Derek A. Pratt

Garlic: Source of the Ultimate Antioxidants—Sulfenic Acids†

ion of an allylic H atom. Clearly, allicin is not directly responsible for the inhibition of ML autoxidation; instead, an allicin decomposition product must be responsible. We also explored the effects of added hydrogen-bondacceptor (HBA) solvents on allicin-inhibited autoxidation reactions. Although the addition of CH3CN (1m) to a solution of allicin in chlorobenzene had little effect on the rate of decomposition of allicin (Figure 1b), it markedly reduced the ability of allicin to inhibit the autoxidation of ML (Figure 1a). Thus, the use of freshly purified (HPLC) allicin in chlorobenzene in the presence of CH3CN (1m) led to an almost eightfold increase in the rate of inhibited autoxidation, and the stoichiometric factor dropped from n= 0.95 to 0.69. Since allicin itself is not the radical-trapping antioxidant in these autoxidation reactions, we cannot derive an inhibition rate constant (kinh) from the autoxidation data in the customary way; however, the drop in antioxidant activity in HBA solvents is informative. This type of solvent effect is well documented for reactions of peroxyl radicals with H-atom donors that are HBDs, such as phenols. The decomposition product of allicin, 2-propenesulfenic acid, is expected to be a strong HBD that will form strong hydrogen bonds with CH3CN (Scheme 1) and thus lead to a decrease in the rate of the inhibited autoxidation, as observed. Although these experiments demonstrate that 2-propenesulfenic acid is highly likely to be the peroxyl-radical scavenger in allicin-inhibited autoxidation reactions, the reason that sulfenic acids appear to be such effective antioxidants is currently unclear. Since the O H bonddissociation enthalpy (BDE) of an H-atom donor, XOH (e.g., phenols, X=Ar), is the key thermodynamic parameter connected with the rate of reaction of XOH with peroxyl radicals, it would be desirable to compare the O H BDE of 2-propenesulfenic acid and other sulfenic acids with the O H BDEs of their isoelectronic cousins, the hydroperoxides (which react with peroxyl radicals much more slowly, with Figure 1. a) Thermally initiated (azobisisobutyronitrile (AIBN), 40 mm) autoxidation of methyl linoleate (91 mm) at 37 8C in chlorobenzene containing allicin (50 mm) and HFPA (0.15m, *), CH3CN (1m, &), or no additive (~). b) Decomposition of allicin (50 mm) at 37 8C in chlorobenzene containing HFPA (0.15m, *), CH3CN (1m, &), or no additive (~). Scheme 1. Mechanistic scheme to explain the observed increase in the rate of allicin-inhibited autoxidation in the presence of acetonitrile. Zuschriften 164 www.angewandte.de 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2009, 121, 163 –166 rate constants of approximately 10–10m 1 s ). Unfortunately, no sulfenic acid O HBDE has ever been reported. As the reliable determination of a sulfenic acid O H BDE appeared to be experimentally impractical (owing to the transient nature of most known sulfenic acids), we calculated these BDEs by using a method known to accurately predict O H BDEs, the complete-basis-set approach of Petersson and co-workers (Table 1). Table 1 reveals two important features of the O H BDEs of sulfenic acids. First, the values are much lower than those for the analogous hydroperoxides. For the simplest members of the series (hydrosulfenic acid and hydrogen peroxide), the difference is 14.2 kcalmol . For the alkanesulfenic acids and alkyl hydroperoxides, the difference is even larger: almost 18 kcalmol 1 for methane-, 2-propene-, and a-toluenesulfenic acids and the corresponding hydroperoxides. Thus, the sulfur atom imposes two effects: It stabilizes the radical to a greater extent, largely by delocalizing the unpaired electron onto itself (the unpaired spin distribution is close to 50:50 between the oxygen and sulfur atoms in sulfinyl radicals and 70:30 between the terminal and nonterminal oxygen atoms in peroxyl radicals), and this results in greater interactions with substituents that are bonded to the sulfur atom. This effect leads to a greater difference between hydrosulfenic acid and the alkanesulfenic acids (ca. 4.5 kcalmol ) than between hydrogen peroxide and the alkyl hydroperoxides (ca. 1 kcal mol ). The O H BDEs of sulfenic acids are among the weakest known and comparable to those of hydroxylamines, such as TEMPO-H (70.7 kcalmol ; TEMPO= 2,2,6,6-tetramethylpiperidin-1-oxyl). 20] To provide additional insight into the reactions of sulfenic acids with peroxyl radicals, we also calculated the transitionstate (TS) structures (Figure 2) and associated activation energies of some representative reactions (Table 2). Two TS structures were identified: one with a cisoid geometry with respect to the oxygen atoms between which the H atom is being transferred (Figure 2a) and one with a transoid geometry (Figure 2b). The cisoid TSs were found to be lower in energy by approximately 6–7 kcalmol 1 than the transoid TSs (Table 2). The highest occupied molecular orbitals (HOMOs) of the cisoid and transoid TS structures (which comprise the four possible combinations of the two p* orbitals on the sulfinyl and peroxyl fragments between which the H atom is transferred) reveal why the cisoid geometry is favored. Whereas the singly occupied (SO) HOMO and HOMO-1 in the cisoid (Figure 2c) and transoid TS structures are very similar, HOMO-2 and HOMO-3 are quite different in the two TS geometries: There is significant bonding overlap between the sulfenic acid sulfur atom and the internal oxygen atom of the peroxyl radical in the cisoid structure, an interaction that is absent in the transoid structure. This bonding overlap suggests that the electron to be transferred from the sulfenic acid to the peroxyl radical is partly localized on the peroxyl radical in the cisoid TS and thus indicates a mechanism based on proton-coupled electron transfer (PCET). As in other PCETreactions, H-atom transfer between a sulfenic acid and a peroxyl radical is predicted to involve initial formation of a hydrogen-bonded complex, RSOH··· COOR’. For the reactions we investigated, in which R and R’ were alkyl groups, these complexes lie some 4.5 to 5.0 kcal mol 1 lower in energy than the separated reactants. This result implies that the corresponding TSs are slightly lower in energy than the separated reactants. These reactions are, therefore, predicted to be diffusion-controlled, consistent with Koelewijn and Berger s estimate that the rate constant for the reaction of 2-methyl-2-propanesulfenic acid with Table 1: O H BDEs (in kcalmol ) calculated by the CBS-QB3 method for some sulfenic acids and hydroperoxides.

