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The C-heteroatom cleavage reactions of substituted dibenzyl sulfides and substituted dibenzylcyclo- hexylamines promoted by singlet oxygen in MeCN have been investigated. In both systems, the cleavage reactions (leading to benzaldehyde and substituted benzaldehyde) were slightly favored by electron-withdrawing substituents with ρ values of +0.47 (sulfides) and +0.27 (amines). With dibenzyl sulfides, sulfones were also obtained whereas sulfoxide formation became negligible when the reactions were carried out in the presence of a base. Through a careful product study for the oxidation of dibenzyl sulfide, in the presence and in the absence of Ph2SO, it was established that sulfone and cleavage product (benzaldehyde) do not come by the same route (involving the persulfoxide and the hydroperoxysulfonium ylide) as required by the generally accepted mechanism (Scheme 1) for C—heteroatom cleavage reactions of sulfides promoted by singlet oxygen. On this basis and in light of the similar structural effects noted above it is suggested that dibenzyl sulfides and dibenzylamines form benzaldehydes by a very similar mechanism. The reaction with singlet oxygen leads to an exciplex that can undergo an intracomplex hydrogen atom transfer to produce a radical pair. With sulfides, collapse of the radical pair leads to an α-hydroperoxy sulfide than can give benzaldehyde by an intramolecular path as described in Scheme 3. With amines, the radical pair undergoes an electron-transfer reaction to form an iminiumcation that hydrolyzes to benzaldehyde. From a kinetic study it has been established that the fraction of exciplex converted to aldehyde is ca. 20% with sulfides and Ca. 7% with amines. [ABSTRACT FROM AUTHOR]

Ketyl radicals with lignin related structures have been generated by means of radiation chemical and photochemical techniques. In the former studies ketyl radicals are produced by reaction of a-carbonyl-β-aryl ether lignin models with the solvated electron produced by pulse radiolysis of an aqueous solution at pH 6.0. The UV-vis spectra of ketyl radicals are characterized by three main absorption bands. The shape and position of these bands slightly change when the spectra are recorded in alkaline solution (pH 11.0) being now assigned to the ketyl radical anions and a pKa = 9.5 is determined for the 1-(3,4,5-trimethoxyphenyl)-2-phenoxyethanol-1-yl radical. Decay rates of ketyl radicals are found to be dose dependent and, at low doses, lie in the range (1.7-2.7) × 10³ s-1. In the presence of oxygen a fast decay of the ketyl radicals is observed (k2 = 1.8-2.7 × 109 M-1 s-1) that is accompanied by the formation of stable products, i.e., the starting ketones. In the photochemical studies ketyl radicals have been produced by charge-transfer (CT) photoactivation of the electron donor-acceptor salts of methyl viologen (MV2+ with a-hydroxy-ct-phenoxymethyl- aryl acetates. This process leads to the instantaneous formation of the reduced acceptor (methyl viologen radical cation, MV2·, as is clearly shown in a laser flash photolysis experiment by the two absorption bands centered at 390 and 605 nm, and an acyloxyl radical [ArC(CO2·)(OH)CH2- (OC6H5)], which undergoes a very fast decarboxylation with formation of the ketyl radicals. Steady- state photoirradiation of the CT ion pairs indicates that 1-aryl-2-phenoxyethanones are formed as primary photoproducts by oxidation of ketyl radicals by MV2+ (under argon) or by molecular oxygen. Small amounts of acetophenones are formed by further photolysis of 1-aryl-2-phenoxyethanones and not by β-fragmentation of the ketyl radicals. The high reactivity of ketyl radicals with oxygen coupled with the low rates of β-fragmentation of the same species have an important bearing in the context of the photoyellowing of lignin containing pulps and papers. [ABSTRACT FROM AUTHOR]

The reactivity of the phthalimide N-oxyl radical (PINO) toward the OH bond of a series of substituted phenols was kinetically investigated in CH3CN. The reaction selectivity and the deuterium kinetic isotope effect were determined. Information on the kinetic solvent effect was also obtained with phenol as the substrate. [ABSTRACT FROM AUTHOR]

A series of aryl-substituted N-hydroxyphthalimides (X-NHPIs) containing either electron-withdrawing groups (4-CH3OCO, 3-F) or electron-donating groups (4-CH3, 4-CH3O, 3-CH3O, 3,6-(CH3O)2) have been used as catalysts in the aerobic oxidation of primary and secondary benzylic alcohols. The selective formation of aromatic aldehydes was observed in the oxidation of primary alcohols; aromatic ketones were the exclusive products in the oxidation of secondary alcohols. O-H bond dissociation enthalpies (BDEs) of X-NHPIs have been determined by using the EPR radical equilibration technique. BDEs increase with increasing the electron-withdrawing properties of the aryl substituent. Kinetic isotope effect studies and the increase of the substrate oxidation rate by increasing the electron-withdrawing power of the NHPI aryl substituent indicate a rate-determining benzylic hydrogen atom transfer (HAT) from the alcohol to the aryl-substituted phthalimide-Noxyl radical (X-PINO). Besides enthalpic effects, polar effects also play a role in the HAT process, as shown by the negative ρ values of the Hammett correlation with σ+ and by the decrease of the ρ values (from -0.54 to -0.70) by increasing the electron-withdrawing properties of the NHPI aryl substituent. The relative reactivity of 3-CH3O-C6H4CH2OH and 3,4-(CH3O)2-C6H3CH2OH, which is higher than expected on the basis of the σ+ values, the small values of relative reactivity of primary vs secondary benzylic alcohols, and the decrease of the p values by increasing the electronwithdrawing properties of the NHPI aryl substituent, suggest that the HAT process takes place inside a charge-transfer (CT) complex formed by the X-PINO and the benzylic alcohol. [ABSTRACT FROM AUTHOR]

