This article describes our recent results of electroorganic synthesis, especially focusing on new organic electrolytic systems toward green sustainable chemistry as follows: (a) Electroorganic synthesis using recyclable solid-supported bases, (b) electrocatalytic hydrogenation and dehalogenation using new electrolytic systems, (c) electrosynthesis of organofluorine compounds, conducting polymers, and others in ionic liquids.
Polyphenylene (PP) was electrochemically synthesized from biphenyl under a centrifugal field (315 g) and Earth’s gravity (1 g). The polymerization rate of biphenyl (BP) was found to be increased by the applied gravitational force. PP films thus synthesized on electrodes with and without centrifugation also exhibited clear differences in their physical and chemical properties. In addition, morphological structure of the film was also affected by the centrifugal field. A mechanism for the electrochemical deposition of PP film under the centrifugal force has also been proposed.
Structural effects on dechlorination of chloroform (CHCl3) have been studied on the low index planes of Ag in acetonitrile using macro electrolysis. All the electrodes produce methane (CH4). Partial current density of methane depends on crystal orientation at −1.83 V (Fc/Fc+) remarkably: Ag(100)<Ag(111)<Ag(110). No structural effect is obtained at −2.58 V (Fc/Fc+). The potential dependence of the structural effect is discussed according to a film formation on the surfaces during the electrolysis.
Chiral catalytic film, PLPy (Pd) electrodes, which were coated with poly (N-substituted pyrrole) film having l-(+)-lactic acid moiety as an optically active group and were incorporated with palladium catalytic metal were prepared. By use of these electrodes, the electrocatalytic hydrogenation of α-keto ester compounds, methyl benzoylformate (1a) and butyl benzoylformate (1b) in an ethanol-HCl buffer solution led to efficient formation of the corresponded hydrogenated products, (R)-(−)-methyl mandelate (2a) and (R)-(−)-butyl mandelate (2b) with appreciable enantioselectivities, respectively.
Rate constants for decomposition reaction were measured for several phenols, i.e., bisphenol A, p-chlorophenol, p-octylphenol, 2,4-dichlorophenol, p-tert-butylphenol and for two other chemicals, benzophenone and PCB compounds in waste oil. The following methods were adopted: electrolysis, photolysis, sonolysis, photolytic electrolysis, and sonolytic electrolysis. The initial concentration was fixed at 50 mg/L except for the PCB, for which the concentration for all the isomers was 500 ng/L. The solvent was water, but for chemicals with insufficient solubility, mixtures with alcohols were used. Additional application of ultrasound led to remarkable enhancement for p-chlorophenol and 2,4-dichlorophenol. The electrolysis rate constant for PCB compounds was comparable to that for the phenols. Benzophenone was decomposed 10 times as slow as the phenols partly because of the solvent containing alcohols.
Electroreduction of methylphenylpolysilane (MPPS) with Mg electrode led to the formation of silyl anion. The reactions of this silyl anion with electrophiles such as dichlorosilanes and dichlorogermanes were found to form new Si–Si and Si–Ge bond, respectively. Namely, the reaction of this silyl anion with methylphenyldichlorosilanes resulted in the increase of the molecular weight of MPPS and that with dichlorogermanes led to the formation of Si–Ge copolymers.
Peroxycitric acid (PCA) was successfully synthesized for the first time as an equilibrium mixture with citric acid and hydrogen peroxide using sulfuric acid (homogeneous) and Nafion (heterogeneous) catalysts. The formation of PCA in the equilibrium mixture was detected by the potentiometric measurement based on the triiodide/iodide redox couple potential buffer and the concentration of PCA was determined from the obtained potential change. The equilibrium % yield of PCA was found to largely depend on the concentration and the nature of catalysts. The peculiar roles of the heterogeneous and homogeneous catalysts in the formation of PCA were discussed briefly.
