Absolute Rate Constants for the β-Scission and Hydrogen Abstraction Reactions of the tert-Butoxyl Radical and for Several Radical Rearrangements: Evaluating Delayed Radical Formations by Time-Resolved Electron Spin Resonance

Aryl-aryl cross-coupling reactions without reagents or catalysts: photocyclization of ortho-iodoaryl ethers and related compounds via triplet aryl cation intermediates

Cyclisations of benzyl ortho-iodoaryl ethers to benzo[c]chromenes can be effected without reagents or catalysts by irradiation with UVC under flow. Reactions proceed via triplet aryl cation generation, 5-exo and 3-exo-cyclisations, and rearomatisation. They have wide scope, are easy to effect and extend to a myriad of related ring systems.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Silyl Radical Cascade Cyclization of 2-Isocyanothioanisole toward 2-Silylbenzothiazoles through Radical Initiator-Inhibitor Symbiosis

A demethylative silyl radical cascade cyclization of 2-isocyanothioanisoles toward 2-silylated benzothiazole building blocks has been developed. The development of a "radical initiator-inhibitor symbiosis" system solves the challenge of otherwise dominant methyl radical-triggered side reactions brought about by kinetically unfavored generation of reactive silyl radical species. The products accessed in this protocol are amendable to various downstream functionalization reactions, including the quick construction of a topoisomerase II inhibitor via a Hiyama cross-coupling reaction and of an antiviral agent via a fluoride-/hydroxide-free nucleophilic substitution to acyl chloride.

Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, for HAT from the C-H bonds of 56 substrates to CumO•, spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C-H bond dissociation enthalpies (BDEs) spanning 27 kcal mol-1 has been calculated using the (RO)CBS-QB3 method. The log kH' vs C-H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C-H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG⧧HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol-1, indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi's principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)-H donors.

Iron-Catalyzed Photoinduced LMCT: a 1° C-H Abstraction Enables Skeletal Rearrangements and C(sp3)-H Alkylation

Herein we disclose an iron-catalyzed method to access skeletal rearrangement reactions akin to the Dowd-Beckwith ring expansion from unactivated C(sp³)-H bonds. Photoinduced ligand-to-metal charge transfer at the iron center generates a chlorine radical, which abstracts electron-rich C(sp³)-H bonds. The resulting unstable alkyl radicals can undergo rearrangement in the presence of suitable functionality. Addition to an electron deficient olefin, recombination with a photoreduced iron complex, and subsequent protodemetallation allows for redox-neutral alkylation of the resulting radical. Simple adjustments to the reaction conditions enable the selective synthesis of the directly alkylated or the rearranged-alkylated products. As a radical clock, these rearrangements also enable the measurement of rate constants of addition into various electron deficient olefins in the Giese reaction.

Azocompounds as generators of defined radical species: Contributions and challenges for free radical research

Peroxyl radicals participate in multiple processes involved in critical changes to cells, tissues, pharmacueticals and foods. Some of these reactions explain their association with degenerative pathologies, including cardiovascular and neurological diseases, as well as cancer development. Azocompounds, and particularly AAPH (2,2'-Azobis(2-methylpropionamidine) dihydrochloride), a cationic water-soluble derivative, have been employed extensively as sources of model peroxyl radicals. A considerable number of studies have reported mechanistic data on the oxidation of biologically-relevant targets, the scavenging activity of foods and natural products, and the reactions with, and responses of, cultured cells. However, despite the (supposed) experimental simplicity of using azocompounds, the chemistry of peroxyl radical production and subsequent reactions is complicated, and not always considered in sufficient depth when analyzing experimental data. The present work discusses the chemical aspects of azocompounds as generators of peroxyl (and other) radicals, together with their contribution to our understanding of biochemistry, pharmaceutical and food chemistry research. The evidence supporting a role for the formation of alkoxyl (RO•) and other radicals during thermal and photochemical decomposition of azocompounds is assessed, together with the potential influence of such species on the reactions under study.