The H[sub 2]O[sub 2]-promoted oxidations of the two nonphenolic β-O-aryl lignin model trimers 1 and 2, catalyzed by lignin peroxidase (LiP) at pH = 3.5, have been studied. The results have been compared with those obtained in the oxidation of 1 and 2 with the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate. These models present a different substitution pattern of the three aromatic rings, and by one-electron oxidation, they form radical cations with the positive charge, which is localized in the dialkoxylated ring as also evidenced by a pulse radiolysis study. Both the oxidations with the enzymatic and with the chemical systems lead to the formation of products deriving from the cleavage of C-C and C-H bonds in a fi position with respect to the radical cation with the charge residing in the dialkoxylated ring (3,4-dimethoxybenzaldehyde (5) and a trimeric ketone 6 in the oxidation of 1 and a dimeric aldehyde 8 and a trimeric ketone 9 in the oxidation of 2). These products are accompanied by a dimeric aldehyde 7 in the oxidation of 1 and 4-methoxybenzaldehyde (10) in the oxidation of 2. The unexpected formation of these two products has been explained by suggesting that 1[sup •+] and 2[sup •+] can also undergo an intramolecular electron transfer leading to the radical cations 1a[sup •+] and 2a[sup •+] with the charge residing in a monoalkoxylated ring. The fast cleavage of a C-C bond β to this ring, leading to 7 from 1[sup •+] and to 10 from 2[sup •+], is the driving force of the endoergonic electron transfer. A kinetic steady-state investigation of the LiP-catalyzed oxidation of the trimer 2, the dimeric model 1-(3,4-dimethoxyphenyl)-2-phenoxy-1-ethanol (4), and 3,4dimethoxybenzyl alcohol (3) has indicated that the turnover number (k[sub cat]) and the affinity for the enzyme decrease significantly by increasing the size of the model compound. In contrast, the three substrates exhibited a very similar reactivity toward a chemical oxidant [Con[sup III]W]. This suggests a size-dependent interaction of the enzyme with the substrate which may influence the efficiency of the electron transfer. [ABSTRACT FROM AUTHOR]

A large number of substances have been tested as redox mediators in the LiP-catalysed oxidation of 4-methoxymandelic acid (4-MMA) to anisaldehyde. In some cases (i.e. thioanisole), the mediation efficiency is almost equal to the maximum value displayed by the natural mediator veratryl alcohol. The mediation efficiency is a function of the redox potential of the mediator and also appears to depend on the kinetic effectiveness with which the mediator is oxidised by the enzyme. In contrast, the lifetime of the mediator radical cation seems not to play any significant role, which would support the idea that the redox mediation is actually accomplished by a complex between the mediator radical cation and the enzyme. [Copyright &y& Elsevier]

Examines the prochiral selectivity in hydrogen peroxide-promoted oxidation of arylalkanols catalysed by chloroperoxidase (CPO). Characterization of the undeuterated arylalkanols; Importance of the enzymatic oxidation of alcohols; Kinetics of the CPO-catalysed oxidations of 4-methoxybenzyl alcohol.

The satisfactory rationalization of complex reactive pathways in solution chemistry may greatly benefit from the combined use of advanced experimental and theoretical complementary methods of analysis. In this work, we combine X‐Ray Absorption and 1H NMR spectroscopies with state‐of‐the‐art Multivariate Curve Resolution and theoretical analyses to gain a comprehensive view on a prototypical reaction involving the variation of the oxidation state and local structure environment of a selected metal ion coordinated by organic ligands. Specifically, we investigate the 2‐cyano‐2‐phenylpropanoic acid reduction of the octahedral complex established by the Cu2+ ion with terpyridine to the tetrahedral complex formed by Cu+ and neocuproine. Through our interdisciplinary approach we gain insights into the nature, concentration time evolution and structures of the key metal (XAS measurements) and organic (1H NMR measurements) species under reaction. We believe our method may prove to be useful in the toolbox necessary to understand the mechanisms of reactive processes of interest in solution. [ABSTRACT FROM AUTHOR]

A kinetic and product study of the 3‐cyano‐N‐methyl‐quinolinium photoinduced monoelectronic oxidation of a series of β‐hydroxysulfoxides has been carried out to investigate the competition between Cα‐S and Cα‐Cβ bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λmax ≈ 520 nm) and that of 3‐CN‐NMQ• (λmax ≈ 390 nm). Steady‐state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo Cα‐S bond cleavage, the presence of a β‐hydroxy group makes, in some cases, the Cα‐Cβ scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the β‐OH group with ‐OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the Cα‐Cβ bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission. [ABSTRACT FROM AUTHOR]

