Electrochemical studies of weak carbon and nitrogen acids: Fluorene and p-cyanoaniline in dimethylformamide

The electrochemical reduction of fluorene and p-cyanoaniline in DMF at a platinum electrode is initially a one-electron process which affords the corresponding readical anions. In the absence of an added proton donor, decomposition of the radical anions occurs by carbonhydrogen bond cleavage to give the conjugate bases of the starting materials; the anions subsequently slowly abstract a proton from the tetraalkylammonium cation of the supporting electrolyte to regenerate the original electroactive species. In the presence of dimethylmalonate, both radical anions rapidly electron transfer to the added proton donor. Neither self-protonation nor protonation by the added donor was observed for either radical anion. In addition to proton abstraction, 9-fluorenyl anion reacts with oxygen to give fluorene and hydroxide ion. Abstraction of a proton from fluorene by the latter species then effects a chain reaction in which 9-fluorenyl anion is the chain-carrying species. Reduction of bifluorenyl occurs with carbon-carbon bond cleavage to give 9-fluorenyl anion as the initial product. Subsequent proton transfer from bifluorenyl to 9-fluorenyl anion then yields the final products, 9-bifluorenyl anion and fluorene, in equimolar amounts.     

Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2

The paradigm that the cleavage of the radical anion of benzyl halides occurs in such a way that the negative charge ends up on the departing halide leaving behind a benzyl radical is well rooted in chemistry. By studying the kinetics of the reaction of substituted benzylbromides and chlorides with SmI₂ in THF it was found that substrates para‐substituted with electron‐withdrawing groups (CN and CO₂Me), which are capable of forming hydrogen bonds with a proton donor and coordinating to samarium cation, react in a reversed electron apportionment mode. Namely, the halide departs as a radical. This conclusion is based on the found convex Hammett plots, element effects, proton donor effects, and the effect of tosylate (OTs) as a leaving group. The latter does not tend to tolerate radical character on the oxygen atom. In the presence of a proton donor, the tolyl derivatives were the sole product, whereas in its absence, the coupling dimer was obtained by a S_N2 reaction of the benzyl anion on the neutral substrate. The data also suggest that for the para‐CN and CO₂Me derivatives in the presence of a proton donor, the first electron transfer is coupled with the proton transfer.     

Mechanism of electrodimerization of retinal and cinnamaldehyde

Electrochemical reduction of either cinnamaldehyde or retinal in acetonitrile containing 0.5 M. TBAP and water or phenol leads to a mixture of dimeric products. However, electroreduction of either of these aldehydes in the presence of the carbon acid, diethyl malonate, leads to increased yield of pinacol formation. The mechanism of these electrodimerization reactions has been discerned using cyclic voltammetry-variations in peak potential as a function of scan rate, proton donor concentration and concentration of aldehyde have been compared to published diagnostic criteria. Retinal and cinnamaldehyde form dimeric products in a radical-radical coupling pathway. The major mechanistic feature that correlates with product distribution is the association (solvation) of oxygen acids, phenol and water with the radical anion. In contrast, diethyl malonate reacts with the radical anion of either cinnamaldehyde or retinal by a direct proton transfer.     

Interception of Deaminatively Generated Benzyl Carbenium Ions by Acetone

Essentially free benzyl carbenium ions were generated via protonation of phenyldiazomethane with benzoic acid in acetone. Interestingly, no proton transfer occurred below -20 degrees C. After protonation and dediazoniation of the diazoalkane at -20 degrees C, the solvent was found to intercept the deaminatively generated carbocations to yield initially the corresponding O-benzyl oxonium ion and benzyl benzoate. The onium ion, however, was labile under the reaction conditions, and decomposed into a cascade of products whose concentrations as a function of time were used to trace the reaction pathway. Thus, the O-benzyl oxonium ion reacted with benzoate ion to yield (2-benzyloxy)isopropyl benzoate; subsequent decomposition of this O-benzyl-O-benzoyl ketal produced 2,2-dibenzyloxypropane (a dibenzyl ketal), 2-benzyloxypropene, and benzyl alcohol. In a related study, benzyl cations were generated via thermolyses of N-benzyl-N-nitroso-O-benzoyl hydroxylamine at 0 and -70 degrees C. The product distributions were found to be temperature-dependent and different from that in the PhCHN(2) + PhCO(2)H case.     

Reductions with SmI2: mechanistic probe for distinguishing between two operational modes of proton transfer

The reduction of 1,1-diaryl-2,2-dicyanoethylenes with SmI(2) in THF was studied in the presence of four proton donors: H(2)O, MeOH, i-PrOH and trifluoroethanol (TFE). The kinetic order for the first two is nearly unity at low proton donor concentrations and approaches four at high concentrations, whereas, for i-PrOH and TFE, the log-log plot is linear with a slope smaller than one. Detailed analysis shows that a curved log-log plot such as for H(2)O and MeOH is indicative of a major contribution by protonation within an ion pair of the radical anion and Sm(+3) complexed to a variable number of proton donor molecules, whereas a linear plot is a result of protonation from the bulk solution.     