Reactivity and Product Analysis of a Pair of Cumyloxyl and tert-Butoxyl Radicals Generated in Photolysis of tert-Butyl Cumyl Peroxide

Alkoxyl radicals play important roles in various fields of chemistry. Understanding their reactivity is essential to applying their chemistry for industrial and biological purposes. Hydrogen-atom transfer and C-C β-scission reactions have been reported from alkoxyl radicals. The ratios of these two processes were investigated using cumyloxyl (CumO^•) and tert-butoxyl radicals (t-BuO^•), respectively. However, the products generated from the pair of radicals have not been investigated in detail. In this study, CumO^• and t-BuO^• were simultaneously generated from the photolysis of tert-butyl cumyl peroxide to understand the chemical behavior of the pair of radicals by analyzing the products and their distribution. Electron paramagnetic resonance and/or transient absorption spectroscopy analyses of radicals, including CumO^• and t-BuO^•, provide more information about the radicals generated during the photolysis of tert-butyl cumyl peroxide. Furthermore, the photoproducts of (3-(tert-butylperoxy)pentane-3-yl)benzene demonstrated that the ether products were formed in in-cage reactions. The triplet-sensitized reaction induced by acetophenone, which is produced from CumO^•, clarified that the spin state did not affect the product distribution.

Copper-Catalyzed Amination of C(sp3)-H bonds: From Anilides to Indolines

Copper-catalyzed oxidative intramolecular cyclization of o-alkylated anilines via cleavage of C(sp³)-H and N-H bonds for the production of indolines is described. This approach provides a straightforward strategy for the synthesis of nitrogen-containing heterocyclic compounds through the functionalization of unactivated C(sp³)-H bonds with high site selectivity. The present catalytic system shows high preference for functionalization of unactivated C(sp³)-H bonds over C(sp²)-H bonds, leading to C-N bond formation.

Remote Allylation of Unactivated C(sp3)-H Bonds Triggered by Photogenerated Amidyl Radicals

The allylation reaction is a highly versatile transformation in chemical synthesis. While many elegant direct C(sp²)-H allylation reactions have been developed, the direct allylation of unactivated C(sp³)-H bonds is underdeveloped. By applying photoredox catalysis and a [1,5]-HAT process, herein we report a direct allylation of unactivated C(sp³)‒H bonds. This photocatalyzed transformation is tolerant of several functional groups in the amide and allylic chloride substrates. Various allyl-substituted amide products were obtained with good yields and high δ-selectivity.

Hydrogen Atom Transfer from Alkanols and Alkanediols to the Cumyloxyl Radical: Kinetic Evaluation of the Contribution of α-C-H Activation and β-C-H Deactivation

A kinetic study on the reactions of the cumyloxyl radical (CumO^•) with a series of alkanols and alkanediols has been carried out. Predominant hydrogen atom transfer (HAT) from the α-C-H bonds of these substrates, activated by the presence of the OH group, is observed. The comparable k_H values measured for ethanol and 1-propanol and the increase in k_H measured upon going from 1,2-diols to structurally related 1,3- and 1,4-diols is indicative of β-C-H deactivation toward HAT to the electrophilic CumO^•, determined by the electron-withdrawing character of the OH group. No analogous deactivation is observed for the corresponding diamines, in agreement with the weaker electron-withdrawing character of the NH₂ group. The significantly lower k_H values measured for reaction of CumO^• with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of β-C-H deactivation. The contribution of torsional effects on reactivity is evidenced by the ∼2-fold increase in k_H observed upon going from the trans isomers of 4- tert-butylcyclohexanol and 1,2- and 1,4-cyclohexanediol to the corresponding cis isomers. These results provide an evaluation of the role of electronic and torsional effects on HAT reactions from alcohols and diols to CumO^•, uncovering moreover β-C-H deactivation as a relevant contributor in defining site selectivity.

Fine Control over Site and Substrate Selectivity in Hydrogen Atom Transfer-Based Functionalization of Aliphatic C-H Bonds

The selective functionalization of unactivated aliphatic C-H bonds over intrinsically more reactive ones represents an ongoing challenge of synthetic chemistry. Here we show that in hydrogen atom transfer (HAT) from the aliphatic C-H bonds of alkane, ether, alcohol, amide, and amine substrates to the cumyloxyl radical (CumO^•) fine control over site and substrate selectivity is achieved by means of acid-base interactions. Protonation of the amines and metal ion binding to amines and amides strongly deactivates the C-H bonds of these substrates toward HAT to CumO^•, providing a powerful method for selective functionalization of unactivated or intrinsically less reactive C-H bonds. With 5-amino-1-pentanol, site-selectivity has been drastically changed through protonation of the strongly activating NH₂ group, with HAT that shifts to the C-H bonds that are adjacent to the OH group. In the intermolecular selectivity studies, trifluoroacetic acid, Mg(ClO₄)₂, and LiClO₄ have been employed in a orthogonal fashion for selective functionalization of alkane, ether, alcohol, and amide (or amine) substrates in the presence of an amine (or amide) one. Ca(ClO₄)₂, that promotes deactivation of amines and amides by Ca²⁺ binding, offers, moreover, the opportunity to selectively functionalize the C-H bonds of alkane, ether, and alcohol substrates in the presence of both amines and amides.