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C–H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis. [ABSTRACT FROM AUTHOR]

In this work, we obtain detailed mechanistic and structural information on bimolecular chemical reactions occurring in solution on the second to millisecond time scales through the combination of a statistical, multivariate and theoretical analysis of time-resolved coupled X-ray Absorption Spectroscopy (XAS) and UV–Vis data. We apply this innovative method to investigate the sulfoxidation of p-cyanothioanisole and p-methoxythioanisole by the nonheme FeIV oxo complex [N4Py·FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in acetonitrile at room temperature. By employing statistical and multivariate techniques we determine the number of key chemical species involved along the reaction paths and derive spectral and concentration profiles for the reaction intermediates. From the quantitative analysis of the XAS spectra we obtain accurate structural information for all reaction intermediates and provide the first structural characterization in solution of complex [N4Py·FeIII(OH)]2+. The employed strategy is promising for the spectroscopic characterization of transient species formed in redox reactions. [ABSTRACT FROM AUTHOR]

Predictability is a key requirement to encompass late‐stage C−H functionalization in synthetic routes. However, prediction (and control) of reaction selectivity is usually challenging, especially for complex substrate structures and elusive transformations such as remote C(sp³)−H oxidation, as it requires distinguishing a specific C−H bond from many others with similar reactivity. Developed here is a strategy for predictable, remote C−H oxidation that entails substrate binding to a supramolecular Mn or Fe catalyst followed by elucidation of the conformation of the host‐guest adduct by NMR analysis. These analyses indicate which remote C−H bonds are suitably oriented for the oxidation before carrying out the reaction, enabling prediction of site selectivity. This strategy was applied to late‐stage C(sp³)−H oxidation of amino‐steroids at C15 (or C16) positions, with a selectivity tunable by modification of catalyst chirality and metal. [ABSTRACT FROM AUTHOR]

Predictability is a key requirement to encompass late‐stage C−H functionalization in synthetic routes. However, prediction (and control) of reaction selectivity is usually challenging, especially for complex substrate structures and elusive transformations such as remote C(sp3)−H oxidation, as it requires distinguishing a specific C−H bond from many others with similar reactivity. Developed here is a strategy for predictable, remote C−H oxidation that entails substrate binding to a supramolecular Mn or Fe catalyst followed by elucidation of the conformation of the host‐guest adduct by NMR analysis. These analyses indicate which remote C−H bonds are suitably oriented for the oxidation before carrying out the reaction, enabling prediction of site selectivity. This strategy was applied to late‐stage C(sp3)−H oxidation of amino‐steroids at C15 (or C16) positions, with a selectivity tunable by modification of catalyst chirality and metal. [ABSTRACT FROM AUTHOR]

Herein we report that an imine‐based iron complex decorated with crown‐ether moieties self‐assembles in CH3CN solution from three building blocks (namely Fe(CH3CN)2(OTf)2, 2‐picolylamine and aldehyde 6a) added in the proper molar ratio (1:2:2, respectively). Aldehyde 6a, endowed with a benzocrown‐ether receptor, can be prepared in two steps from commercial materials with high yields (79 %) and no need of chromatographic separations. The self‐assembled complex is highly stable in CH3CN solution. The complex has been tested as catalyst for the oxidation of aromatic compounds endowed with primary ammonium anchoring group with H2O2. An unexpected yet moderate selectivity for remote meta over ortho hydroxylation was obtained, which is mainly due to the steric hindrance of the catalyst. [ABSTRACT FROM AUTHOR]

The article titled "Cover Feature: Signal Transduction Allows Temporal Control of the Potential of a Concentration Cell Driven by the Decarboxylation of an Activated Carboxylic Acid" explores the concept of using signal transduction to control the potential of a concentration cell. The authors demonstrate this concept by using an anthropomorphic nitroacetic acid to turn a light bulb on and off. The degradation of the acid to nitromethane and CO2 controls the connection between the light bulb and the socket. The article suggests that this time-controllable pH signal can be used to govern electric devices. [Extracted from the article]

Transition metal chemistry in solution is central to key processes in biology and catalysis. Achieving accurate knowledge on the structural and electronic properties of the relevant reaction intermediates requires the use of innovative and complementary spectroscopic probes. Herein, we combine X-ray Absorption Spectroscopy (XAS) and 1 H-NMR to investigate the exchange reaction between two prototypical Cu coordination complexes in solution. By means of multivariate and theoretical analyses, we identify the number, nature and concentration time evolution of the relevant reaction intermediates and quantitatively determine their structures. The XAS and NMR techniques are proven to be complementary experimental tools to track the fate of both the metal and organic portions of reactive systems involving transition metals. The employed combined experimental and theoretical method may be useful in the study of reactive systems involving intermediates that may be labile or silent to conventional detection. • A combined XAS/NMR analysis has been used to investigate a reaction in solution. • Multivariate and theoretical analyses have provided mechanistic information. • The approach is promising to study the reactivity of transition metal complexes. [ABSTRACT FROM AUTHOR]

