, relative to the phosphate group, and displaced the R¹ and OCH₃ groups, respectively, to produce phosphorus containing six-membered rings fused to the aryl moiety. When the (CH₂R¹)⁺ substituents were in the meta position relative to the phosphate group, the ‘ortho effect’ reactions were not observed. However, when the C?O(OCH₃) substituent was in the meta position relative to the phosphate group, an abundant fragment ion containing a five-membered phosphate ring fused to the aryl ring was detected with the original phosphoryl oxygen ortho to both the phosphate oxygen and a formyl group, formed from the original C?O(OCH₃) substituent. All other fragmentations not involving the ‘ortho effect’ reactions were nearly identical for the different structural isomers of the substituted bisaryl phosphate compounds.     

1H NMR spectra. Part 30: 1H chemical shifts in amides and the magnetic anisotropy,electric field and steric effects of the amide group

The ¹H spectra of 37 amides in CDCl₃ solvent were analysed and the chemical shifts obtained. The molecular geometries and conformational analysis of these amides were considered in detail. The NMR spectral assignments are of interest, e.g. the assignments of the formamide NH₂ protons reverse in going from CDCl₃ to more polar solvents. The substituent chemical shifts of the amide group in both aliphatic and aromatic amides were analysed using an approach based on neural network data for near (≤3 bonds removed) protons and the electric field, magnetic anisotropy, steric and for aromatic systems π effects of the amide group for more distant protons. The electric field is calculated from the partial atomic charges on the N.C═O atoms of the amide group. The magnetic anisotropy of the carbonyl group was reproduced with the asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic amides 10.53 and ?23.67 (×10^?6 ų/molecule) and for the aromatic amides 2.12 and ?10.43 (×10^?6 ų/molecule). The nitrogen anisotropy was 7.62 (×10^?6 ų/molecule). These values are compared with previous literature values. The ¹H chemical shifts were calculated from the semi‐empirical approach and also by gauge‐independent atomic orbital calculations with the density functional theory method and B3LYP/6–31G⁺⁺ (d,p) basis set. The semi‐empirical approach gave good agreement with root mean square error of 0.081 ppm for the data set of 280 entries. The gauge‐independent atomic orbital approach was generally acceptable, but significant errors (ca. 1 ppm) were found for the NH and CHO protons and also for some other protons. Copyright © 2013 John Wiley & Sons, Ltd.     

Photoredox Catalytic α‐Alkoxypentafluorosulfanylation of α‐Methyl‐ and α‐Phenylstyrene Using SF6

SF₆ was applied as pentafluorosulfanylation reagent to prepare ethers with a vicinal SF₅ substituent through a one‐step method involving photoredox catalysis. This method shows a broad substrate scope with respect to applicable alcohols for the conversion of α‐methyl and α‐phenyl styrenes. The products bear a new structural motif with two functional groups installed in one step. The alkoxy group allows elimination and azidation as further transformations into valuable pentafluorosulfanylated compounds. These results confirm that non‐toxic SF₆ is a useful SF₅ transfer reagent if properly activated by photoredox catalysis, and toxic reagents are completely avoided. In combination with light as an energy source, a high level of sustainability is achieved. Through this method, the proposed potential of the SF₅ substituent in medicinal chemistry, agrochemistry, and materials chemistry may be exploited in the future.     

The Role of Pyridoxal Phosphate in the Catalysis of Glycogen Phosphorylases

Glycogen phosphorylases catalyze the degradation of glycogen by phosphate (or arsenate) to glucose 1-phosphate (or glucose + arsenate). All glycogen phosphorylases that have been studied so far contain pyridoxal 5′-phosphate, a vitamin B₆-derivative, as cofactor. Removal of the cofactor results in an inactive apoenzyme. However, reduction of the azomethine bond linking pyridoxal phosphate to an ?-aminolysyl side chain of the enzyme with NaBH₄ does not inactivate glycogen phosphorylase. If therefore the cofactor should be involved in catalysis in glycogen phosphorylase it must function differently from all other classical pyridoxal phosphate dependent enzymes, for these are inactivated by reduction. ³¹P-NMR spectroscopy has revealed that the 5′-phosphate group of pyridoxal phosphate is present in catalytically active forms of glycogen phosphorylases as dianion in a hydrophobic environment shielded from aqueous solvent. Covalent and/or allosteric activation of muscle glycogen phosphorylases is accompanied by a transition of the monoprotonated form to the dianionic form of the phosphate group of the cofactor. We now report on such ionization changes in unregulated active potato- and E. coli maltodextrin phosphorylases on binding of glucose and oligosaccharides and following catalytic turnover, i.e. arsenolysis of α-1,4-glycosidic bonds. (Like glycogen phosphorylases, maltodextrin phosphorylases belong to the class of α-glucan phosphorylases.) The results of experiments carried out by our group together with recent findings on the three dimensional structure of crystalline muscle glycogen phosphorylases indicate a participation of the dianionic phosphate group as proton acceptor for the glucosyl transfer to and from the glucosyl acceptor. Although other interpretations are not excluded, at present little doubt remains that in the case of glycogen phosphorylases the dianionic phosphate group of the cofactor functions in catalysis.     