The Reaction of Sulfenic Acids with Peroxyl Radicals: Insights into the Radical‐Trapping Antioxidant Activity of Plant‐Derived Thiosulfinates

Sulfenic acids play a prominent role in biology as key participants in cellular signaling relating to redox homeostasis, in the formation of protein-disulfide linkages, and as the central players in the fascinating organosulfur chemistry of the Allium species (e.g., garlic). Despite their relevance, direct measurements of their reaction kinetics have proven difficult owing to their high reactivity. Herein, we describe the results of hydrocarbon autoxidations inhibited by the persistent 9-triptycenesulfenic acid, which yields a second order rate constant of 3.0×10(6) u2009M(-1) u2009s(-1) for its reaction with peroxyl radicals in PhCl at 30u2009°C. This rate constant drops 19-fold in CH(3)CN, and is subject to a significant primary deuterium kinetic isotope effect, k(H)/k(D) = 6.1, supporting a formal H-atom transfer (HAT) mechanism. Analogous autoxidations inhibited by the Allium-derived (S)-benzyl phenylmethanethiosulfinate and a corresponding deuterium-labeled derivative unequivocally demonstrate the role of sulfenic acids in the radical-trapping antioxidant activity of thiosulfinates, through the rate-determining Cope elimination of phenylmethanesulfenic acid (k(H)/k(D) ≈ 4.5) and its subsequent formal HAT reaction with peroxyl radicals (k(H)/k(D) ≈ 3.5). The rate constant that we derived from these experiments for the reaction of phenylmethanesulfenic acid with peroxyl radicals was 2.8×10(7) u2009M(-1) u2009s(-1); a value 10-fold larger than that we measured for the reaction of 9-triptycenesulfenic acid with peroxyl radicals. We propose that whereas phenylmethanesulfenic acid can adopt the optimal syn geometry for a 5-centre proton-coupled electron-transfer reaction with a peroxyl radical, the 9-triptycenesulfenic is too sterically hindered, and undergoes the reaction instead through the less-energetically favorable anti geometry, which is reminiscent of a conventional HAT.

A Simple Cu-Catalyzed Coupling Approach to Substituted 3-Pyridinol and 5-Pyrimidinol Antioxidants

A convenient approach to 3-pyridinols and 5-pyrimidinols via a two-step Cu-catalyzed benzyloxylation/catalytic hydrogenation sequence is presented. The corresponding 3-pyridinamines and 5-pyrimidinamines can be prepared in an analogous sequence utilizing benzylamine in lieu of benzyl alcohol. The radical-scavenging ability of these derivatives are preliminarily explored and reveal that the increased acidities of the pyridinols and pyrimidinols render them susceptible to more significant kinetic solvent effects when compared to phenols.

Preparation and Investigation of Vitamin B6-Derived Aminopyridinol Antioxidants

3-Pyridinols bearing amine substitution para to the hydroxylic moiety have previously been shown to inhibit lipid peroxidation more effectively than typical phenolic antioxidants, for example, α-tocopherol. We report here high-yielding, large-scale syntheses of mono- and bicyclic aminopyridinols from pyridoxine hydrochloride (i.e., vitamin B(6)). This approach provides straightforward, scaleable access to novel, potent, molecular scaffolds whose antioxidant properties have been investigated in homogeneous solutions and in liposomal vesicles. These molecular aggregates mimic cell membranes that are the targets of oxidative damage in vivo.