Abstract: Back and forth motions of the acid–base‐operated molecular switch 1 are photo‐controlled by irradiation of a solution, which also contains the photolabile pre‐fuel 4. The photo‐stimulated deprotection of the pre‐fuel produces controlled amounts of acid 2, the base‐promoted decarboxylation of which fuels the back and forth motions of the Sauvage‐type [2]‐catenane‐based molecular switch. Because switch 1 and pre‐fuel 4 do not interact in the absence of irradiation, an excess of the latter with respect to 1 can be added to the solution from the beginning. In principle, photocontrol of the back and forth motions of any molecular machine, the operation of which is guided by protonation/deprotonation, could be attained by use of pre‐fuel 4, or of any other protected acid that undergoes deprotection by irradiation with light at a proper wavelength, followed by decarboxylation under conveniently mild conditions. [ABSTRACT FROM AUTHOR]

Site-selective C−H functionalization of aliphatic alkyl chains is a longstanding challenge in oxidation catalysis, given the comparable relative reactivity of the different methylenes. A supramolecular, bioinspired approach is described to address this challenge. A Mn complex able to catalyze C(sp3)-H hydroxylation with H2O2 is equipped with 18-benzocrown-6 ether receptors that bind ammonium substrates via hydrogen bonding. Reversible pre-association of protonated primary aliphatic amines with the crown ether selectively exposes remote positions (C8 and C9) to the oxidizing unit, resulting in a site-selective oxidation. Remarkably, such control of selectivity retains its efficiency for a whole series of linear amines, overriding the intrinsic reactivity of C−H bonds, no matter the chain length. [ABSTRACT FROM AUTHOR]

The oxidation of aryl 1-methyl-1-phenylethyl sulfides promoted by the nonheme iron(IV)-oxo complexes [(N4Py)FeIVO]2+ and [(Bn-TPEN)FeIVO]2+ occurs by an electron transfer-oxygen rebound (ET-OT) mechanism leading to aryl 1-methyl-1-phenylethyl sulfoxides accompanied by products derived from Cα-S fragmentation of sulfide radical cations (2-phenyl-2-propanol and diaryl disulfides). For the first time, the rate constants for the oxygen rebound process (kOT), which are in the range of <0.8 × 104 to 3.5 × 104 s-1, were determined from the fragmentation rate constants of the radical cations (kf) and the S oxidation/fragmentation product ratios. [ABSTRACT FROM AUTHOR]

The oxidation of a series of aryl diphenylmethyl sulfides (4-X-C6H4SCH(C6H5)2, where X = OCH3 (1), X = CH3 (2), X = H (3), and X = CF3 (4)) promoted by the nonheme iron(IV)-oxo complex [(N4Py)FeIV-O]2+ occurs by an electron transfer-oxygen transfer (ET-OT) mechanism as supported by the observation of products (diphenylmethanol, benzophenone, and diaryl disulfides) deriving from α-C-S and α-C-H fragmentation of radical cations 1+-4+, formed besides the S-oxidation products (aryl diphenylmethyl sulfoxides). The fragmentation/S-oxidation product ratios regularly increase through a decrease in the electron-donating power of the aryl substituents, that is, by increasing the fragmentation rate constants of the radical cations as indicated by a laser flash photolysis (LFP) study of the photochemical oxidation of 1-4 carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate (MeOP+PF6-). [ABSTRACT FROM AUTHOR]

A family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized, and employed as C–H oxidation catalysts. Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe(OTf)2 in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (L1)2Fe(OTf)(ClO4) and (L3)2Fe(OTf)2 are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. ¹H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (LX)2Fe(OTf)2 efficiently catalyze the oxidation of alkyl C–H bonds employing H2O2 as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C–H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C–H bonds. Isotopic labeling analyses show that H2O2 is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O–O cleavage, without the assistance of a cis-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H2O2 at (LX)2Fe(OTf)2 differs from that occurring in biomimetic iron catalysts described to date. [ABSTRACT FROM AUTHOR]

Abstract: Biochemical studies suggest a direct relationship between the radical scavenging activity of paracetamol (I) and its antipyretic and analgesic action. To evaluate the effect of chemical modifications on the radical scavenging activity of compounds of type I, analogues 1–14 were prepared and submitted to a stable free radical (DPPH; 1,1-diphenyl-2-picryl-hydrazyl) assay. All paracetamol derivatives showed a significant higher efficiency than the parent compound. This study showed that radical scavenging activity can be increased by decreasing the phenolic ortho substituents steric hindrance or by introducing substituents on the acyl moiety like an indazole ring or the ionic N-methyl morpholinium group. A significant activating effect was also observed by replacing the 1,4-acylamidophenol with a 1,4-acylamidonaphthol system. [Copyright &y& Elsevier]