Quantum yields of the initiation step and chain propagation turnovers in S(RN)1 reactions: photostimulated reaction of 1-iodo-2-methyl-2-phenyl propane with carbanions in DMSO

Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 x 10(5) M(-)(1) s(-)(1), by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 10(6) M(-)(1) s(-)(1). Although the overall quantum yield determined (lambda = 350 nm) for the studied reactions is below 1, the chain lengths (Phi(propagation)) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.     

Photochemical primary process of photo-Fries rearrangement reaction of 1-naphthyl acetate as studied by MFE probe

Photo-Fries rearrangement reactions of 1-naphthyl acetate (NA) in n-hexane and in cyclohexane were studied by the magnetic field effect probe (MFE probe) under magnetic fields (B) of 0 to 7 T. Transient absorptions of the 1-naphthoxyl radical, T-T absorption of NA, and a short-lifetime intermediate (τ = 24 ns) were observed by a nanosecond laser flash photolysis technique. In n-hexane, the yield of escaped 1-naphthoxyl radicals dropped dramatically upon application of a 3 mT field, but then the yield increased with increasing B for 3 mT < B≤ 7 T. These observed MFEs can be explained by the hyperfine coupling and the Δg mechanisms through the singlet radical pair. The fact that MFEs were observed for the present photo-Fries rearrangement reaction indicates the presence of a singlet radical pair intermediate with a lifetime as long as several tens of nanoseconds.     

The redox behavior of gem-dihalo compounds: 9,9-dibromofluorene, 9,9-dichlorofluorene and dichlorodiphenylmethane

The redox behavior of several gem-dihalo compounds has been examined at platinum and vitreous carbon electrodes in dimethylformamide. The reduction of 9,9-dichlorofluorene is initially an overall two-electron process which involves cleavage of chloride ion and the formation of 9-chlorofluorenyl anion. The final products and their distribution are then dependent upon the relative rates of reduction of the parent compound, nucleophilic attack of 9-chlorofluorenyl anion on the parent compound, and proton availability. If reaction by substitution is permitted to predominate, 9,9′-dichlorobifluorenyl results. This species is electroactive at the applied potential and undergoes reductive dechlorination to give bifluorenylidene. In contrast, if either the rate of reduction of 9,9-dichlorofluorene of the rate of protonation of 9-chlorofluorenyl anion exceeds the rate of substitution, the predominant product becomes 9-chlorofluorene. Reduction of this species then gives a mixture of fluorene and bifluorenyl when electrolysis is effected in an aprotic medium and fluorene when electrolysis is performed in the presence of diethyl malonate, a weak proton donor. Dichlorodiphenylmethane and 9,9-dibromofluorene also undergo reductive dehalogenation to give monomeric and dimeric products by pathways analogous to those observed for dichlorofluorene. In the case of dibromofluorene, however, the product distribution is also potential dependent since the intermediate 9-bromofluorenyl radical may not be reduced at the applied potential. No evidence was obtained in these studies to support previous claims of carbenes and/or carbene radical anions in these reductions.     

Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction

The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions.     

Charge-transfer derivatization in fast-atom-bombardment mass spectrometry

The formation of charge-transfer complexes as derivatization reactions for fast-atom- bombardment (f.a.b.) mass spectrometry has been investigated. The donor N,N,N′,N′- tetramethyl-1,4-phenylenediamine (TMPD) and the acceptor 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) were studied. The f.a.b, spectrum of this complex in glycerol first yielded the cation radical, TMPD⁺ at m/z 164. However, with time the dominant ion becomes (TMPD+H)⁺. Results suggest that although a strong charge-transfer complex is formed, protonation of the donor molecule occurs whenever possible. In dimethylsulfoxide, initial f.a.b, spectra contain (TMPD+H)⁺, but as solvent evaporates, this ion is supplanted by the cation radical. Increased abundances of the cation radicals for charge- transfer complexes are observed in the absence of solvent or with the use of aprotic solvents. Characteristic ultraviolet/visible spectra of charge-transfer complexes clarify the competing processes of electron and proton transfer, and their time dependence. Charge- transfer derivatization is used to increase signals for the donor cation radicals of anthracene/picric acid, pyrene/picric acid, and indole/trinitrofluorenone.     