Ortho effects in organic molecules on electron impact. 14—Concerted and stepwise ejections of SO2 and N2 from N-arylidene 2-nitrobenzenesulphenamides

Unexpected ortho interaction of the nitro group has been noticed during the mass spectral fragmentations of N-arylidene 2-nitrobenzenesulphenamides, where the molecular ions expel SO₂ and N₂ both in concerted and stepwise processes. Loss of a hydrogen or the substituent from this fragment leads to a very abundant ion in all the compounds studied. Based on chemical evidence and linked-scan studies, a 1,2-phenylenetropylium cation structure has been postulated for the [M–SO₂–N₂–H/substituent]⁺ ion.     

Reactions of arylcarbamoyl chlorides—II : Reaction with aromatic amines

The reaction of N-arylcarbamoyl chlorides with anilines in acetonitrile initially gives isocyanates, which can react with primary or secondary amines to give N,N-diaryl ureas. The rate of HCl elimination fits the Bronsted catalysis law with β ≈ 1, suggesting that there is extensive proton transfer in the transition state. Plots of log k against σ have ? ≈ 0 when the aryl substituent has a negative σ and ? ≈ 1 when it has a positive σ value. These observations and the kinetic deuterium isotope effect suggest that with electron attracting substituents, an initial proton transfer gives an ion pair which then loses chloride ion, but with electron releasing substituents, these steps become more concerted.     

Mécanisme de l'addition nucléophile au groupement carbonyle—I: Influence de la nature du substituant polaire sur la stéréochimie et la réactivité dans les réactions d'addition de bromure de méthylmagnésium aux aryl-1 phényl-2 propanones

The influence of a m- or p-polar substituent in 1-aryl 2-phenyl propanones, on addition reactions with CH₃MgBr in ether at 30° has been investigated. The invariability of asymmetric induction as a function of substituent, and the correlation of rate constant ratios with Hammett σ constants (π = 0,24, r = 0,97), are consistent with a four-centre pericyclic concerted mechanism with concomitant departure of a solvent molecule.     

Aldol condensation of amides using phosphazene-based catalysis

We have developed a new method for the direct aldol condensation of unactivated amides using 1,3,5-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-hexachloride (TAPC)-based phosphorous/SO₄²⁻ catalysis. The SO₄²⁻ species in a reaction mixture enhances the reaction rate of the catalysis. In principle, no metal sources are required for the generation of the catalyst, and there is no requirement for the use of stoichiometric quantities of an acid or base. This catalyst system is operative under relatively acidic conditions. One major advantage of carrying out the reaction under acidic conditions is that both aldehydes and acetals are capable of undergoing carbon-carbon bond formation at the α-carbon of amide carbonyl groups through dehydration.     

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.     

The use of freshwater fish scale of the species Leporinus elongatus as adsorbent for anionic dyes

Fish scale of the species Leporinus elongatus was tested as an adsorbent for anionic Remazol dyes. Characterization has suggested that hydroxyl, phosphate, amides I, II, and III, and carbonate groups are the potential sites of adsorption. From solution calorimetry, values of thermal effects, Q _int, and amount of dye that interacts, n _int, were determined. The adsorption order observed was Yellow-dye/scale?>?Red-dye/scale?>?Blue-dye/scale. The Q _int and n _int data were successfully adjusted to the Langmuir isotherm model. The dyes removals by fish scale are exothermic processes (from ?83 to ?199?kJ?mol^?1) with negative entropies and are thermodynamically spontaneous. The thermodynamic results suggest that the interactions at scale/anionic dye interfaces occur mainly by surface reactions. It was finding that fish scale is a new and suitable sorbent material for recovery and biosorption/adsorption of anionic dyes from aqueous solutions.     

The Mechanism of Stereospecific C?H Oxidation by Fe(Pytacn) Complexes: Bioinspired Non‐Heme Iron Catalysts Containing cis‐Labile Exchangeable Sites

A detailed mechanistic study of the hydroxylation of alkane C? H bonds using H₂O₂ by a family of mononuclear non heme iron catalysts with the formula [Fe^II(CF₃SO₃)₂(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the α and γ positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis‐labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The C? H hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that C? H hydroxylation is performed by highly electrophilic [Fe^V(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C? H bond, followed by rebound of the hydroxyl ligand. The [Fe^V(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic‐labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.