A kinetic study of the one-electron oxidation of a series of ferrocenes (FcX: X = H, CO2Et, CONH2, CH2CN, CH2OH, Et, and Me2) by PINO generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with the cumyloxyl radical produced by 355 nm laser flash photolysis of dicumyl peroxide has been carried out. Ferrocenium cations were formed, and the reaction rate was determined by following the decay of PINO radical at 380 nm as a function of the FcX concentration. Rate constants were very sensitive to the oxidation potential of the substrates and exhibited a good fit with the Marcus equation, from which a λ value of 38.3 kcal mol-1 was calculated for the reorganization energy required in the PINO/ferrocenes electron-transfer process. Knowing the ferrocene/ferrocenium self-exchange reorganization energy it was possible to calculate a value of 49.1 kcal mol-1 for the PINO/PINO self-exchange reaction in CH3CN. Moreover, from the Marcus cross relation and the self-exchange rates of ferrocene and dimethylferrocene, the intrinsic reactivity of PINO in electron-transfer reactions has been calculated as 7.6 x 102 M-1 The implications of these values and the comparison with the electron-transfer self- exchange reorganization energies of peroxyl radicals are briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: The N-methylquinolinium tetrafluoroborate (NMQ+)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a–c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH3CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O2. By laser irradiation, the formation of sulfide radical cations 1a + –c + in the monomeric form (λ max=520nm) and of 2 + in both the monomeric (λ max=520nm) and dimeric form (λ max=780nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C–S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ ). A different situation was found in the presence of O2 since steady-state photolysis produced substantial amounts of C–S bond cleavage products (alcohols, alkenes, and ketones from 1a–c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O2 − generated by NMQ and O2. For the tert-alkyl phenyl sulfides, 1a–c, these results have been interpreted by suggesting that C–S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O2 − . In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O2 − to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O2 − of 2 n+ . [Copyright &y& Elsevier]

A product and time-resolved kinetic study of the effect of ring substitution on the reactivity of 1,1-diarylalkoxyl radicals has been carried out. The radicals undergo an O-neophyl shift to give the isomeric 1-aryl-l-aryloxyalkyl radicals from which the corresponding aromatic ketones are formed. The rearrangement rate constants are influenced by ring substitution, increasing in the presence of electron-withdrawing substituents and decreasing in the presence of electron-donating ones. From the results of product and kinetic studies, the following migratory aptitudes have been obtained: 4-trifluoromethylphenyl > phenyl ≅ 4-methylphenyl > 4-methoxyphenyl. Excellent Hammett-type correlations between the σ+ substituent constants and both the visible absorption band maxima and the rearrangement rate constants have been obtained. The experimental results indicate that the rearrangement is governed by electronic effects in the starting 1,1-diarylalkoxyl radicals, whereas the stability of the rearranged carbon-centered radical plays a minor role, in line with a reactant-like transition state, strongly supporting the hypothesis that the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals proceeds through a concerted mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Aryl substituted N-hydroxyphthalimides (NHPIs) have been tested as mediators in the laccase-promoted oxidation of non-phenolic monomeric and dimeric lignin model compounds and in the delignification of kraft pulp samples. In the oxidation of the model compounds a significant increase in the product yields was observed upon increasing the electron donating properties of the NHPI ring substituent. Product yields also increased, but to a smaller extent, by increasing the electron donating properties of the aromatic substituent in the lignin models. These results suggest the contribution to the overall reactivity of both the oxidation of the aryl substituted NHPI to the N-oxyl radical (X-PINO) by laccase and the hydrogen atom transfer step from the substrate to the X-PINO, with a major contribution of the former process. When applied to the delignification of kraft pulps again the mediation efficiency increased by increasing the electron donating properties of the NHPI aryl substituent, the best mediators being 4-Me-NHPI and 4-MeO-NHPI. Thus, the use of non-phenolic lignin model compounds in the oxidation promoted by the laccase/X-NHPI system is well suited to mimic the behaviour of the lignin polymer. [Copyright &y& Elsevier]

The kinetics of decay of veratryl alcohol radical cation, generated by cerium(IV) ammonium nitrate induced oxidation of veratryl alcohol, have been followed spectrophotometrically in a stopped-flow apparatus. In acidic aqueous acetonitrile the radical cation was found to decay by a first-order process, due to deprotonation from the α-carbon leading to an α-hydroxybenzyl radical with the rate constant of 17.1±0.5 s−1. This value is in full agreement with those obtained by pulse radiolysis studies but much lower than the value (1.2×103 s−1) indirectly determined by EPR experiments. The implications of these results with respect to the possible role of veratryl alcohol as a mediator in the oxidative biodegradation of lignin catalysed by lignin peroxidase are discussed. [Copyright &y& Elsevier]

The outstanding efficiency and selectivity of enzymatic reactions, such as C-H oxidation by nonheme iron oxygenases, stems from a precise control of substrate positioning inside the active site. The resulting proximity between a specific moiety (a certain C-H bond) and the reactant (a Fe IV (O) active species) translates into higher rates and selectivity, that can be in part replicated also with artificial supramolecular catalysts. However, structural modification of the position and orientation of the binding site both in enzymes and in artificial catalysts often leads to significant variations in reactivity that can be difficult to rationalize due to the system's complexity. Herein, we quantitatively analyzed the impact of such a structural modification (namely receptor orientation) on the C-H oxidation reactivity (kinetics, Effective Molarity) and selectivity by comparing simple supramolecular Fe IV (O) models. Overall, we did not observe significant differences in reaction rates, but we noticed slight changes in the selectivity profile. These results indicate that, when a crown-ether is employed as a recognition site, the key ingredient for enhanced reactivity is the presence of the supramolecular receptor itself rather than its exact orientation, providing a guide for the rational design of supramolecular catalysts., (© 2024 Wiley-VCH GmbH.)

