Oxygen Versus Carbon Acidity in the Side-Chain Fragmentation of 2-, 3-, and 4-Arylalkanol Radical Cations in Aqueous Solution: The Influence of the Distance between the OH Group and the Aromatic Ring1

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Catalytic Ring-Opening of Cyclic Alcohols Enabled by PCET Activation of Strong O-H Bonds

We report a new photocatalytic protocol for the redox-neutral isomerization of cyclic alcohols to linear ketones via C-C bond scission. Mechanistic studies demonstrate that key alkoxy radical intermediates in this reaction are generated via the direct homolytic activation of alcohol O-H bonds in an unusual intramolecular PCET process, wherein the electron travels to a proximal radical cation in concert with proton transfer to a weak Brønsted base. Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.

Reactions of the tetraoxidosulfate(˙-) and hydroxyl radicals with poly(sodium α-methylstyrene sulfonate)

Poly(α-methylstyrene sulfonic acid) (PAMS) represents a class of polymers that can form the protogenic constituent in electrolyte membranes for fuel cells. Oxidative stress is thought to play an important role in the degradation of the fuel cell membranes. Having previously established that damage may be mediated via abstraction of a benzylic hydrogen, we examined model compounds similar to those used before, but with a methyl group at the α-position. We studied the reaction of HO˙ and SO4(˙-), generated by pulse radiolysis, with model compounds in aqueous solution, and measured k = (2 ± 0.5) × 10(10) M(-1) s(-1) and (2 - 3) × 10(10) M(-1) s(-1) for the reaction of HO˙ with PAMS with average molecular weights of 2640 Da (PAMS-2640) and 6440 Da (PAMS-6440), respectively, at room temperature. At low pH, the decay of the hydroxycyclohexadienyl radical thus formed is accompanied by the formation of an absorption band in the visible region of the spectrum, which we tentatively assign to the radical cation of PAMS-2640 and -6440. The radical cation of PAMS-2640, formed by the reaction of SO4(˙-) with k = (6 ± 1) × 10(8) M(-1) s(-1), has a local absorption maximum at 560 nm, with ε560 ≥ 1400 M(-1) cm(-1). For the reaction of HO˙ and SO4(˙-) with the model compound benzenesulfonate, we measured k = (4-5) × 10(9) M(-1) s(-1) and (1.0 ± 0.3) × 10(8) M(-1) s(-1), respectively, while the reaction of SO4(˙-) with PAMS-6440 proceeds with (0.8-1) × 10(9) M(-1) s(-1). The 4-sulfophenoxyl radical was generated via the reaction of N3˙ with 4-hydroxybenzenesulfonate; ε410 ≥ 2300 M(-1) cm(-1). Not unexpectedly, the radical cation of PAMS is longer-lived than that of polystyrene sulfonic acid. Furthermore, fragmentation may result in desulfonation.

Theoretical investigations on the formation of nitrobenzanthrone-DNA adducts

3-Nitrobenzanthrone (3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust. The thermochemical formation cascades were calculated for six 3-NBA-derived DNA adducts employing its arylnitrenium ion as precursor using density functional theory (DFT). Clear exothermic pathways were found for four adducts, i.e., 2-(2'-deoxyadenosin-N(6)-yl)-3-aminobenzanthrone, 2-(2'-deoxyguanosin-N(2)-yl)-3-aminobenzanthrone, N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone and 2-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone. All four have been observed to be formed in cell-free experimental systems. The formation of N-(2'-deoxyadenosin-8-yl)-3-aminobenzanthrone is predicted to be not thermochemically viable explaining its absence in either in vitro or in vivo model systems. However, 2-(2'-deoxyadenosin-8-yl)-3-aminobenzanthrone, can be formed, albeit not as a major product, and is a viable candidate for an unknown adenine adduct observed experimentally. 2-nitrobenzanthrone (2-NBA), an isomer of 3-NBA, was also included in the calculations; it has a higher abundance in ambient air than 3-NBA, but a much lower genotoxic potency. Similar thermochemical profiles were obtained for the calculated 2-NBA-derived DNA adducts. This leads to the conclusion that enzymatic activation as well as the stability of its arylnitrenium ion are important determinants of 2-NBA genotoxicity.

Reaction of SO4˙⁻ with an oligomer of poly(sodium styrene sulfonate). Probing the mechanism of damage to fuel cell membranes

Clarification of the mechanism of degradation of model compounds for polymers used in polymer electrolyte fuel cells may identify intermediates that propagate damage; such knowledge can be used to improve the lifetime of fuel cell membranes, a central issue to continued progress in fuel cell technology. In proton-exchange membranes based on poly(styrene sulfonic acid), hydroxycyclohexadienyl radicals are formed after reaction with HO˙ and thought to decay to short-lived radical cations at low pH. To clarify subsequent reactions, we generated radical cations by reaction of SO(4)˙(-) with oligomers of poly(styrene sulfonic acid) (MW ≈ 1100 Da). At 295 K, this reaction proceeds with k = (4.5 ± 0.6) × 10(8) M(-1) s(-1), both at pH 2.4 and 3.4, and yields benzyl radicals with an estimated yield of ≤60% relative to [SO(4)˙(-)]. The radical cation is too short-lived to be observed: based on a benzyl radical yield of 60%, a lower limit of k > 6.8 × 10(5) s(-1) for the intramolecular transformation of the aromatic radical cation of the oligomer to a benzyl radical is deduced. Our results show that formation of the benzyl radical, an important precursor in the breakdown of the polymer, is irreversible.

