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

Near-UV and Visible Light Degradation of Iron (III)-Containing Citrate Buffer: Formation of Carbon Dioxide Radical Anion via Fragmentation of a Sterically Hindered Alkoxyl Radical

Citrate is a commonly used buffer in pharmaceutical formulations which forms complexes with adventitious metals such as Fe³⁺. Fe³⁺-citrate complexes can act as potent photosensitizers under near-UV and visible light exposure, and recent studies reported evidence for the photo-production of a powerful reductant, carbon dioxide radical anion (^•CO₂^-), from Fe³⁺-citrate complexes (Subelzu, N.; Schöneich, N., Mol. Pharm. 2020, 17, 4163-4179). The mechanisms of ^•CO₂^- formation are currently unknown but must be established to devise strategies against ^•CO₂^- formation in pharmaceutical formulations which rely on the use of citrate buffer. In this study, we first established complementary evidence for the photolytic generation of ^•CO₂^- from Fe³⁺-citrate through spin trapping and electron paramagnetic resonance (EPR) spectroscopy, and subsequently used spin trapping in conjunction with tandem mass spectrometry (MS/MS) for mechanistic studies on the pathways of ^•CO₂^- formation. Experiments with stable isotope-labeled citrate suggest that the central carboxylate group of citrate is the major source of ^•CO₂^-. Competition studies with various inhibitors (alcohols and dimethyl sulfoxide) reveal two mechanisms of ^•CO₂^- formation, where one pathway involves β-cleavage of a sterically hindered alkoxyl radical generated from the hydroxyl group of citrate.

Factors Governing Reactivity and Selectivity in Hydrogen Atom Transfer from C(sp3)-H Bonds of Nitrogen-Containing Heterocycles to the Cumyloxyl Radical

A kinetic study of the hydrogen atom transfer (HAT) reactions from nitrogen-containing heterocycles (secondary and tertiary lactams, 2-imidazolidinones, 2-oxazolidinones, and succinimides) to the cumyloxyl radical has been carried out employing laser flash photolysis with ns time resolution. HAT occurs from the C–H bonds that are α to nitrogen, activated by hyperconjugative overlap with the N–C=O π system. In the lactam series, the second-order HAT rate constant (k_H) was observed to decrease by a factor of ∼4 going from the five- and six-membered ring derivatives to the eight-membered ones, a behavior that was rationalized on the basis of a reduced extent of hyperconjugative activation associated to the greater flexibility of the larger rings compared to the smaller ones. In the five-membered-ring substrate series, the k_H values were observed to increase by >3 orders of magnitude on going from succinimide to 2-imidazolidinones, a behavior that was explained in terms of the divergent contribution of hyperconjugative activation and deactivating electronic effects determined by ring functionalities. The results are discussed in the framework of the development of HAT-based C–H bond functionalization procedures.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

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.

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.

Hydrogen atom transfer from 1,2- and 1,3-diols to the cumyloxyl radical. The role of structural effects on metal-ion induced C-H bond deactivation

Strong C-H bond deactivation toward HAT has been observed in the reactions of the cumyloxyl radical with 1,2- and 1,3-diols, following addition of Li⁺ and Ca²⁺. Weaker effects have been observed with Mg²⁺. The role of the substrate structure and of the metal ion in the formation of Lewis acid-base complexes is discussed.

Activation and Deactivation Strategies Promoted by Medium Effects for Selective Aliphatic C-H Bond Functionalization

Selective functionalization of unactivated aliphatic C-H bonds represents an important goal of modern synthetic chemistry. Differentiating between such bonds in organic molecules with high levels of selectivity remains a crucial issue, and a profound understanding of even the subtlest reactivity trends is needed. Among the methods that have been developed, those based on hydrogen atom transfer (HAT) have attracted considerable interest. Within this framework, medium effects have proved effective in altering the reactivity and site selectivity in synthetically useful C-H functionalization procedures. In this Review, the mechanistic features behind the available strategies are discussed. It is shown that hydrogen bonding and acid-base interactions can promote C-H bond activation or deactivation toward HAT reagents, thereby providing fine-control over the site selectivity and product chemoselectivity as well as useful guidelines for future development and applications.

The Quest for Selectivity in Hydrogen Atom Transfer Based Aliphatic C-H Bond Oxygenation

Aliphatic C-H bond functionalization is at the frontline of research because it can provide straightforward access to simplified and cost-effective synthetic procedures. A number of these methodologies are based on hydrogen atom transfer (HAT), which, as a consequence of the inert character of C-H bonds, often represents the most challenging step of the overall process. Because the majority of organic molecules contain multiple nonequivalent C-H bonds that display similar chemical properties, differentiating between these bonds with high levels of selectivity represents one of the most challenging issues. Clarification of the factors that govern the relative reactivity of C-H bonds toward HAT reagents is thus of primary importance in order to develop selective functionalization procedures. In this Account we describe, through the combination of kinetic studies employing a genuine HAT reagent such as the cumyloxyl radical, along with oxidations performed with H₂O₂ and iron or manganese catalysts, our contribution toward the development of selective C-H functionalization methodologies. Despite the different nature of these reagents, an oxygen-centered radical and a metal-oxo species, congruent reactivity and selectivity patterns have emerged, providing strong evidence that both reactions proceed via HAT. Consequently, selectivity in this class of metal catalyzed C-H oxidations can be reasonably predicted and synthetically exploited. Amides have been identified as preferential functional groups for governing selectivity on the basis of electronic, steric, and stereoelectronic effects. Torsional effects have proven moreover to be particularly important C-H directing factors in the oxidation of cyclohexane scaffolds where a delicate balance of these effects, in synergistic combination with catalyst design, enables highly chemoselective and enantioselective oxidations. Medium effects have been also shown to govern the relative HAT reactivity of C-H bonds in proximity to polar, hydrogen bond acceptor (HBA) functional groups. By engaging in hydrogen bonding with these groups, fluorinated alcohols strongly deactivate proximal C-H bonds toward HAT-based oxidation. As a result, alcohols, ethers, amines, and amides, which are electron rich and effective proximal C-H activating groups toward HAT reagents in conventional solvents, become oxidatively robust deactivating functionalities that direct C-H oxidation toward remote positions. These deactivating effects enable moreover the accomplishment of product chemoselective methylenic hydroxylations. Overall, clarification of the factors that govern HAT-based reactions has served to provide unique examples of catalytic methodologies for chemoselective and enantioselective oxidation of nonactivated aliphatic C-H bonds of potential utility in organic synthesis.