Oscillating reactions are among the most intriguing phenomena in chemistry, but many questions on their mechanisms still remain unanswered, due to their intrinsic complexity and to the low sensitivity of the most common spectroscopic techniques toward the reaction brominated species. In this work, we investigate the cerium ion-catalyzed Belousov-Zhabotinsky (BZ) oscillating reaction by means of time-resolved X-ray absorption spectroscopy (XAS), in combination with UV-vis spectroscopy and unsupervised machine learning, multivariate curve resolution, and kinetic analyses. Altogether, we provide new insights into the collective oscillatory behavior of the key brominated species involved in the classical BZ reaction and measure previously unreported oscillations in their concentrations through Br K-edge XAS, while simultaneously tracking the oscillatory Ce 4+ -to-Ce 3+ transformation by coupling XAS with UV-vis spectroscopy. Our work evidences the potential of the XAS technique to investigate the mechanisms of oscillatory chemical systems whose species are often not detectable with conventional experimental methods.

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp 3 )-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models., (© 2024 Wiley-VCH GmbH.)

The use of Activated Carboxylic Acids (ACAs) allows the time-controlled operation of dissipative chemical systems based on the acid-base reaction by providing both the stimulus that temporarily drives a physicochemical change and, subsequently, the counter-stimulus with a single reagent addition. However, their application is inherently limited to acid-sensitive systems. To overcome this limitation, we herein develop a straightforward device for the transduction of the acid-base stimuli delivered by an ACA into a voltage signal that, in turn, is used to control voltage-sensitive circuits that are not pH-responsive by themselves. The signal transductor can be easily assembled from common laboratory equipment and employs aqueous solutions of readily available chemicals. Furthermore, the operator can simply and intuitively tune the amplitude of the voltage signal, as well as its duration and offset by varying the concentration of the chemical species involved in the transduction process., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Site-selective oxidation of aliphatic C-H bonds is a powerful synthetic tool because it enables rapid build-up of product complexity and diversity from simple precursors. Besides the poor reactivity of alkyl C-H bonds, the main challenge in this reaction consists in differentiating between the multiple similar sites present in most organic molecules. Herein, a manganese oxidation catalyst equipped with two 18-benzo-6-crown ether receptors has been employed in the oxidation of the long chain tetradecane-1,14-diamine. 1 H-NMR studies evidence simultaneous binding of the two protonated amine moieties to the crown ether receptors. This recognition has been used to pursue site-selective oxidation of a methylenic site, using hydrogen peroxide as oxidant in the presence of carboxylic acids as co-ligands. Excellent site-selectivity towards the central methylenic sites (C6 and C7) is observed, overcoming selectivity parameters derived from polar deactivation by simple amine protonation and selectivity observed in the oxidation of related monoprotonated amines.

The dissipative translocation of the Zn 2+ ion between two prototypical coordination complexes has been investigated by combining X-ray absorption and 1 H NMR spectroscopy. An integrated experimental and theoretical approach, based on state-of-the-art Multivariate Curve Resolution and DFT based theoretical analyses, is presented as a means to understand the concentration time evolution of all relevant Zn and organic species in the investigated processes, and accurately characterize the solution structures of the key metal coordination complexes. Specifically, we investigate the dissipative translocation of the Zn 2+ cation from hexaaza-18-crown-6 to two terpyridine moieties and back again to hexaaza-18-crown-6 using 2-cyano-2-phenylpropanoic acid and its para -chloro derivative as fuels. Our interdisciplinary approach has been proven to be a valuable tool to shed light on reactive systems containing metal ions that are silent to other spectroscopic methods. These combined experimental approaches will enable future applications to chemical and biological systems in a predictive manner.

