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

Quinone method dimers, (bio)synthetic intermediates en route to many naturally products derived from resveratrol, are potent radical-trapping antioxidants, besting the phenols from which they are derived and to which they can be converted.

Hydrogen atom transfer (HAT) is the mechanism by which the vast majority of radical-trapping antioxidants (RTAs), such as hindered phenols, inhibit autoxidation. As such, at least one weak O–H bond is the key structural feature which underlies the reactivity of phenolic RTAs. We recently observed that quinone methide dimers (QMDs) synthesized from hindered phenols are significantly more reactive RTAs than the phenols themselves despite lacking O–H bonds. Herein we describe our efforts to elucidate the mechanism by which they inhibit autoxidation. Four possible reaction paths were considered: (1) HAT from the C–H bonds on the carbon atoms which link the quinone methide moieties; (2) tautomerization or hydration of the quinone methide(s) in situ followed by HAT from the resultant phenolic O–H; (3) direct addition of peroxyl radicals to the quinone methide(s), and (4) homolysis of the weak central C–C bond in the QMD followed by combination of the resultant persistent phenoxyl radicals with peroxyl radicals. The insensitivity of the reactivity of the QMDs to substituent effects, solvent effects and a lack of kinetic isotope effects rule out the HAT reactions (mechanisms 1 and 2). Simple (monomeric) quinone methides, to which peroxyl radicals add, were found to be ca. 100-fold less reactive than the QMDs, ruling out mechanism 3. These facts, combined with the poor RTA activity we observe for a QMD with a stronger central C–C bond, support mechanism 4. The lack of solvent effects on the RTA activity of QMDs suggests that they may find application as additives to materials which contain H-bonding accepting moieties that can dramatically suppress the reactivity of conventional RTAs, such as phenols. This reactivity does not extend to biological membranes owing to the increased microviscosity of the phospholipid bilayer, which suppresses QMD dissociation in favour of recombination. Interestingly, the simple QMs were found to be very good RTAs in phospholipid bilayers – besting even the most potent form of vitamin E.

Base-Promoted C-C Bond Activation Enables Radical Allylation with Homoallylic Alcohols

The C_α–C_β bond in homoallylic alcohols can be activated under basic conditions, qualifying these nonstrained acyclic systems as radical allylation reagents. This reactivity is exemplified by photoinitiated (with visible light and/or blue LEDs) allylation of perfluoroalkyl and alkyl radicals generated from perfluoroalkyl iodides and alkylpyridinium salts, respectively, with homoallylic alcohols. C-radical addition to the double bond of the title reagents and subsequent base-promoted homolytic C_α–C_β cleavage leads to the formation of the corresponding allylated products along with ketyl radicals that act as single electron reductants to sustain the chain reactions. Substrate scope is documented and the role of base in the C–C bond activation is studied by computation.

Electrochemical Dimerization of Phenylpropenoids and the Surprising Antioxidant Activity of the Resultant Quinone Methide Dimers

A simple method for the dimerization of phenylpropenoid derivatives is reported. It leverages electrochemical oxidation of p-unsaturated phenols to access the dimeric materials in a biomimetic fashion. The mild nature of the transformation provides excellent functional group tolerance, resulting in a unified approach for the synthesis of a range of natural products and related analogues with excellent regiocontrol. The operational simplicity of the method allows for greater efficiency in the synthesis of complex natural products. Interestingly, the quinone methide dimer intermediates are potent radical-trapping antioxidants; more so than the phenols from which they are derived-or transformed to-despite the fact that they do not possess a labile H-atom for transfer to the peroxyl radicals that propagate autoxidation.

Trifunctional metal ion-catalyzed solvolysis: Cu(II)-promoted methanolysis of N,N-bis(2-picolyl) benzamides involves unusual Lewis acid activation of substrate, delivery of coordinated nucleophile, powerful assistance of the leaving group departure

The methanolyses of Cu(II) complexes of a series of N,N-bis(2-picolyl) benzamides (4a-g) bearing substituents X on the aromatic ring were studied under (s)(s)pH-controlled conditions at 25 °C. The active form of the complexes at neutral (s)(s)pH has a stoichiometry of 4:Cu(II):((-)OCH(3))(HOCH(3)) and decomposes unimolecularly with a rate constant k(x). A Hammett plot of log(k(x)) vs σ(x) values has a ρ(x) of 0.80 ± 0.05. Solvent deuterium kinetic isotope effects of 1.12 and 1.20 were determined for decomposition of the 4-nitro and 4-methoxy derivatives, 4b:Cu(II):((-)OCH(3))(HOCH(3)) and 4g:Cu(II):((-)OCH(3))(HOCH(3)), in the plateau region of the (s)(s)pH/log(k(x)) profiles in both CH(3)OH and CH(3)OD. Activation parameters for decomposition of these complexes are ΔH(++) = 19.1 and 21.3 kcal mol(-1) respectively and ΔS(++) = -5.1 and -2 cal K(-1) mol(-1). Density functional theory (DFT) calculations for the reactions of the Cu(II):((-)OCH(3))(HOCH(3)) complexes of 4a,b and g (4a, X = 3,5-dinitro) were conducted to probe the relative transition state energies and geometries of the different states. The experimental and computational data support a mechanism where the metal ion is coordinated to the N,N-bis(2-picolyl) amide unit and positioned so that it permits delivery of a coordinated Cu(II):((-)OCH(3)) nucleophile to the C═O in the rate-limiting transition state (TS) of the reaction. This proceeds to a tetrahedral intermediate INT, occupying a shallow minimum on the free energy surface with the Cu(II) coordinated to both the methoxide and the amidic N. Breakdown of INT is a virtually barrierless process, involving a Cu(II)-assisted departure of the bis(2-picolyl)amide anion. The analysis of the data points to a trifunctional role for the metal ion in the solvolysis mechanism where it activates intramolecular nucleophilic attack on the C═O group by coordination to an amidic N in the first step of the reaction and subsequently assists leaving group departure in the second step. The catalysis is very large; compared with the second order rate constant for methoxide attack on 4b, the computed reaction of CH3O(-) and 4b:Cu(II):(HOCH(3))(2) is accelerated by roughly 2.0 × 10(16) times.