Oxidation of α-mangostin 1, γ-mangostin 2, and related derivatives with electrolysis or active species electrochemically generated in situ from iodobenzene was performed. γ-Mangostin 2 and its derivatives provided the dienone-type products, 5, 5a–5c, although 1 and its derivatives gave a complex mixtures. The methoxy derivative 6 was obtained by oxidation of 1 using the iodobenzene-derived active species. In contrast to these oxidations, reactions of 1 and 2 with [bis(trifluoroacetoxy)iodobenzene] (PIFA) provided complex mixtures.
A constant current electrolysis of saturated aliphatic ketones in a CH3CN or DMF solution containing 0.1 mol dm−3 Bu4NBF4 with a platinum cathode and a magnesium anode under an atmospheric pressure of CO2 resulted in an efficient fixation of CO2 at α-position of carbonyl group in the ketones to give the corresponding β-keto carboxylic acids in moderate to good yields.
We have successfully developed a novel mixed-Kolbe electrolysis system using solid-supported bases. Mixed-Kolbe electrolysis of carboxylic acids using solid-supported bases was carried out to provide the corresponding mixed-Kolbe products in moderate to good yields. In this system, solid-supported bases were recyclable even under high current density.
We have successfully developed a novel electrolytic system using KBr as a mediator and solid-supported acids as a supporting electrolyte. In this system, solid-supported acids such as Amberlyst 15Dry (–SO3H) serve as not only a promoter of the dissociation of KBr but also a supporting electrolyte. Anodic oxidation of some benzyl alcohols using KBr and Amberlyst 15Dry (–SO3H) in the presence of 2,6-lutidine was successfully carried out to provide the corresponding benzaldehydes in good to excellent yields.
Liquefiable micelle-like micro-particles possessing maleimide moieties on the surface, which allow reversible capture of electrogenerated iminium cation intermediates of pyrrolidine derivatives, were prepared. It was found that the particles efficiently captured the aimed intermediates, allowing separation from the electrolyte solution, and could be made to release the tagged product simply by thermal liquefaction. The tag introduced to the pyrrolidine was easily removed by electro-oxidative carbon-sulfur bond cleavage under mild condition.
Combination of cyclohexane and nitriles in the presence of electrolytes were found to realize an effective, biphasic thermomorphic system that enable practical applications of electrochemical reactions featuring high current density followed by rapid product/electrolyte separation.
To achieve accurate real-time 2D H2O2 distribution imaging, we fabricated an H2O2 sensor array by dispensing small quantities of enzyme solution very accurately onto electrodes in an electrode array. We confirmed the accuracy of the dispensing technique. We then investigated the responses of each sensor to H2O2 injection using a 64-channel multipotentiostat and found they varied by 20%. Real-time 2D H2O2 imaging was therefore successfully realized by using our flow cell system. This sensor array will be useful for the non-invasive, real-time monitoring of the H2O2 distribution in biological samples.
Contact glow discharge electrolysis (CGDE) of alkanesulfonates in a neutral phosphate buffer solution was investigated. Methanesulfonic acid was oxidatively degraded to inorganic carbon and sulfate ion as the final products. Also in CGDE of ethanesulfonate and 1-propanesulfonate, the oxidation of alkyl and sulfo groups smoothly proceeded and various carboxylic acids such as oxalate and formate were found as the intermediate products. On the basis of product distribution and kinetic studies, it was assumed that hydroxyl radicals would play a crucial role in the oxidative degradation of aqueous alkanesulfonates.
The 1,4-addition reaction of several β-ketoesters with vinyl ketones has been accomplished using iron(II)tetrafluoroborate (Fe(BF4)2·6H2O) as catalyst under air atmospheric conditions at room temperature in an acetonitrile or an ionic liquid solvent system. Recycled use of the catalyst was possible while maintaining excellent yield in the ionic liquid solvent system.
Steady-state kinetic parameters between PQQ-dependent soluble glucose dehydrogenase (PQQ-sGDH) and mediators were evaluated based on a theory of bioelectrocatalysis. Quinones showed bi-molecular reaction constants in the order of 1010 M−1 s−1 as the limiting values, while the values of Os-complexes were two or three orders smaller. The magnitude of the limiting values was interpreted in terms of the long-range electron transfer in the active center as well as the diffusion-controlled reaction model.
