M. Dale Hawley

Electron-transfer processes: The electrochemical and chemical behavior of nitrosobenzene

Summary The electrochemical and chemical reduction of nitrosobenzene has been studied in acetonitrile and dimethylformamide. The reduction has been shown to be a kinetically controlled process which initially gives rise to the corresponding radical anion. In the absence of proton donors the radical anion is suggested to dimerize to a dianion intermediate; the dianion subsequently abstracts a proton from a component of the solvent system and expels hydroxide ion to yield the final product, azoxybenzene. Exhaustive, controlled-potential electrolyses of nitrosobenzene produce azoxybenzene in approximately 73% yield in acetonitrile and 85% yield in dimethylformamide; the corresponding n -values are typically 0.3 and 0.7. The unusually small n -values have been shown to result from the reaction of nitrosobenzene with the base which is generated during the formation of azoxybenzene. Reaction of nitrosobenzene with hydroxide ion alone affords azoxybenzene in approximately two-thirds yield in both solvent systems. Complete consumption of nitrosobenzene in acetonitrile could be effected by a mole ratio of hydroxide ion to nitroso compound of 0.3.

Electrochemical and enzymatic oxidation of catecholamines involved in sclerotization and melanization of insect cuticle

Abstract The electrochemical properties of catecholamines that occur in the haemolymph and cuticle of Manduca sexta (L.) during development were studied. The cyclic voltammetric behaviour for dopamine, N- acetyldopamine (NADA) and N-β-alanyldopamine (NBAD) was similar except that the open chain o- quinones of the latter two compounds cyclized significantly more slowly than dopamine o- quinone . Tyrosinase from pharate pupal cuticle oxidized the three catecholamines with NBAD and NADA being the preferred substrates. Oxidation to the corresponding o- quinone , indolization and reoxidation to p- quinone imine was the pathway observed for both electrochemical and enzyme catalyzed reactions. The possible roles of these metabolites in cuticle sclerotization and melanization are discussed.

Electrochemical studies of the formation and decomposition of p-nitrobenzenesulfonamide radical anions

Summary The redox behavior of N,N-dimethyl-, N-methyl-, and p-nitrobenzenesulfonamide has been studied in DMF by cyclic voltammetry, chronoamperometry, controlled-potential coulometry, and electron spin resonance spectroscopy. The tertiary compound, N,N-dimethyl-p-nitrobenzenesulfonamide, undergoes reversible, stepwise reduction to form the corresponding dianion. In contrast, the acidic primary and secondary sulfonamides exhibit three cathodic waves. The first of these processes (Ep.c=−0.93 V) is attributed to the one-electron reduction of the sulfonamide to its radical anion. This species is unstable, however, and decomposes with loss of hydrogen during the course of the cyclic voltammetric experiment to form the corresponding sulfonamide anion. Reduction of this sulfonamide anion to its dianion radical occurs at slightly more negative potential (Ep.c=−1.16 V). The third cathodic process (Ep.c=−1.44 V) arises when any remaining sulfonamide radical anion is reduced to its dianion. This dianion is much less stable than the corresponding radical anion and rapidly undergoes nitrogen-hydrogen bond cleavage to form the dianion radical and hydrogen.

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

Abstract 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.

The effect of kenetic vs. thermodynamic acidity on the redox behavior of several 9-hydroxy- and 9-methoxyfluorenes

The electrochemical reductions of 9-methoxyfluorene (FlHOCH3), 9-methylfluorenol (Fl(CH3)OH) and 9-fluorenol (FlHOH) have been studied at a platinum electrode in DMF−0.1 M (n-Bu)4NClO4 over the temperature range −51°⩽T⩽22°C. The anion radicals of these compounds undergo heterolytic cleavage of the carbon—oxygen bond to give a fluorenyl radical (FlR, where R=H or CH3) and OR−. The subsequent redox behavior and follow-up solution reactions differ for each compound and depend upon the availability of a C9 and/or a hydroxylic proton. In the case of Fl(CH3)OH, the electrogenerated OH− abstracts a hydroxylic proton from unreacted starting material to give Fl(CH3)O−. Although this species is not oxidized, it is reduced at more negative potential to an unstable dianion radical that rapidly and indiscriminately abstracts proton. When a hydroxylic proton is not available, as in the case of FlHOCH3, the electrogenerated CH 3O− abstracts a proton from a component of the solvent-electrolyte system rather than from the C9 position. FlOCH3− is then formed slowly, but this is the result of C9 proton transfer from FlHOCH3 to the electrogenerated FlH−. Here, FlOCH3−, like FlH−, is oxidized irreversibly to its radical, but is not reduced in this solvent-electrolyte system. The OH− produced in the decomposition of FlHOH− abstracts only the hydroxylic proton from unreacted FlHOH. The cathodic wave for the reduction of FlHO− and the anodic wave for the oxidation of FlR− are kinetically controlled and result from the relatively slow abstraction of the thermodynamically more acidic C9 proton from FlHOH by FlHO− and FlH respectively.