A kinetic analysis of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the iron(IV)-oxo complex [(N4Py)Fe IV (O)] 2+ and to the phthalimide N -oxyl radical (PINO) has been carried out. The results indicate that a higher activating effect of α-heteroatoms toward the HAT from C-H bonds is observed with the more electrophilic PINO radical. When the N -hydroxy precursor of PINO, N- hydroxyphthalimide (NHPI), is used as a HAT mediator in the oxidation promoted by [(N4Py)Fe IV (O)] 2+ , significant differences in terms of selectivity have been found. Product studies of the competitive oxidations of primary and secondary aliphatic alcohols (1-decanol, cyclopentanol, and cyclohexanol) with alkylaromatics (ethylbenzene and diphenylmethane) demonstrated that it is possible to modify the selectivity of the oxidations promoted by [(N4Py)Fe IV (O)] 2+ in the presence of NHPI. In fact, alkylaromatic substrates are more reactive in the absence of the mediator while alcohols are preferably oxidized in the presence of NHPI., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The understanding of reactive processes involving organic substrates is crucial to chemical knowledge and requires multidisciplinary efforts for its advancement. Herein, we apply a combined multivariate, statistical and theoretical analysis of coupled time-resolved X-ray absorption (XAS)/UV-Vis data to obtain detailed mechanistic information for on the C-H bond activation of 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph2CH2) by the nonheme FeIV-oxo complex [N4Py·FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in CH3CN at room temperature. Within this approach, we determine the number of key chemical species present in the reaction mixtures and derive spectral and concentration profiles for the reaction intermediates. From the quantitative analysis of the XAS spectra the transient intermediate species are structurally determined. As a result, it is suggested that, while DHA is oxidized by [N4Py·FeIV(O)]2+ with a hydrogen atom transfer-electron transfer (HAT-ET) mechanism, Ph2CH2 is oxidized by the nonheme iron-oxo complex through a HAT-radical dissociation pathway. In the latter process, we prove that the intermediate FeIII complex [N4Py·FeIII(OH)]2+ is not able to oxidize the diphenylmethyl radical and we provide its structural characterization in solution. The employed combined experimental and theoretical strategy is promising for the spectroscopic characterization of transient intermediates as well as for the mechanistic investigation of redox chemical transformations on the second to millisecond time scales.

Sterically hindered imine-based non-heme complexes 4 and 5 rapidly self-assemble in acetonitrile at 25 °C, when the corresponding building blocks are added in solution in the proper ratios. Such complexes are investigated as catalysts for the H 2 O 2 oxidation of a series of substrates in order to ascertain the role and the importance of the ligand steric hindrance on the action of the catalytic core 1, previously shown to be an efficient catalyst for aliphatic and aromatic C-H bond oxidation. The study reveals a modest dependence of the output of the oxidation reactions on the presence of bulky substituents in the backbone of the catalyst, both in terms of activity and selectivity. This result supports a previously hypothesized catalytic mechanism, which is based on the hemi-lability of the metal complex. In the active form of the catalyst, one of the pyridine arms temporarily leaves the iron centre, freeing up a lot of room for the access of the substrate., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this work, we propose a method to directly determine the mechanism of the reaction between the nonheme complex Fe II (tris(2-pyridylmethyl)amine) ([Fe II (TPA)(CH 3 CN) 2 ] 2+ ) and peracetic acid (AcOOH) in CH 3 CN, working at room temperature. A multivariate analysis is applied to the time-resolved coupled energy-dispersive X-ray absorption spectroscopy (EDXAS) reaction data, from which a set of spectral and concentration profiles for the reaction key species is derived. These "pure" extracted EDXAS spectra are then quantitatively characterized by full multiple scattering (MS) calculations. As a result, structural information for the elusive reaction intermediates [Fe III (TPA)(κ 2 -OOAc)] 2+ and [Fe IV (TPA)(O)(X)] +/2+ is obtained, and it is suggested that X = AcO - in opposition to X = CH 3 CN. The employed strategy is promising both for the spectroscopic characterization of reaction intermediates that are labile or silent to the conventional spectroscopic techniques, as well as for the mechanistic understanding of complex redox reactions involving organic substrates.

The oxidation of a series of hydrocarbons by the nonheme iron(IV)-oxo complex [(N4Py)Fe IV ═O] 2+ is efficiently mediated by N -hydroxyphthalimide. The increase of reactivity is associated to the oxidation of the mediator to the phthalimide N -oxyl radical, which efficiently abstracts a hydrogen atom from the substrates, regenerating the mediator in its reduced form.

The upgrading of complex bio-renewable feedstock, such as lignocellulose, through depolymerisation benefits from the selective reactions at key functional groups. Applying homogeneous catalysts developed for selective organic oxidative transformations to complex feedstock such as lignin is challenged by the presence of interfering components. The selection of appropriate model compounds is essential in applying new catalytic systems and identifying such interferences. Here, it was shown by using as an example the oxidation of a model substrate containing a β-O-4 linkage with H 2 O 2 and an in situ-prepared manganese-based catalyst, capable of efficient oxidation of benzylic alcohols, that interference from compounds liberated during the reaction can prevent its application to lignocellulose depolymerisation., (© 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Evaluation of polar effects in hydrogen atom transfer (HAT) processes is made difficult by the fact that in most cases substrates characterized by lower bond dissociation energies (BDEs), activated from an enthalpic point of view, are also more activated by polar effects. In search of an exception to this general rule, we found that the introduction of a methoxy substituent in the 3-position of 2,6-dimethylphenol results in a small increase in the O-H BDE and a decrease of the ionization potential of the phenol. These findings suggest that the enthalpic effect associated with the addition of the m-methoxy group to 2,6-dimethylphenol will decrease reaction rates, while the polar effects will increase reaction rates. Our model analysis of polar effects has been experimentally validated by comparing the reactivity of 2,6-dimethylphenol with that of 2,6-dimethyl-3-methoxyphenol in HAT promoted by a series of radicals (cumyloxyl, galvinoxyl, 2,2-diphenylpycrylhydrazyl, phthalimide- N-oxyl, and benzotriazole- N-oxyl radicals). In line with our predictions, the ratio of HAT rate constants ( k H mOMe / k H H ) is larger in cases where there is a greater contribution of polar effects in the HAT reaction, i.e., in HAT promoted by N-oxyl radicals containing electron-withdrawing groups or when more polar solvents are employed.