Catalytic Ring-Opening of Cyclic Alcohols Enabled by PCET Activation of Strong O-H Bonds

We report a new photocatalytic protocol for the redox-neutral isomerization of cyclic alcohols to linear ketones via C-C bond scission. Mechanistic studies demonstrate that key alkoxy radical intermediates in this reaction are generated via the direct homolytic activation of alcohol O-H bonds in an unusual intramolecular PCET process, wherein the electron travels to a proximal radical cation in concert with proton transfer to a weak Brønsted base. Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.

Reactivity and selectivity patterns in hydrogen atom transfer from amino acid C-H bonds to the cumyloxyl radical: polar effects as a rationale for the preferential reaction at proline residues

Absolute rate constants for hydrogen atom transfer (HAT) from the C-H bonds of N-Boc-protected amino acids to the cumyloxyl radical (CumO(•)) were measured by laser flash photolysis. With glycine, alanine, valine, norvaline, and tert-leucine, HAT occurs from the α-C-H bonds, and the stability of the α-carbon radical product plays a negligible role. With leucine, HAT from the α- and γ-C-H bonds was observed. The higher kH value measured for proline was explained in terms of polar effects, with HAT that predominantly occurs from the δ-C-H bonds, providing a rationale for the previous observation that proline residues represent favored HAT sites in the reactions of peptides and proteins with (•)OH. Preferential HAT from proline was also observed in the reactions of CumO(•) with the dipeptides N-BocProGlyOH and N-BocGlyGlyOH. The rate constants measured for CumO(•) were compared with the relative rates obtained previously for the corresponding reactions of different hydrogen-abstracting species. The behavior of CumO(•) falls between those observed for the highly reactive radicals Cl(•) and (•)OH and the significantly more stable Br(•). Taken together, these results provide a general framework for the description of the factors that govern reactivity and selectivity patterns in HAT reactions from amino acid C-H bonds.

Radical aryl migration reactions and synthetic applications

This review provides a comprehensive summary of radical aryl migration reactions, highlighting their mechanistic studies and synthetic applications.

Mechanistic insights from reaction of α-oxiranyl-aldehydes with cyanobacterial aldehyde deformylating oxygenase

The biosynthesis of long-chain aliphatic hydrocarbons, which are derived from fatty acids, is widespread in Nature. The last step in this pathway involves the decarbonylation of fatty aldehydes to the corresponding alkanes or alkenes. In cyanobacteria, this is catalyzed by an aldehyde deformylating oxygenase. We have investigated the mechanism of this enzyme using substrates bearing an oxirane ring adjacent to the aldehyde carbon. The enzyme catalyzed the deformylation of these substrates to produce the corresponding oxiranes. Performing the reaction in D2O allowed the facial selectivity of proton addition to be examined by (1)H NMR spectroscopy. The proton is delivered with equal probability to either face of the oxirane ring, indicating the formation of an oxiranyl radical intermediate that is free to rotate during the reaction. Unexpectedly, the enzyme also catalyzes a side reaction in which oxiranyl-aldehydes undergo tandem deformylation to furnish alkanes two carbons shorter. We present evidence that this involves the rearrangement of the intermediate oxiranyl radical formed in the first step, resulting in aldehyde that is further deformylated in a second step. These observations provide support for a radical mechanism for deformylation and, furthermore, allow the lifetime of the radical intermediate to be estimated based on prior measurements of rate constants for the rearrangement of oxiranyl radicals.

Hydrogen abstraction from cyclic amines by the cumyloxyl and benzyloxyl radicals. The role of stereoelectronic effects and of substrate/radical hydrogen bonding

A kinetic study on the hydrogen abstraction reactions from cyclic amines and diamines (pyrrolidines, piperidines, morpholines, and piperazines) by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. The reactions with CumO(•) were described in all cases as direct hydrogen abstractions. The differences in the hydrogen abstraction rate constant (k(H)) were explained in terms of the different number of abstractable hydrogen atoms, the operation of stereoelectronic effects, and, with the morpholines, on the basis of polar effects. Significantly higher k(H) values were measured for the reactions of the amines with BnO(•). This behavior was explained on the basis of a mechanism that proceeds through the rate-determining formation of a hydrogen bonded pre-reaction complex between the radical α-C-H and the nitrogen lone pair followed by hydrogen abstraction within the complex. A decrease in k(H) was observed going from secondary to tertiary amines and, with tertiary amines, on increasing steric hindrance at nitrogen, pointing toward the important role of steric and electronic effects on pre-reaction complex formation. These results expand previous findings contributing to a detailed mechanistic description of the reactions of alkoxyl radicals with amines, showing that structural effects in both the substrate and the radical can play a dramatic role and providing new information on the role of substrate/radical interactions on these processes.