Adsorption behavior of bilirubin oxidase (BOD) on Au electrodes was studied using quartz crystal microbalance (QCM) measurements and electrochemical measurements of BOD-catalyzed reduction current of O2 without mediators. The QCM measurements supported the monolayer adsorption of BOD on Au electrodes at pH 5 to 9. The catalytic current density was, however, much smaller than that at hydrophilic edge plane-type carbon electrodes or than the calculated value for active BOD. Some denaturation is suggested on Au electrodes, which was supported by electrochemical QCM (EQCM) measurements.
1-Alkoxy-2,2,2-trifluoroethyl group was found to be a new protecting group for electrochemical oxidation of L-prolinol. The result is a first example for the introduction of a nucleophile to α-position of L-prolinol without a loss of the optical activity through an iminium ion intermediate. This exploited method is complementary for methods in which an electrophile is introduced to the α-position of L-proline derivative through a carbanion intermediate.
The reactions of 3-substituted-diphenylamine cation radicals in acetonitrile were studied using an electron transfer stopped-flow method. In the reactions of the 3-chloro-diphenylamine cation radicals (mCl-DPA·+), the main reaction route was the formation of the benzidine dimer, which was similar to the case of the diphenylamine cation radical (DPA·+). Although the reaction of DPA·+ proceeded via the cation radical-cation radical coupling as verified from the rate law of −d[DPA·+]/dt=k[DPA·+]2, the present kinetic analysis has revealed that the decay rate of mCl-DPA·+ was dependent on the concentration of the neutral molecules, i.e., the rate law was expressed as −d[mCl-DPA·+]/dt=k[mCl-DPA·+]2 [mCl-DPA]. In contrast, the reaction of the 3-methoxy-diphenylamine cation radical (mMeO-DPA·+) was too fast to be observed using the stopped-flow method, which is quite in contrast to the 4-methoxy-diphenylamine cation radical (pMeO-DPA·+) which was very stable in acetonitrile. In the case of mMeO-DPA·+, the cyclization reaction was confirmed to proceed soon after the generation of mMeO-DPA·+, which is similar to the case of the 3-methyl-diphenylamine cation radical (mMe-DPA·+). Thus, it was found that the substituent on the 3-position changed the reaction pathways of DPA·+ significantly, as well as their reactivity.
Electrolysis of 2-azetidinone 3 in AcOH/CH3CN containing NaBr (2–10 equiv.) was carried out in an undivided cell fitted with two platinum electrodes to afford the corresponding N-bromo-2-azetidinones 2a, while a similar electrolysis in a divided cell afforded no appreciable amount of 2 but a small amount of 4-acetoxy-2-azetidinone 1 together with a complex mixture. Ring-expansion of 2-azetidinone 3 leading to 4 took place exclusively by electrolysis in methanol containing AcONa. On the other hand, N-iodination proceeded efficiently only in a divided cell in the presence of 2.5 equiv. of NaI. Reaction of N-bromo-2-azetidinone 2 with various nucleophiles, e.g., acetate, thiolate, and alkoxides, afforded no substitution products, resulting in the reductive debromination leading to 3, whereas with diphenyl disulfide, N-phenylsulfenyl-2-azetidinone 5 was mainly obtained.
The modified platinum electrodes having complexes of sodium with crown ether type ligands were prepared and utilized in the electroreduction of methyl decanoate (1) in the presence of t-BuOH as a proton donor and Bu4NClO4 as a supporting electrolyte. The product yield by using the modified platinum electrode having the complex of lithium or sodium ion with the monomeric ligand LA (MPt(A)–Na) was 46%. The use of the modified platinum electrodes having the complex of sodium ion with the polymeric ligands LB (MPt(B)–Na) or LC (MPt(C)–Na) gave the reduced product in 52% or 47% yield respectively. Durability of the electrodes, MPt(A)–Na and MPt(B)–Na, under the electroreduction conditions was not so high, however, the activity of the electrodes was found to be recovered by the re-complexation of sodium ion with the ligand on the surface of the used modified electrodes. The durability of the electrode MPt(C)–Na was satisfactory to use repeatedly, and the use of MPt(C)–Na as a cathode and NaClO4 instead of Bu4NClO4 as a supporting electrolyte gave the reduced product in 69% yield. Moreover, using the modified electrodes in the presence of anodically dissolved magnesium ion the reduced product was obtained in >90% yield.
