Kinetic Solvent Effects on Hydrogen Abstraction from Phenol by the Cumyloxyl Radical. Toward an Understanding of the Role of Protic Solvents

Phenoxazine: A Privileged Scaffold for Radical-Trapping Antioxidants

Diphenylamines are widely used to protect petroleum-derived products from autoxidation. Their efficacy as radical-trapping antioxidants (RTAs) relies on a balance of fast H-atom transfer kinetics and stability to one-electron oxidation by peroxidic species. Both H-atom transfer and one-electron oxidation are enhanced by substitution with electron-donating substituents, such as the S-atom in phenothiazines, another important class of RTA. Herein we report the results of our investigations of the RTA activity of the structurally related, but essentially ignored, phenoxazines. We find that the H-atom transfer reactivity of substituted phenoxazines follows an excellent Evans-Polanyi correlation spanning k_inh = 4.5 × 10⁶ M^-1 s^-1 and N-H BDE = 77.4 kcal mol^-1 for 3-CN,7-NO₂-phenoxazine to k_inh = 6.6 × 10⁸ M^-1 s^-1 and N-H BDE = 71.8 kcal mol^-1 for 3,7-(OMe)₂-phenoxazine (37 °C). The reactivity of the latter compound is the greatest of any RTA ever reported and is likely to represent a reaction without an enthalpic barrier since log A for this reaction is likely ∼8.5. The very high reactivity of most of the phenoxazines studied required the determination of their kinetic parameters by inhibited autoxidations in the presence of a very strong H-bonding cosolvent (DMSO), which slowed the observed rates by up to 2 orders of magnitude by dynamically reducing the equilibrium concentration of (free) phenoxazine as an H-atom donor. Despite their remarkably high reactivity toward peroxyl radicals, the phenoxazines were found to be comparatively stable to one-electron oxidation relative to diphenylamines and phenothiazines (E° ranging from 0.59 to 1.38 V vs NHE). Thus, phenoxazines with comparable oxidative stability to commonly used diphenylamine and phenothiazine RTAs had significantly greater reactivity (by up to 2 orders of magnitude). Computations suggest that this remarkable balance in H-atom transfer kinetics and stability to one-electron oxidation results from the ability of the bridging oxygen atom in phenoxazine to serve as both a π-electron donor to stabilize the aminyl radical and σ-electron acceptor to destabilize the aminyl radical cation. Perhaps most excitingly, phenoxazines have "non-classical" RTA activity, where they trap >2 peroxyl radicals each, at ambient temperatures.

Tuning reactivity and selectivity in hydrogen atom transfer from aliphatic C-H bonds to alkoxyl radicals: role of structural and medium effects

Hydrogen atom transfer (HAT) is a fundamental reaction that takes part in a wide variety of chemical and biological processes, with relevant examples that include the action of antioxidants, damage to biomolecules and polymers, and enzymatic and biomimetic reactions. Moreover, great attention is currently devoted to the selective functionalization of unactivated aliphatic C-H bonds, where HAT based procedures have been shown to play an important role. In this Account, we describe the results of our recent studies on the role of structural and medium effects on HAT from aliphatic C-H bonds to the cumyloxyl radical (CumO(•)). Quantitative information on the reactivity and selectivity patterns observed in these reactions has been obtained by time-resolved kinetic studies, providing a deeper understanding of the factors that govern HAT from carbon and leading to the definition of useful guidelines for the activation or deactivation of aliphatic C-H bonds toward HAT. In keeping with the electrophilic character of alkoxyl radicals, polar effects can play an important role in the reactions of CumO(•). Electron-rich C-H bonds are activated whereas those that are α to electron withdrawing groups are deactivated toward HAT, with these effects being able to override the thermodynamic preference for HAT from the weakest C-H bond. Stereoelectronic effects can also influence the reactivity of the C-H bonds of ethers, amines, and amides. HAT is most rapid when these bonds can be eclipsed with a lone pair on an adjacent heteroatom or with the π-system of an amide functionality, thus allowing for optimal orbital overlap. In HAT from cyclohexane derivatives, tertiary axial C-H bond deactivation and tertiary equatorial C-H bond activation have been observed. These effects have been explained on the basis of an increase in torsional strain or a release in 1,3-diaxial strain in the HAT transition states, with kH(eq)/kH(ax) ratios that have been shown to exceed one order of magnitude. Medium effects on HAT from aliphatic C-H bonds to CumO(•) have been also investigated. With basic substrates, from large to very large decreases in kH have been measured with increasing solvent hydrogen bond donor (HBD) ability or after addition of protic acids or alkali and alkaline earth metal ions, with kinetic effects that exceed 2 orders of magnitude in the reactions of tertiary alkylamines and alkanamides. Solvent hydrogen bonding, protonation, and metal ion binding increase the electron deficiency and the strength of the C-H bonds of these substrates deactivating these bonds toward HAT, with the extent of this deactivation being modulated by varying the nature of the substrate, solvent, protic acid, and metal ion. These results indicate that through these interactions careful control over the HAT reactivity of basic substrates toward CumO(•) and other electrophilic radicals can be achieved, suggesting moreover that these effects can be exploited in an orthogonal fashion for selective C-H bond functionalization of substrates bearing different basic functionalities.

Model studies on the antioxidant activity of common terpenoid constituents of essential oils by means of the 2,2-diphenyl-1-picrylhydrazyl method

The research aims to indicate which of the structural elements of monoterpenes are responsible for their antioxidant activity. The activity was determined spectrophotometrically with the use of the DPPH(•) assay. It has been shown that π bonds are responsible for the chain-breaking antioxidant activity of monoterpenes. It has been proved, for the first time, that blocking of conjugated double bonds leads to a decrease of the antioxidant activity of monoterpenes. A probable reaction mechanism between monoterpenes and DPPH(•) has been proposed. It has been indicated that the antioxidant activity of monoterpenes strongly depends on the polarity of solvent used in the experiments. The presented results may stimulate additional research in the field of terpenoid antioxidants.

Anthrone and related hydroxyarenes: tautomerization and hydrogen bonding

The keto-enolization of hydroxyl-substituted naphthols and 9-anthrols has been investigated by means of CBS-QB3 calculations. An excellent agreement between experiment and theory is found for the energetics for the anthrone (5) ⇌ anthrol (6) equilibrium, with an enthalpy of tautomerization, Δ(t)H, of 3.8 kcal mol(-1). In contrast, 1-naphthol is the preferred tautomer with a Δ(t)H = -9.0 kcal mol(-1). Substitution of the hydrogens at the adjacent carbons by hydroxyl groups leads to the formation of strong intramolecular hydrogen bonds within a six-membered ring in the enones and the enols. Due to the difference in the intramolecular hydrogen bond enthalpy, Δ(HB)H(intra), the equilibrium shifts further to the enone. Thus, for 1,8-dihydroxy-anthrone (anthralin, dithranol) Δ(t)H increases to 12.7 kcal mol(-1) with an enol/enone ratio of 10(-10). The solvent effect on the 5 ⇌ 6 equilibrium has been quantified by considering the formation of intermolecular hydrogen bond(s), leading to an acidity parameter α₂(H) for anthrol of 0.42. It is shown that the hydrogen bond donating ability of bulk methanol is greatly attenuated through the formation of cyclic oligomers. The benzylic and phenolic bond dissociation enthalpies for anthrone up to anthralin suggest some antioxidant potency but the precise (radical) mechanism of action remains uncertain.