Hydrogen atom abstraction selectivity in the reactions of alkylamines with the benzyloxyl and cumyloxyl radicals. The importance of structure and of substrate radical hydrogen bonding

A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(•), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO(•), where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO(•) were in all cases significantly higher than those measured for the corresponding reactions of CumO(•), and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive α-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO(•))/k(H)(CumO(•)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(•) were described in all cases as direct hydrogen atom abstractions. With BnO(•) the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical α-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate α-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.

Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(•))/k(H)(CumO(•)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(•))/k(H)(CumO(•)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(•) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(•), BnO(•)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.

Kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical. The role of hydrogen bonding

A kinetic study of the H-atom abstraction reactions from 1,4-cyclohexadiene and triethylamine by the cumyloxyl radical has been carried out in different solvents. Negligible effects are observed with 1,4-cyclohexadiene, whereas with triethylamine a significant decrease in rate constant (k(H)) is observed on going from benzene to MeOH. A good correlation between log k(H) and the solvent hydrogen bond donor parameter alpha is observed, indicative of an H-bonding interaction between the amine lone pair and the solvent.

The role of one-electron reduction of lipid hydroperoxides in causing DNA damage

The in vivo metabolism of plasma lipids generates lipid hydroperoxides that, upon one-electron reduction, give rise to a wide spectrum of genotoxic unsaturated aldehydes and epoxides. These metabolites react with cellular DNA to form a variety of pre-mutagenic DNA lesions. The mechanisms of action of the radical precursors of these genotoxic electrophiles are poorly understood. In this work we investigated the nature of DNA products formed by a one-electron reduction of (13S)-hydroperoxy-(9Z,11E)-octadecadienoic acid (13S-HPODE), a typical lipid molecule, and the reactions of the free radicals thus generated with neutral guanine radicals, G(-H)(*). A novel approach was devised to generate these intermediates in solution. The two-photon-induced ionization of 2-aminopurine (2AP) within the 2'-deoxyoligonucleotide 5'-d(CC[2AP]TCGCTACC) by intense nanosecond 308 nm excimer laser pulses was employed to simultaneously generate hydrated electrons and radical cations 2AP(*+). The latter radicals either in cationic or neutral forms, rapidly oxidize the nearby G base to form G(-H)(*). In deoxygenated buffer solutions (pH 7.5), the hydrated electrons rapidly reduce 13S-HPODE and the highly unstable alkoxyl radicals formed undergo a prompt beta-scission to pentyl radicals that readily combine with G(-H)(*). Two novel guanine products in these oligonucleotides, 8-pentyl- and N(2)-pentylguanine, were identified. It is shown that the DNA secondary structure significantly affects the ratio of 8-pentyl- and N(2)-pentylguanine lesions that changes from 0.9:1 in single-stranded, to 1:0.2 in double-stranded oligonucleotides. The alkylation of guanine by alkyl radicals derived from lipid hydroperoxides might contribute to the genotoxic modification of cellular DNA under hypoxic conditions. Thus, further research is warranted on the detection of pentylguanine lesions and other alkylguanines in vivo.

Decomposition of Tertiary Alkoxy Radicals

Rate coefficients of β-scission reactions in tertiary alkoxy radicals, R(CH3)2CO (R = methyl, ethyl, tert-butyl and neo-pentyl) have been estimated via density functional theory (DFT) calculations in conjunction with statistical unimolecular rate theory. For tert-butoxy, results obtained by employing different basis sets are compared with experimental values, indicating that UB3LYP/6-31G(d,p) excellently predicts kinetic data. Rate coefficients for inter- and intramolecular hydrogen abstraction are also reported. Depending on R, the β-scission rate may vary by orders of magnitude. The predicted temperature dependence of the alcohol-to-ketone product ratios for alkoxy radical decomposition in a hydrocarbon environment is in remarkably close agreement with the corresponding ratios measured on the product mixtures from decomposition of tert-butyl and tert-amyl peroxyacetates in solution of n-heptane.