One-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic and 1-(4-methoxyphenyl)cyclopropanecarboxylic acids in aqueous solution. the involvement of radical cations and the influence of structural effects and pH on the side-chain fragmentation reactivity

A product and time-resolved kinetic study on the one-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic acid (2), 1-(4-methoxyphenyl)cyclopropanecarboxylic acid (3), and of the corresponding methyl esters (substrates 4 and 5, respectively) has been carried out in aqueous solution. With 2, no direct evidence for the formation of an intermediate radical cation 2*+ but only of the decarboxylated 4-methoxycumyl radical has been obtained, indicating either that 2*+ is not formed or that its decarboxylation is too fast to allow detection under the experimental conditions employed (k > 1 x 10(7) s(-1)). With 3, oxidation leads to the formation of the corresponding radical cation 3*+ or radical zwitterion -3*+ depending on pH. At pH 1.0 and 6.7, 3*+ and -3*+ have been observed to undergo decarboxylation as the exclusive side-chain fragmentation pathway with rate constants k = 4.6 x 10(3) and 2.3 x 10(4) s(-1), respectively. With methyl esters 4 and 5, direct evidence for the formation of the corresponding radical cations 4*+ and 5*+ has been obtained. Both radical cations have been observed to display a very low reactivity and an upper limit for their decay rate constants has been determined as k < 10(3) s(-1). Comparison between the one-electron oxidation reactions of 2 and 3 shows that the replacement of the C(CH3)2 moiety with a cyclopropyl group determines a decrease in decarboxylation rate constant of more than 3 orders of magnitude. This large difference in reactivity has been qualitatively explained in terms of three main contributions: substrate oxidation potential, stability of the carbon-centered radical formed after decarboxylation, and stereoelectronic effects. In basic solution, -3*+ and 5*+ have been observed to react with -OH in a process that is assigned to the -OH-induced ring-opening of the cyclopropane ring, and the corresponding second-order rate constants (k-OH) have been obtained. With -3*+, competition between decarboxylation and -OH-induced cyclopropane ring-opening is observed at pH >or=10, with the latter process that becomes the major fragmentation pathway around pH 12.

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.

Steady-State and Laser Flash Photolysis Study of the Carbon−Carbon Bond Fragmentation Reactions of 2-Arylsulfanyl Alcohol Radical Cations

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products.

Structural effects on the OH-promoted fragmentation of methoxy-substituted 1-arylalkanol radical cations in aqueous solution: the role of oxygen acidity

A kinetic and product study of the OH- -induced decay in H2O of the radical cations generated from some di-and tri-methoxy-substituted 1-arylalkanols (ArCH(OH)R*+) and 2- and 3-(3,4-dimethoxyphenyl)alkanols has been carried out by using pulse- and gamma-radiolysis techniques. In the 1-arylalkanol system, the radical cation 3,4-(MeO)2C6H3CH2-OH*+ decay at a rate more than two orders of magnitude higher than that of its methyl ether; this indicates the key role of the side-chain OH group in the decay process (oxygen acidity). However, quite a large deuterium kinetic isotope effect (3.7) is present for this radical cation compared with its a-dideuterated counterpart. A mechanism is suggested in which a fast OH deprotonation leads to a radical zwitterion which then undergoes a rate-determining 1,2-H shift, coupled to a side-chain-to-ring intramolecular electron transfer (ET) step. This concept also attributes an important role to the energy barrier for this ET, which should depend on the stability of the positive charge in the ring and, hence, on the number and position of methoxy groups. On a similar experimental basis, the same mechanism is suggested for 2,5-(MeO)2C6H3CH2OH*+ as for 3,4-(MeO)2C6H3CH2OH*+, in which some contribution from direct C-H deprotonation (carbon acidity) is possible. In fact, the latter process dominates the decay of the trimethoxylated system 2,4,5-(MeO)3C6H2CH2-OH*+, which, accordingly, reacts with OH- at the same rate as that of its methyl ether. Thus, a shift from oxygen to carbon acidity is observed as the positive charge is increasingly stabilized in the ring; this is attributed to a corresponding increase in the energy barrier for the intramolecular ET. When R=tBu, the OH- -promoted decay of the radical cation ArCH(OH)R*+ leads to products of C-C bond cleavage. With both Ar = 3,4- and 2,5-dimethoxyphenyl the reactivity is three orders of magnitude higher than that of the corresponding cumyl alcohol radical cations; this suggests a mechanism in which a key role is played by the oxygen acidity as well as by the strength of the scissile C-C bond: a radical zwitterion is formed which undergoes a rate-determining C-C bond cleavage, coupled with the intramolecular ET. Finally, oxygen acidity also determines the reactivity of the radical cations of 2-(3,4-dimethoxyphenyl)ethanol and 3-(3,4-dimethoxyphenyl)propanol. In the former the decay involves C-C bond cleavage, in the latter it leads to 3-(3,4-dimethoxyphenyl)propanal. In both cases no products of C-H deprotonation were observed. Possible mechanisms, again involving the initial formation of a radical zwitterion, are discussed.