Time-resolved X-ray absorption (XAS) and UV-vis spectroscopies with millisecond resolution are used simultaneously to investigate oxidation reactions of organic substrates by nonheme iron activated species. In particular, the oxidation processes of arylsulfides and benzyl alcohols by a nonheme iron-oxo complex have been studied. We show for the first time that the pseudo-first-order rate constants of fast bimolecular processes in solution (milliseconds and above) can be determined by time-resolved XAS technique. By following the Fe K-edge energy shift, it is possible to detect the rate of iron oxidation state evolution that matches that of the bimolecular reaction in solution. The kinetic constant values obtained by XAS are in perfect agreement with those obtained by means of the concomitant UV-vis detection. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase that involve changes of the oxidation state of a metal center, and it is particularly useful in complex chemical systems where possible interferences from species present in solution could make it impossible to use other detection techniques.

Substrate-selectivity stemming from recognition is a key feature of enzymes that has been seldom observed in artificial catalysts. Herein, we report a recognition-driven, substrate-selective C-H oxidation that inverts the intrinsic reactivity of the competing C-H bonds. Analysis of this selectivity highlights an unexpectedly high reactivity enhancement imparted by intramolecularity.

The oxidation of a series of N -acetyl amino acid methyl esters with H 2 O 2 catalyzed by a very simple iminopyridine iron(ii) complex 1 easily obtainable in situ by self-assembly of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf) 2 was investigated. Oxidation of protected aliphatic amino acids occurs at the α-C-H bond exclusively ( N -AcAlaOMe) or in competition with the side-chain functionalization ( N -AcValOMe and N -AcLeuOMe). N -AcProOMe is smoothly and cleanly oxidized with high regioselectivity affording exclusively C-5 oxidation products. Remarkably, complex 1 is also able to catalyze the oxidation of the aromatic N -AcPheOMe. A marked preference for the aromatic ring hydroxylation over Cα-H and benzylic C-H oxidation was observed, leading to the clean formation of tyrosine and its phenolic isomers., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

1,8-Naphthalenediol (dihydroxynaphthalene, 1,8-DHN) has been shown to be a potent hydrogen atom transfer (HAT) antioxidant compound because of the strong stabilization of the resulting free radical by intramolecular hydrogen bonding. However, the properties, reactivity, and fate of the 1,8-DHN phenoxyl radical have remained so far uncharted. Herein, we report an integrated experimental and computational characterization of the early intermediates and dimer products that arise by the oxidation of 1,8-DHN. Laser flash photolysis (LFP) studies of HAT from 1,8-DHN to the cumyloxyl and aminoxyl radicals showed the generation of a transient species absorbing at 350, 400, and >600 nm attributable to the 1,8-DHN phenoxyl radical. Peroxidase/H 2 O 2 oxidation of 1,8-DHN was found to proceed via an intense blue intermediate (λ max 654 nm) preceding precipitation of a black melanin-like polymer. By halting the reaction in the early stages, three main dimers featuring 2,2'-, 2,4'-, and 4,4'-bondings could be isolated and characterized in pure form. Density functional theory calculations supported the generation of the 1,8-DHN phenoxyl radical and its subsequent coupling via the 2- and 4-positions giving extended quinone dimers with intense transitions in the visible range, consistent with UV-vis and LFP data. Overall, these results allowed to elucidate the mechanism of oxidative polymerization of 1,8-DHN of possible relevance to melanogenesis in fungi and other processes of environmental and astrochemical relevance., Competing Interests: The authors declare no competing financial interest.

An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of Fe II into two subsequent Fe III species and the latter involving the more complex Fe II -Fe III -Fe IV -Fe III sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.

A change in regioselectivity has been observed in the hydrogen atom transfer (HAT) reactions from 4-alkyl-N,N-dimethylbenzylamines (alkyl = ethyl, isopropyl, and benzyl) to the phthalimide N-oxyl radical (PINO) by effect of protonation. This result can be rationalized on the basis of an acid-induced deactivation of the C-H bonds α to nitrogen toward HAT to PINO as evidenced by the 10 4 -10 7 -fold decrease in the HAT rate constants in acetonitrile following addition of 0.1 M HClO 4 . This acid-induced change in regioselectivity has been successfully applied for selective functionalization of the less activated benzylic C-H bonds para to the CH 2 N(CH 3 ) 2 group in the aerobic oxidation of 4-alkyl-N,N-dimethylbenzylamines catalyzed by N-hydroxyphthalimide in acetic acid.