Chain reactions in several 9-substituted fluorenes and bifluorenyls induced by electrogenerated bases

Abstract Three types of chain reactions that are induced by electrogenerated bases were studied. In the first series radical β scission of the carbon-oxygen bond in the electrogenerated anion radical of 9-methoxybifluorenyl (FlHFlOCH3) affords OCH3· and FlHFl−. The latter species then abstracts the C9 proton from unreacted FlHFlOCH3 to give bifluorenyl (FIH)2) and the corresponding conjugate base, Fl(OCH3)Fl−. The propagation cycle involves the slow loss of OCH3− from Fl(OCH3)Fl−, followed by the rapid abstraction of a proton from FlHFlOCH3 by OCH3− to regenerate Fl(OCH3)Fl−. The final products of the base-induced transformation are bifluorenylidene (Fl=Fl) and CH3OH. In the second series radical β scission of the carbon-oxygen bond in the electrogenerated anion radicals of 9-hydroxybifluorenyl (FlHFlOH) and 9,9′-dihydroxybifluorenyl ((FlOH)2) gives OH· and FlHFl− and Fl(OH)Fl− respectively. Subsequent steps include the abstraction of a hydroxylic proton from unreacted starting material by the electrogenerated base, followed by the heterolytic cleavage of the carbon-carbon bond in the resulting anions, FlHFlO− and Fl(OH)FlO−. Fluorenone (Fl=0) and fluorene (FlH2) are formed in equimolar quantities as the final products from FlHFlOH while (FlOH)2 yields Fl=0 and fluorenol (FlHOH). The third type of chain process is initiated when electrogenerated azobenzene anion radical abstracts a proton from either FlHOH or 9-aminofluorene (FlHNH2). Electron transfer from the conjugate bases, FlOH− and FlNH2−, to unreduced PhN=NPh then causes a series of proton and electron transfer reactions to ensue which results in the reduction of PhN=NPh to PhNHNHPh and the oxidation of FlHOH and FlHNH2 to Fl=0) and Fl=NH respectively.

The electrochemical reduction of fluorenone hydrazone, fluorenone fluorenylhydrazone, fluorenone azine and their benzophenone analogs in DMF

Abstract The electrochemical reductions of fluorenone hydrazone (Fl=NNH2), fluorenone fluorenylhydrazone (FIHNHN=Fl), fluorenone azine (Fl=N−N=Fl) and benzophenone analogs have been studied at platinum cathodes in DMF−0.1 M(n-Bu)4NClO4 in the absence and presence of an added proton donor. Fl=NNH2 undergoes reduction to give an unstable anion radical which decomposes by an unidentified pathway to afford Fl=NH. The latter species is electroactive at the applied potential and is reduced to the corresponding amine, FlHNH2. Four electrons per molecule of Fl=NNH2 are required for this process when electroreduction is effected in the presence of diethyl malonate (DEM), an electroinactive proton donor. Unreacted Fl=NNH2 serves as the source of protons when electroreduction is conducted in the absence of DEM. Dual reaction channels are observed for the reduction of FlHNHN=Fl. If the corresponding anion radical is not protonated by either an added proton donor or unreacted starting material, decomposition occurs by carbon-nitrogen bond cleavage to give FlH2 and Fl=NNH2 as products. Reduction of the latter species occurs at the same potential as FlHNHN=Fl, ultimately affording FlHNH2. The reaction channel involving carbon-nitrogen bond cleavage is replaced by a pathway involving nitrogen-nitrogen bond cleavage in the presence of an added proton donor. The final reduction product by this route is FlHNH2. Fl=N−N=Fl is reduced stepwise and reversibly to its dianion in an aprotic medium. In the presence of a relatively strong proton donor such as hexafluoro-2-propanol, reduction gives FlHNHN=Fl. The redox behavior of the benzophenone analogs, Ph2C=N−N=CPh2, Ph2CHNHN=CPh2 and Ph2C=NNH2, parallels that of their counterparts in the fluorene series.