Structures and energetics of the deprotonated adenine-uracil base pair, including proton-transferred systems

The B3LYP/DZP++ level of theory has been employed to investigate the structures and energetics of the deprotonated adenine-uracil base pairs, (AU-H)-. Formation of the lowest-energy structure, [A(N9)-U]- (which corresponds to deprotonation at the N9 atom of adenine), through electron attachment to the corresponding neutral is accompanied by proton transfer from the uracil N3 atom to the adenine N1 atom. The driving force for this proton transfer is a significant stabilization from the base pairing in the proton transferred form. Such proton transfer upon electron attachment is also observed for the [A(N6b)-U]- and [A(C2)-U]- anions. Electron attachment to the A-U(N3) radical causes strong lone pair repulsion between the adenine N1 and the uracil N3 atoms, driving the two bases apart. Similarly, lone pair repulsion in the anion A(N6a)-U causes the loss of coplanarity of the two base units. The computed adiabatic electron attachment energies for nine AU-H radicals range from 1.86 to 3.75 eV, implying that the corresponding (AU-H)- anions are strongly bound. Because of the large AEAs of the (AU-H) radicals, the C-H and N-H bond dissociation in the AU- base pair anions requires less energy than the neutral AU base pair. The computed C-H and N-H bond dissociation energies for the AU- anion (i.e., the AU base pair plus one electron) are in the range 1.0-3.2 eV, while those for neutral AU are 4.08 eV or higher.     

Competitive proton and hydride transfer reactions via ion‐neutral complexes: fragmentation of deprotonated benzyl N‐phenylcarbamates in mass spectrometry

The gas‐phase chemistry of deprotonated benzyl N‐phenylcarbamates was investigated by electrospray ionization tandem mass spectrometry. Characteristic losses of a substituted phenylcarbinol and a benzaldehyde from the precursor ion were proposed to be derived from an ion‐neutral complex (INC)‐mediated competitive proton and hydride transfer reactions. The intermediacy of the INC consisting of a substituted benzyloxy anion and a phenyl isocyanate was supported by both ortho‐site‐blocking experiments and density functional theory calculations. Within the INC, the benzyloxy anion played the role of either a proton abstractor or a hydride donor toward its neutral counterpart. Relative abundances of the product ions were influenced by the nature of the substituents. Electron‐withdrawing groups at the N‐phenyl ring favored the hydrogen transfer process (including proton and hydride transfer), whereas electron‐donating groups favored direct decomposition to generate the benzyloxy anion (or substituted benzyloxy anion). By contrast, electron‐withdrawing and electron‐donating substitutions at the O‐benzyl ring exhibited opposite effects. Copyright © 2015 John Wiley & Sons, Ltd.     

Kinetics and mechanism of sensitized photooxidation of tetramethylammonium salt of 2-(phenylthio)acetic acid in solution: Steady-state and flash photolysis studies

The mechanism for the photo-induced oxidation of the tetramethylammonium salt of 2-(phenylthio)acetic acid was elucidated. The photosensitizer was the benzophenone triplet in acetonitrile solutions. Time-resolved absorption spectra and kinetics were used to follow the intermediates which included the triplet of benzophenone, the ketyl radical of benzophenone, and an ion pair consisting of a radical anion of benzophenone and a tetramethylammonium cation. Rate constants for the growth and decay of the transients were determined along with the quantum yields of the transients. The intermediacy of other radicals was inferred by the products observed following steady-state photolysis. Quantum yields were also determined for photoproducts resulting from the steady-state irradiation. The mechanism was proposed that rationalized the quantitative observations. Of particular note was how the nature of the counter ion effected the secondary reactions of the radicals and the radical ions.     

Transient spectroscopy of ninhydrin

The photochemistry of ninhydrin in benzene and water was studied by laser flash photolysis and electron paramagnetic resonance. Its photochemistry was shown to be dependent on the solvent. In benzene, a triplet excited state was observed, which underwent hydrogen abstraction reactions or reduction to the radical anion. In water, the radical anion of ninhydrin was formed within the laser pulse (15 ns) at neutral pH, whereas the neutral ketyl radical was formed by protonation of the radical anion at low pH. A pKa of 0.77 was determined for the protonation equilibrium. The formation of hydrindantin is proposed to occur through the dimerization of the ketyl radical or the radical anion (or both). In addition, ninhydrin was shown to be a poor precursor for the photogeneration of hydroxyl radicals.     

Direct determination of rate constants for coupling between aromatic radical anions and alkyl and benzyl radicals by laser-flash photolysis

Coupling rates between the radicals methyl, n-, sec-, tert-butyl and benzyl (R.) and the aromatic radical anions of 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene and fluorenone (A-.) have been obtained using a new laser-flash photolysis method. The radicals R. and the radical anions A-. were generated by a photoinduced electron transfer reaction between the aromatic compound A and the alkyl or benzyl triphenylborate anion RB(Ph)3-. For the first time the rate constants of the coupling reaction between methyl and benzyl radicals with aromatic radical anions have been obtained. For all the measured coupling rate constants an average value of k1 = 1.9 x 10(9) M-1 s-1 was found with a relatively small variation in the coupling rates (0.8-2.9 x 10(9) M-1 s-1). The results demonstrate that the coupling rate k1 is insensitive to changes in the steric and electronic properties of the radicals and the structure and standard potentials of the aromatic radical anions.     