Electrochemical synthesis of poly (cyclotetramethylenesilylene) was successfully carried out for the first time by reducing 1,1-dichlorosilacyclopentane in 1,2-dimethoxyethane using a platinum cathode and a silver anode. The resulting polymer had Mw=5–8×103 and a relatively broad absorption with its λmax at ca. 275 nm. The polymer was relatively stable to UV irradiation compared to a typical polysilane, poly (dibutylsilane). Copolymerization of 1,1-dichlorosilacyclopentane and dibutyldichlorosilane were also successful.
A method for sequential introduction of two organic groups on the same or the different α-carbons of nitrogen has been developed based on a combination of the concept of electroauxiliary and the cation pool method. Selective introduction of two carbon nucleophiles into 2,5- or 2,2-position of pyrrolidine has been accomplished via sequential electrochemical oxidation of pyrrolidine derivatives containing two silyl groups as the electroauxiliaries. Introduction of two carbon nucleophiles into 2,6-position of piperidine was also successful. The products having two olefinic groups were effectively converted to nitrogen-containing spiro compounds using ring closing metathesis, and an application to formal synthesis of cephalotaxine was also carried out. These results clearly illustrate the synthetic utility and potential of the present approach, and open a new aspect of the synthesis of nitrogen-containing compounds.
One-pot vicinal double C-carboalkoxylation of styrene derivatives has been developed by electroreduction in the presence of a large excess of N-carboalkoxyimidazoles in N,N-dimethylformamide (DMF) containing tetra-n-butylammonium bromide as a supporting electrolyte using an undivided cell equipped with Zn-plate as the anode and cathode under the constant current conditions until 6 F/mol of electricity passed through the system, to afford the corresponding phenylsuccinic acid esters in good to moderate yields. On the other hand, electroreduction of aromatic imine derivatives under the similar conditions brought about one-pot geminal double C-carboalkoxylation, giving the corresponding aryl aminomalonic esters in moderate yields. As one of the most plausible mechanism, electron transfer to carbon-carbon and carbon-nitrogen double bonds may involved as the first step, followed by electrophilic C-carboalkoxylation.
Kinetics of mono and dimethoxy-substituted benzyl alcohols oxidation by electrogenerated phthalimido-N-oxyl radical were investigated using rotating disk electrode voltammetry. The kinetic data arranged in a Hammett correlation indicated the validity of the peculiar reactivity of veratryl alcohol.
The surface of a gold (Au) disk electrode was modified with a self-assembled monolayer consisting of phenylboronic acid residue to prepare a ribonucleoside-sensitive electrode. The cyclic voltammetric response of the modified electrode to [Fe(CN)6]3− ion dissolved in the sample solution was found to be dependent of the type of ribonucleoside and its concentration. In other words, the peak current in the cyclic voltammogram of [Fe(CN)6]3− ion was significantly decreased when uridine or cytidine was added in the working buffer, while the response to thymidine, 2′-deoxyuridine, and 2′-deoxycytidine was rather small. The voltammetric response was rationalized on the basis of the electrostatic repulsion between [Fe(CN)6]3− ion and the negatively-charged electrode surface, which originates from the addition of OH− ion on the electrode as a result of the formation of phenylboronic acid-ribonucleoside conjugate on the surface of the monolayer.
Indirect electroreductions of 4-methyl-4-trichloromethyl-p-quinone-(1)-arylimines using a sacrificial sulfur-graphite electrode (S-C electrode) caused the elimination of a trichloromethyl group to give the corresponding N-tolylarylamines, whose yields increased largely with an increase in temperature and by the addition of benzoic acid as a proton donor. Similarly, diimines with two trichloromethyl groups underwent the elimination of two trichloromethyl groups. The above indirect electroreduction using the S-C electrode was undertaken at the low cathodic overpotential, and gave high product yields compared with the direct electroreduction.