Quinone methide dimers lacking labile hydrogen atoms are surprisingly excellent radical-trapping antioxidants

Interrupted Homolytic Substitution Enables Organoboron Compounds to Inhibit Radical Chain Reactions Rather than Initiate Them

The reactions of organoboranes with peroxyl radicals are key to their use as radical initiators for a vast array of radical chain reactions, particularly at low temperatures where high stereoselectivity or regioselectivity is desired. Whereas these reactions generally proceed via concerted homolytic substitution (SH2) mechanisms, organoboranes that bear groups that can stabilize tetracoordinate boron radical "ate" complexes (e.g., catecholboranes) undergo this reaction via a stepwise addition/fragmentation sequence and serve as useful stoichiometric alkyl radical precursors. Here we show that arylboronic esters and amides derived from catecholborane and diaminonaphthaleneborane, respectively, are potent radical-trapping antioxidants (RTAs). Mechanistic studies reveal that this is because the radical "ate" complexes derived from peroxyl radical addition to boron are sufficiently persistent to trap another radical in an interrupted SH2 reaction. Remarkably, the reactivity of these organoboranes as inhibitors of autoxidation was shown to translate from simple hydrocarbons to the phospholipids of biological membranes such that they can inhibit ferroptosis, the cell death modality driven by lipid autoxidation and relevant in neurodegeneration and other major pathologies. The unique mechanism of these organoboranes is one of only a handful of RTA mechanisms that are not based on H-atom transfer processes and provide a new dimension to boron chemistry and its applications.

Leveraging the Persistent Radical Effect in the Synthesis of trans-2,3-Diaryl Dihydrobenzofurans

A simple method for accessing trans-2,3-diaryl dihydrobenzofurans is reported. This approach leverages the equilibrium between quinone methide dimers and their persistent radicals. This equilibrium is disrupted by phenols that yield comparatively transient phenoxyl radicals, leading to cross-coupling between the persistent and transient radicals. The resultant quinone methides with pendant phenols rapidly cyclize to form dihydrobenzofurans (DHBs). This putative biomimetic access to dihydrobenzofurans provides superb functional group tolerance and a unified approach for the synthesis of resveratrol-based natural products.

Iron-Catalyzed Oxidative C-O and C-N Coupling Reactions Using Air as Sole Oxidant

We describe the oxygenation of tertiary arylamines, and the amination of tertiary arylamines and phenols. The key step of these coupling reactions is an iron‐catalyzed oxidative C−O or C−N bond formation which generally provides the corresponding products in high yields and with excellent regioselectivity. The transformations are accomplished using hexadecafluorophthalocyanine−iron(II) (FePcF₁₆) as catalyst in the presence of an acid or a base additive and require only ambient air as sole oxidant.

Unexpected Role of pH and Microenvironment on the Antioxidant and Synergistic Activity of Resveratrol in Model Micellar and Liposomal Systems

Experimental and theoretical studies indicate that resveratrol (RSV, dietary polyphenol that effectively reduces cellular oxidative stress) is a good scavenger of hydroxyl, alkoxyl, and peroxyl radicals in homogeneous systems. However, the role of RSV as a chain-breaking antioxidant is still questioned. Here, we describe pH dependent effectiveness of RSV as an inhibitor of peroxidation of methyl linoleate in Triton X-100 micelles and in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes, with the best effectiveness at pH 6 (stoichiometric factors, n, are 4.9 and 5.6, and the rate constants for reaction with peroxyl radicals, k_inh, are 1200 and 3300 M^–1 s^–1 in micellar and liposomal systems, respectively). We propose the mechanism in which RSV-derived radicals are coupled to dimers with recovered ability to trap lipidperoxyl radicals. The formation of such dimers is facilitated due to increased local concentration of RSV at the lipid–water interface. Good synergy of RSV with α-tocopherol analogue in micelles and liposomes is in contrast to the previously reported lack of synergy in non-polar solvents; however, the increased persistency of tocopheroxyl radicals in dispersed lipid/water systems and proximal localization of both antioxidants greatly facilitate the possible recovery of α-TOH by RSV.