Electrochemical reduction of camphorquinone in DMF in the presence of a proton donor

The electroreduction of camphorquinone in DMF, at mercury electrodes, was investigated by a variety of techniques. In DMF, in the absence of proton donor, camphorquinone exhibits two one-electron waves: the first, a one-electron reversible wave to be due to a reversible charge transfer without a coupled chemical reaction. After the first charge transfer, the semidione anion radical is reduced to the dianion. The irreversibility of the second wave derives from a fast irreversible protonation of the dianion. A wide variety of changes in behaviour is observed in the reduction of camphorquinone as increasing amounts of benzoic acid are added: a new two-electron irreversible wave appears at a potential less negative than the original wave. A proton donor to substrate ratio of 2 is required to completely suppress the two original waves. A mechanism for the electroreduction of camphorquinone is proposed and discussed on the basis that the prewave current is controlled by the diffusion of the undissociated acid species and that the undissociated acid, rather than the solvated proton, takes part in the protonation, prior to the charge transfer.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Mechanistic insights on how hydroquinone disarms OH and OOH radicals

Phenol derivatives are distinguished as successful free radical scavengers. We present a detailed analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radical with emphasis on changes that take place in the vicinity of the transition state. Quantum theory of atoms in molecules is employed to elucidate the sequence of positive and negative charge transfer by studying selected properties of the three key atoms (the transferring hydrogen, the donor atom, and the acceptor atom) along intrinsic reaction path. The presented results imply that in both reactions, which are examples of proton coupled electron transfer, proton, and electron get simultaneously transferred to the radical oxygen atom. The fact that the hydrogen's charge and volume do not monotonously change in the vicinity of the transition state in the product valley results from the adjacency of the proton and the electron to the donor and the acceptor oxygen atoms. Obtaining a detailed understanding of mechanisms by which free radicals are disarmed is of paramount importance given the effects of those highly reactive species on biological systems. A comprehensive analysis of hydroxyl hydrogen abstraction from hydroquinone by hydroxyl and hydroperoxyl radicals, based on changes of selected electronic properties of the three most relevant atoms (hydrogen donor, hydrogen acceptor, and the hydrogen itself), along the reaction coordinate, can be obtained by first‐principles calculations.     

Reductive halogen elimination from phenols by organic radicals in aqueous solutions; chain reaction induced by proton-coupled electron transfer

Gamma-radiolysis and measurements of halide ions by means of ion chromatography have been employed to investigate reductive dehalogenation of chloro-, bromo-, and iodophenols by carbon-centered radicals, *CH(CH(3))OH, *CH(2)OH, and *CO(2)-, in oxygen-free aqueous solutions in the presence of ethanol, methanol, or sodium formate. While the reactions of 4-IC(6)H(4)OH with *CH(CH(3))OH and *CH(2)OH radicals are endothermic in water/alcohol solutions, the addition of bicarbonate leads to iodide production in high yields, indicative of a chain reaction. The maximum effect has been observed with about 10 mM sodium bicarbonate present. The complex formed from an alpha-hydroxyalkyl radical and a bicarbonate anion is considered to cause the enhancement of the reduction power of the former to the extent at which the reduction of the iodophenol molecule becomes exothermic. No such effect has been observed with phosphate, which is a buffer with higher proton affinity, when added in the concentration of up to 20 mM at pH 7. This indicates that one-electron reduction reactions by alpha-hydroxyalkyl radicals occur by the concerted proton-coupled electron transfer, PCET, and not by a two-step ET/PT or PT/ET mechanisms. The reason for the negative results with phosphate buffer could be thus ascribed to a less stable complex or to the formation of a complex with a less suitable structure for an adequate support to reduce iodophenol. The reduction power of the carbonate radical anion is shown to be high enough to reduce iodophenols by a one-electron-transfer mechanism. In the presence of formate ions as H-atom donors, the dehalogenation also occurs by a chain reaction. In all systems, the chain lengths depend on the rate of reducing radical reproduction in the propagation step, that is, on the rate of H-atom abstraction from methanol, ethanol, or formate by 4-*C(6)H(4)OH radicals liberated after iodophenol dehalogenation. The rate constants of those reactions were determined from the iodide yield measurements at a constant irradiation dose rate. They were estimated to be 6 M(-1)(s-1) for methanol, 140 M(-1)(s-1) for ethanol, and 2100 M(-1)(s-1) for formate. Neither of the tested reducing C-centered radicals was able to dehalogenate the bromo or chloro derivative of phenol.