TEMPO reacts with oxygen-centered radicals under acidic conditions.

In the presence of organic acids in organic media, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) reacts with peroxyl radicals at nearly diffusion-controlled rates by proton-coupled electron transfer from the protonated nitroxide.

Pyridine and pyrimidine analogs of acetaminophen as inhibitors of lipid peroxidation and cyclooxygenase and lipoxygenase catalysis.

Herein we report an investigation of the efficacy of pyridine and pyrimidine analogs of acetaminophen (ApAP) as peroxyl radical-trapping antioxidants and inhibitors of enzyme-catalyzed lipid peroxidation by cyclooxygenases (COX) and lipoxygenases (LOX). In inhibited autoxidations we find that ApAP, the common analgesic and antipyretic agent, is a very good antioxidant with a rate constant for reaction with peroxyl radicals (k(inh) = 5 x 10(5) M(-1) s(-1)) that is higher than many widely-used phenolic antioxidants, such as the ubiquitous butylated hydroxytoluene (BHT). This reactivity is reduced substantially upon incorporation of nitrogen into the phenolic ring, owing to an increase in the O-H bond dissociation enthalpy of pyridinols and pyrimidinols with respect to phenols. Incorporation of nitrogen into the phenolic ring of ApAP was also found to decrease its efficacy as an inhibitor of prostaglandin biosynthesis by ovine COX-1 (oCOX-1). This is explained on the basis of an increase in its oxidation potential and its reduced reactivity as a reducing co-substrate of the peroxidase protoporphyrin. In contrast, the efficacy of ApAP as an inhibitor of lipid hydroperoxide biosynthesis by soybean LOX-1 (sLOX-1) increased upon incorporation of nitrogen into the ring, suggesting a different mechanism of inhibition dependent on the acidity of the phenolic O-H which may involve chelation of the catalytic non-heme iron atom. The greater stability of the 3-pyridinols and 5-pyrimidinols to air oxidation as compared to phenols allowed us to evaluate some electron-rich pyridinols and pyrimidinols as inhibitors of oCOX-1 and sLOX-1. While the pyridinols had the best combination of activities as antioxidants and inhibitors of oCOX-1 and sLOX-1, they were found to be more toxic than ApAP in preliminary assays in human hepatocellular carcinoma (HepG2) cell culture. The pyrimidinols, however, were up to 17-fold more reactive to peroxyl radicals and up to 25-fold better inhibitors of prostaglandin biosynthesis than ApAP, with similar cytotoxicities to HepG2 cells at high levels of exposure.

Preparation of Highly Reactive Pyridine- and Pyrimidine-Containing Diarylamine Antioxidants

We recently reported a preliminary account of our efforts to develop novel diarylamine radical-trapping antioxidants (Hanthorn, J. J. et al. J. Am. Chem. Soc. 2012, 134, 8306-8309) wherein we demonstrated that the incorporation of ring nitrogens into diphenylamines affords compounds which display a compromise between H-atom transfer reactivity to peroxyl radicals and stability to one-electron oxidation. Herein we provide the details of the synthetic efforts associated with that report, which have been substantially expanded to produce a library of substituted heterocyclic diarylamines that we have used to provide further insight into the structure-reactivity relationships of these compounds as antioxidants (see the accompanying paper, DOI: 10.1021/jo301012x). The diarylamines were prepared in short, modular sequences from 2-aminopyridine and 2-aminopyrimidine wherein aminations of intermediate pyri(mi)dyl bromides and then Pd-catalyzed cross-coupling reactions of the amines and precursor bromides were the key steps to yield the diarylamines. The cross-coupling reactions were found to proceed best with Pd(η(3)-1-PhC(3)H(4))(η(5)-C(5)H(5)) as precatalyst, which gave higher yields than the conventional Pd source, Pd(2)(dba)(3).

Synthesis of Pyrrolnitrin and Related Halogenated Phenylpyrroles

A general approach to halogenated arylpyrroles, including the antifungal natural product pyrrolnitrin, is described using newly synthesized halogenated pyrroles and 2,6-disubstituted nitrobenzenes or 2,6-disubstituted anilines.

On the role of alkylcobalamins in the vitamin B12-catalyzed reductive dehalogenation of perchloroethylene and trichloroethylene

Theoretical studies are presented on the structures and reactivity of chlorinated ethylcobalamins, potential intermediates in the vitamin B12-catalyzed reductive dehalogenation of the environmental pollutants perchloroethylene and trichloroethylene; the results suggest an alternative mechanism of catalysis.