Radical-Trapping Antioxidant Activity of Copper and Nickel Bis(Thiosemicarbazone) Complexes Underlies Their Potency as Inhibitors of Ferroptotic Cell Death

Herein we demonstrate that copper(II)-diacetyl-bis(N⁴-methylthiosemicarbazone)(CuATSM), clinical candidate for the treatment of ALS and Parkinson's disease, is a highly potent radical-trapping antioxidant (RTA) and inhibitor of (phospho)lipid peroxidation. In THF autoxidations, CuATSM reacts with THF-derived peroxyl radicals with k_inh = 2.2 × 10⁶ M^-1 s^-1─roughly 10-fold greater than α-tocopherol (α-TOH), Nature's best RTA. Mechanistic studies reveal no H/D kinetic isotope effects and a lack of rate-suppressing effects from H-bonding interactions, implying a different mechanism from α-TOH and other canonical RTAs, which react by H-atom transfer (HAT). Similar reactivity was observed for the corresponding Ni²⁺ complex and complexes of both Cu²⁺ and Ni²⁺ with other bis(thiosemicarbazone) ligands. Computations corroborate the experimental finding that rate-limiting HAT cannot account for the observed RTA activity and instead suggest that the reversible addition of a peroxyl radical to the bis(thiosemicarbazone) ligand is responsible. Subsequent HAT or combination with another peroxyl radical drives the reaction forward, such that a maximum of four radicals are trapped per molecule of CuATSM. This sequence is supported by spectroscopic and mass spectrometric experiments on isolated intermediates. Importantly, the RTA activity of CuATSM (and its analogues) translates from organic solution to phospholipid bilayers, thereby accounting for its (their) ability to inhibit ferroptosis. Experiments in mouse embryonic fibroblasts and hippocampal cells reveal that lipophilicity as well as inherent RTA activity contribute to the potency of ferroptosis rescue, and that one compound (CuATSP) is almost 20-fold more potent than CuATSM and among the most potent ferroptosis inhibitors reported to date.

Autoxidation vs. antioxidants - the fight for forever

Autoxidation limits the longevity of essentially all hydrocarbons and materials made therefrom - including us. The radical chain reaction responsible often leads to complex mixtures of hydroperoxides, alkyl peroxides, alcohols, carbonyls and carboxylic acids, which change the physical properties of the material - be it a lubricating oil or biological membrane. Autoxidation is inhibited by addtitives such as radical-trapping antioxidants, which intervene directly in the chain reaction. Herein we review the most salient features of autoxidation and its inhibition, emphasizing concepts and mechanistic considerations important in understanding this chemistry across the wide range of contexts in which it is relevant.

Temperature-Dependent Effects of Alkyl Substitution on Diarylamine Antioxidant Reactivity

Alkylated diphenylamines are among the most efficacious radical-trapping antioxidants (RTAs) for applications at elevated temperatures since they are able to trap multiple radical equivalents due to catalytic cycles involving persistent diphenylnitroxide and diphenylaminyl radical intermediates. We have previously shown that some heterocyclic diarylamine RTAs possess markedly greater efficacy than typical alkylated diphenylamines, and herein, report on our efforts to identify optimal alkyl substitution of the scaffold, which we had found to be the ideal compromise between reactivity and stability. Interestingly, the structure-activity relationships differ dramatically with temperature: para-alkyl substitution slightly increased reactivity and stoichiometry at 37 and 100 °C due to more favorable (stereo)electronic effects and corresponding diarylaminyl/diarylnitroxide formation, while ortho-alkyl substitution slightly decreased both reactivity and stoichiometry. No such trends were evident at 160 °C; instead, the compounds were segregated into two groups based on the presence/absence of benzylic C-H bonds. Electron spin resonance spectroscopy indicates that increased efficacy was associated with lesser diarylnitroxide formation, and deuterium-labeling suggests that this is due to abstraction of the benzylic H atom, precluding nitroxide formation. Computations predict that this reaction path is competitive with established fates of the diarylaminyl radical, thereby minimizing the formation of off-cycle products and leading to significant gains in high-temperature RTA efficacy.