Electron-transfer processes: the electrochemical reduction of N,N-dimethyl-and p-cyanobenzene-sulfonamide

Abstract The redox behavior of N,N-dimethyl-and p -cyanobenzenesulfonamide has been studied in dimethylformamide by electroanalytical methods. The tertiary sulfonamide is reduced in successive one-electron steps to give first its stable radical anion and then its dianion. The latter species is unstable and rapidly decomposes by sulfur-nitrogen bond cleavage to give dimethylamide anion and p -cyanobenzenesulfinate ion. One-electron reduction of the sulfinate to its stable dianion radical occurs at the applied potential and completes the reduction pathway. The reduction pathway for the primary sulfonamide is potential dependent. At the potential of the first cathodic wave, the primary sulfonamide is reduced irreversibly to its radical anion. This species undergoes nitrogen-hydrogen bond cleavage and affords hydrogen and the conjugate base of the primary sulfonamide as products. The base is then reduced to its stable dianion radical at more negative potential. Reduction of the primary sulfonamide at the potential of the second cathodic wave gives its dianion, a species which rapidly undergoes decomposition by nitrogen-sulfur bond cleavage to give amide anion and p -cyanobenzenesulfinate. Subsequent chemical and electrochemical reactions involving these species and the primary sulfonamide formally give p -NCC 6 H 4 SO 2 NH −2 as the principal product in an overall two-electron step.

Triple-potential-step chronoamperometry: Application to electrode processes involving a quasi-reversible electron transfer followed by an irreversible chemical reaction

Abstract Triple-potential-step chronoamperometry (TPSCA) has been used to identify either the previously disputed or the unidentified anodic process that arises in the electroreduction of diazodiphenylmethane, diethyl diazomalonate, and fluorenone hydrazone. In each of the three systems, slow heterogeneous electron transfer at a platinum electrode in DMF-0.1 M (n-Bu)4NClO4 causes the separation between the anodic peak for the reoxidation of the unreacted anion radical of the starting material and the corresponding cathodic peak to exceed 0.7 V. The scope and limitations of the TPSCA method for a reaction scheme involving quasi-reversible heterogeneous electron transfer followed by an irreversible homogeneous chemical reaction are discussed.

The electrochemical reduction of fluorenone imine, 9-aminofluorene, and several N-substituted derivatives in dimethylformamide

Abstract The cyclic voltammetric reduction of fluorenone imine (FlNH) and N-phenylfluorenone imine (FlNPh) in DMF occurs in successive one-electron steps. Although FlNH−. and FlNPh−. are relatively stable on the cyclic voltammetric time scale, they react on the coulometric time scale to give the corresponding amines in high yield. FlNH2− and FlNPh2− have half lives of less than 1 ms and react by abstracting a proton to give FlNH2− and FlNHPh−, respectively. Oxidation of FlNHPh− to FlNPh in the presence of potassium t-butoxide involves a kinetically-controlled anodic peak which arises from the catalytic oxidation of FlNHPh− by electrogenerated FlNPh and a second, irreversible, anodic peak at more positive potential which is assigned to the direct electrochemical oxidation of FlNHpH−. Electrocatalytic oxidation is not observed under analogous solution conditions in the FlNH2− system unless fluorenone azine anion radical is also present. FlHNH2−. and FlHNMe2−. decompose by carbon—nitrogen bond cleavage (k =0.8 s−1 and 1.1 s−1, respectively, at −22°C and −51°C, respectively) to give an anionic fragment, presumably an amide, and a radical fragment, presumably FlH.. After reduction of the radical fragment to an anion by an unreacted anion radical, the amide abstracts a proton from the C9 position of the starting material. FlH− also reacts by proton abstraction from starting material, but at a rate which is kinetically controlled on the cyclic voltammetric time scale. The oxidation of FlNMe2− to the corresponding cation occurs in successive, one-electron steps in the absence of FlH−. If reaction of the electrogenerated FlH− with FlHNMe2 is incomplete when oxidation of FlNMe2− is effected, the intermediate radical, FlNMe2., is interdicted by electrocatalytically formed FlH..