Mario C. Foti

Reaction of phenols with the 2,2-diphenyl-1-picrylhydrazyl radical. Kinetics and DFT calculations applied to determine ArO-H bond dissociation enthalpies and reaction mechanism.

The formal H-atom abstraction by the 2,2-diphenyl-1-picrylhydrazyl (dpph(*)) radical from 27 phenols and two unsaturated hydrocarbons has been investigated by a combination of kinetic measurements in apolar solvents and density functional theory (DFT). The computed minimum energy structure of dpph(*) shows that the access to its divalent N is strongly hindered by an ortho H atom on each of the phenyl rings and by the o-NO(2) groups of the picryl ring. Remarkably small Arrhenius pre-exponential factors for the phenols [range (1.3-19) x 10(5) M(-1) s(-1)] are attributed to steric effects. Indeed, the entropy barrier accounts for up to ca. 70% of the free-energy barrier to reaction. Nevertheless, rate differences for different phenols are largely due to differences in the activation energy, E(a,1) (range 2 to 10 kcal/mol). In phenols, electronic effects of the substituents and intramolecular H-bonds have a large influence on the activation energies and on the ArO-H BDEs. There is a linear Evans-Polanyi relationship between E(a,1) and the ArO-H BDEs: E(a,1)/kcal x mol(-1) = 0.918 BDE(ArO-H)/kcal x mol(-1) - 70.273. The proportionality constant, 0.918, is large and implies a late or product-like transition state (TS), a conclusion that is congruent with the small deuterium kinetic isotope effects (range 1.3-3.3). This Evans-Polanyi relationship, though questionable on theoretical grounds, has profitably been used to estimate several ArO-H BDEs. Experimental ArO-H BDEs are generally in good agreement with the DFT calculations. Significant deviations between experimental and DFT calculated ArO-H BDEs were found, however, when an intramolecular H-bond to the O(*) center was present in the phenoxyl radical, e.g., in ortho semiquinone radicals. In these cases, the coupled cluster with single and double excitations correlated wave function technique with complete basis set extrapolation gave excellent results. The TSs for the reactions of dpph(*) with phenol, 3- and 4-methoxyphenol, and 1,4-cyclohexadiene were also computed. Surprisingly, these TS structures for the phenols show that the reactions cannot be described as occurring exclusively by either a HAT or a PCET mechanism, while with 1,4-cyclohexadiene the PCET character in the reaction coordinate is much better defined and shows a strong pi-pi stacking interaction between the incipient cyclohexadienyl radical and a phenyl ring of the dpph(*) radical.

Use and Abuse of the DPPH• Radical

The 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical is approaching 100 years from its discovery in 1922 by Goldschmidt and Renn. This radical is colored and remarkably stable, two properties that have made it one of the most popular radicals in a wide range of studies. First, there is the evaluation of the antioxidant abilities of phenols and other natural compounds (A-H) through a test that-at a closer look-is utterly inappropriate. In fact, the test-derived EC50, that is, the concentration of A-H able to scavenge 50% of the initial DPPH(•), is not a kinetic parameter and hence its purported correlation with the antioxidant properties of chemicals is not justified. Kinetic measurements, such as the second-order rate constants for H-atom abstraction from A-H by DPPH(•), in apolar media, are the only useful parameters to predict the antioxidant ability of A-H. Other applications of DPPH(•) include kinetic and mechanistic studies, kinetic solvent effects, EPR spectroscopy, polymer chemistry, and many more. In this review these applications are evaluated in detail by showing the usefulness of some and the uselessness of others. The chemistry of DPPH(•) is also briefly reviewed.

ANTIOXIDANT ACTIVITY OF PHENOLIC MERODITERPENOIDS FROM MARINE ALGAE

Abstract The overall 1 O 2 quenching rate constants k Q for three meroditerpenoids from Mediterranean marine algae of the genus Cystoseira were measured using 1,4-dimethylnaphthalene 1,4-endoperoxide as a thermal source of 1 O 2 ( 1 Δ g ). The most active of the compounds tested had a k Q value which is comparable with that of α-tocopherol. These metabolites also act as inhibitors of methyl linoleate peroxidation, while their O .− 2 quenching activity is rather low, similar to α-tocopherol.

Kinetics of the Oxidation of Quercetin by 2,2-Diphenyl-1-picrylhydrazyl (dpph•)

In methanol/water, dpph(•) bleaching (519 nm) by quercetin, QH(2), exhibits biphasic kinetics. The dpph(•) reacts completely with the quercetin anion within 100 ms. Subsequent slower bleaching involves solvent and QH(2) addition to quinoid products. The fast reaction is first-order in dpph(•) but only ca. 0.38 order in [QH(2)]. This extraordinary nonintegral order is attributed to reversible formation of π-stacked {QH(-)/dpph(•)} complexes in which electron transfer to products, {QH(•)/dpph(-)}, is slow (k(ET) ≈ 10(5) s(-1)).

Brasilane-type sesquiterpenoids from the mediterranean red alga Laurencia obtusa

Abstract Three new irregular sesquiterpenoids were isolated and characterized, along with epi -brasilenol, from the Mediterranean red alga Laurencia obtusa . They possess the unusual skeleton of brasilane and one of them contains the unprecedented 5, 6-double bond (instead of a 1,6-double bond). Their structures were determined by detailed spectra analysis. The relative configurations were assigned by NOESY spectroscopy, molecular mechanics calculation (MM2) and, in one case, by a quantitative computer simulation of the lanthanide induced shifts in the 1 H NMR spectrum.

A Meta Effect in Nonphotochemical Processes: The Homolytic Chemistry of m-Methoxyphenol

The m-methoxy group is normally electron-withdrawing (EW), sigma(m) = +0.12, sigma(m+) = +0.05. The strong EW activity of a phenoxyl radicals O* atom causes the m-methoxy group to become electron-donating (ED), sigma(m)(+) = -0.14. In valence bond terms, this can be ascribed to the nonclassical resonance structures 1c-e. Although it has long been known that m-methoxy is ED in photoexcited states, it has now been found to be ED for homolytic O-H bond breaking in ground-state 3-methoxyphenol.

The structure of laurobtusol, a new rearranged sesquiterpenoid from the mediterranean red alga laurencia obtusa

Abstract An alcohol with a new tricyclic humulane skeleton, laurobtusol was isolated from the Mediterranean Red Alga Laurencia obtusa. The structure was established mainly by 2 D NMR methods and the relative configuration was assigned by a quantitative computer simulation of the lanthanide induced shifts in the 1 H NMR spectrum and Molecular Mechanics calculation (MM2).