Rearrangement of the 1,1-Diphenylethoxyl Radical Is Not Concerted but Occurs through a Bridged Intermediate

Recent experimental observations cast some doubt on the conclusions of earlier work that indicated that the rearrangement of 1,1-diphenylethoxyl radical occurs through a bridged intermediate. We show, by using carefully benchmarked DFT calculations, that the title rearrangement is indeed a two-step reaction.

Kinetic analysis of the conversion of nonheme (alkylperoxo)iron(III) species to iron(IV) complexes

Low-spin mononuclear (alkylperoxo)iron(III) complexes decompose by peroxide O-O bond homolysis to form iron(IV) species. We examined the kinetics of previously reported homolysis reactions for (alkylperoxo)iron(III) intermediates supported by TPA (tris(2-pyridylmethyl)amine) in CH3CN solution and promoted by pyridine N-oxide, and by BPMCN (N,N-bis(2-pyridylmethyl)-N,N-dimethyl-trans-1,2-diaminocyclohexane) in its cis-beta configuration in CH3CN and CH2Cl2, as well as for the previously unreported chemistry of TPA and 5-Me3TPA intermediates in acetone. Each of these reactions forms an oxoiron(IV) complex, except for the beta-BPMCN reaction in CH2Cl2 that yields a novel (hydroxo)(alkylperoxo)iron(IV) product. Temperature-dependent rate measurements suggest a common reaction trajectory for each of these reactions and verify previous theoretical estimates of a ca. 60 kJ/mol enthalpic barrier to homolysis. However, both the tetradentate supporting ligand and exogenous ligands in the sixth octahedral coordination site significantly perturb the homolyses, such that observed rates can vary over 2 orders of magnitude at a given temperature. This is manifested as a compensation effect in which increasing activation enthalpy is offset by increasingly favorable activation entropy. Moreover, the applied kinetic model is consistent with geometric isomerism in the low-spin (alkylperoxo)iron(III) intermediates, wherein the alkylperoxo ligand is coordinated in either of the inequivalent cis sites afforded by the nonheme ligands.

Autoxidation of Ethylbenzene: The Mechanism Elucidated

Using a combined experimental and theoretical approach, we elucidated the mechanism of ethylbenzene autoxidation, at about 420 K. The generally accepted literature mechanism indeed fails to explain basic experimental observations, such as the high ketone to alcohol ratio. The hitherto overlooked propagation of 1-phenyl-ethylhydroperoxide, the primary chain product, is now unambiguously identified as the source of acetophenone as well as of 1-phenylethanol via a subsequent activated cage reaction. A similar mechanism allowed rationalizing of the cyclohexanone and cyclohexanol formation in the autoxidation of cyclohexane. The primary hydroperoxide product is found to react about 10 times faster than the arylalkane substrate with the chain carrying peroxyl radicals, whereas in cyclohexane autoxidation, this reactivity ratio is as high as 55. In combination with a lower efficiency of the above-mentioned cage reaction, this results in a rather high 1-phenyl-ethylhydroperoxide yield and causes a high ketone/alcohol ratio. Radicals are shown to be predominantly generated via a concerted bimolecular reaction of the hydroperoxide with the arylalkane substrate, producing alkyl and hydrated alkoxy free radicals. In this autoxidation system, no reaction product exhibits a major initiation-enhancing autocatalytic effect, as is the case with cyclohexanone in cyclohexane autoxidation. As a result, the conversion rate increases less sharply in time compared to cyclohexane autoxidation. In fact, even some slight inhibition can be observed, due to the formation of chain-terminating HO2* radicals in the alcohol co-oxidation. At 418 K, the chain length is estimated to be about 300-500 for conversions up to 10%.

Enhanced Activity and Selectivity in Cyclohexane Autoxidation by Inert H-Bond Acceptor Catalysts

Herein, we demonstrate that the chain-initiating dissociation of cyclohexyl hydroperoxide, CyOOH, is substantially accelerated by H-bond acceptors (e.g. Teflon), which assist O-O bond breaking by stabilising the leaving *OH radical. This is a completely new approach to boost the chain-propagating radical concentration. Indeed, up to now, literature has remained focussed on transition metal catalysis. In addition to this initiation effect, we demonstrate how inert perfluorinated compounds are also able to steer the selectivity at the molecular level, by promoting the conversion of the intermediate cyclohexyl hydroperoxide to the most desired end-product, cyclohexanone. This effect is explained by an enhanced, H-bond-assisted, hydroperoxide propagation. This hitherto overlooked hydroperoxide propagation was recently presented by us as the dominant cyclohexanone and cyclohexanol source. We herein thus confirm our previously reported autoxidation scheme, and illustrate its usefulness as a solid basis for designing new catalytic systems.