Solid-state solvolysis of thiophene-substituted trityl-type alcohols: nucleophilic substitution induced by gas-solid contact

The nucleophilic substitution reaction by gas-solid contact has been investigated. When 9-thienothienylfluoren-9-ol derivatives were coground with dichlorodicyanoquinone (DDQ) and then exposed to methanol vapor, the corresponding 9-methoxyfluorenes were obtained in 15-70% yields. Throughout the whole procedure the solid state was retained. The generation of a radical cation in the coground solids via charge-transfer interaction between the substrate alcohol and DDQ was suggested by the ESR spectrum. The mechanism involving the collapse of the radical cation to generate a proton, which acts as a catalyst to afford the carbocation, was deduced based on the electrochemical oxidation of the substrate in solution. The propagation of the substitution reaction in the solid state has been shown for the carbocation upon contact with methanol vapor. The crystalline inclusion compounds of 9-thienothienylfluoren-9-ol derivatives incorporating methanol as a guest were exposed to HCl gas. This gas-solid reaction also led to the formation of the corresponding methoxy compounds maintaining the solid state. Through this work a new consequence of solid-state cogrinding is deduced.

Rate-determining steps in phenacetin oxidations by human cytochrome P450 1A2 and selected mutants

Mutants with altered activities were obtained from random libraries of human cytochrome P450 (P450) 1A2 with the putative substrate recognition sequences (SRS) mutated [Parikh, A., Josephy, P. D., and Guengerich, F. P. (1999) Biochemistry 38, 5283-5289]. Six mutants from SRS 2 (E225I, E225N, F226I, and F226Y) and 4 (D320A and V322A) regions were expressed as oligohistidine-tagged proteins, purified to homogeneity, and used to analyze kinetics of individual steps in the catalytic cycle, to determine which reaction steps have been altered. When the wild-type, E225I, E225N, F226I, F226Y, D320A, and V322A proteins were reconstituted with NADPH-P450 reductase, rates of 7-ethoxyresorufin O-deethylation and phenacetin O-deethylation were in accord with those expected from membrane preparations. Within each assay, the values of k(cat)/K(m) varied by 2-3 orders of magnitude, and in the case of E225I and E225N, these parameters were 7-8-fold higher than for the wild-type enzyme. The coupling efficiency obtained from the rates of product formation and NADPH oxidation was low (<20%) in all enzymes. No correlation was found between activities and several individual steps in the catalytic cycle examined, including substrate binding, reduction kinetics, NADPH oxidation, and H(2)O(2) formation. Quench reactions did not show a burst for either phenacetin O-deethylation or formation of the acetol, a minor product, indicating that rate-determining steps occur prior to product formation. Inter- and intramolecular kinetic deuterium isotope effects for phenacetin O-deethylation were 2-3. In the case of phenacetin acetyl hydroxylation (acetol formation), large isotope effects [(D)k(cat) or (D)(k(cat)/K(m)) > 10] were observed, providing evidence for rate-limiting C-H bond cleavage. We suggest that the very high isotope effect for acetol formation reflects rate-limiting hydrogen atom abstraction; the lower isotope effect for O-deethylation may be a consequence of a 1-electron transfer pathway resulting from the low oxidation potential of the substrate phenacetin. These pre-steady-state, steady-state, and kinetic hydrogen isotope effect studies indicate that the rate-limiting steps are relatively unchanged over an 800-fold range of catalytic activity. We hypothesize that these SRS mutations alter steps leading to the formation of the activated Michaelis complex following the introduction of the first electron.

Mechanistic aspects of beta-bond-cleavage reactions of aromatic radical cations

The mesolytic cleavage of a beta-C-X bond (ArCR(2)-X(*+) --> ArCR(2)(*/+) + X(+/*)) is one of the most important reactions of alkylaromatic radical cations. In this Account, our group's results concerning some fundamental aspects of this process (cleavage mode, structural and stereoelectronic effects, competitive breaking of different beta-bonds, nucleophilic assistance, possible stereochemistry, carbon vs oxygen acidity in arylalkanol radical cations) are presented and critically discussed for reactions where X = H, CR(3), SR, and SiR(3). Several examples illustrating how this information was exploited as a tool to detect electron-transfer mechanisms in chemical and enzymatic oxidations are also reported.