Yoshitomo Kashiwagi

Enantioselective electrocatalytic oxidation of racemic sec-alcohols using a chiral 1-azaspiro[5.5]undecane-N-oxyl radical

Nitroxyl radical (6S,7R,10R)-4-acetylamino-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]-undecane-N-oxyl reveals a reversible redox peak in cyclic voltammetry at + 0.62 V vs. Ag/AgCl. A preparative electrocatalytic oxidation of racemic sec-alcohols on the nitroxyl radical yielded mixtures of 51.4 – 63.9 % ketones and 36.1 – 48.6 % alcohols by 10 h of electrolysis. The current efficiency and turnover number of the reactions were 85.6 – 87.9 % and 20.6 – 25.6, respectively. The enantiopurity of the remaining (R)-isomers was 50 – 70 % and the S values as a selective factor was 4.1 – 4.6.

Enantioselective, electrocatalytic oxidative coupling of naphthol, naphthyl ether and phenanthrol on a TEMPO-modified graphite felt electrode in the presence of (–)-sparteine (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yloxyl)

Constant potential electrolysis of 2-naphthol, 2-methoxynaphthalene and 10-hydroxyphenanthrene at 0.6 V vs. Ag/AgCl on a TEMPO-modified graphite felt electrode in the presence of (–)-sparteine in acetonitrile yields, enantioselectively, (S)-binaphthyl type dimers in >92% isolated yield and with >88% current efficiency with enantiomeric excesses of 98, 91 and 97% respectively for each dimer, respectively.

Preparative, electroenzymatic reduction of ketones on an all components-immobilized graphite felt electrode

Abstract A thin cation-exchange polymer (Nafion)- and poly(acrylic acid) layers-coated graphite felt electrode immobilizing all mediation components of methyl viologen, NAD + , diaphorase and alcohol dehydrogenase to construct a complete bioelectrochemical reactor was prepared and applied to electroenzymatic reduction of ketones in a phosphate buffer at constant potential of −0.80 V vs sce . S -Isomers of 2- and 3-methylcyclohexanone were enantioselectively reduced to the corresponding (1 S , 2 S )-(+)-2- and (1 S , 3 S )-(−)-3-methylcyclohexanol, respectively, in high current efficiency (98.6 and 96.5%), high reduction yield (almost quantitatively) and high enantiomeric excess (ee) (100 and 93.1%), respectively. The unreacted ketones, ( R )-(−)-2- and ( R )-(+)-3-methylcyclohexanone, were recovered in high recovery yield (100 and 96%) and high ee (99.6 and 95.8%), respectively.

Voltammetric behavior of β-phosphonylated nitroxide and its application to electrocatalytic oxidation of alcohol

Abstract N - tert -Butyl- N -[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (DEPN) as β-phosphonylated nitroxide was prepared using tert -butylamine, pivalaldehyde and phosphonic acid diethyl ether as starting materials. This nitroxide revealed a reversible redox peak in cyclic voltammetry at +0.81 V versus Ag/AgCl, and showed electrocatalysis for the oxidation of alcohol. A preparative electrocatalytic oxidation of 4-methylbenzyl alcohol on the nitroxide yielded 98% 4-methylbenzaldehyde by 10 h of electrolysis. The current efficiency, turnover number and pseudo-first-order kinetic constant of the reaction were 96.5%, 39 and 0.95×10 −5 s −1 , respectively.

Electrocatalytic dehalogenation of organohalides on a nickel(II) tetraazamacrocyclic complex-modified graphite felt electrode

Abstract Electrocatalytic dehalogenation of organohalides was studied using a nickel(II) tetraazamacrocyclic complex-modified graphite felt electrode. The nickel(II) tetraazamacrocyclic complex-modified graphite felt electrode was prepared by attaching nickel(II) (6-(2′-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane)perchlorate chemically to the carboxyl groups of a thin poly(acrylic acid) layer coated on the graphite felt. The modified electrode gave a reversible electron transfer for the nickel(II)/nickel(I) redox couple in cyclic voltammetry at −0.95 V versus Ag/AgCl. A preparative electrocatalytic dehalogenation of organohalides was successfully achieved on the modified electrode with an adequate current efficiency (55.6–94.8%), conversion (34.2–100%) and turnover number of the Ni catalyst (667–3333).

Enantioselective electrocatalytic oxidation of racemic alcohols on a TEMPO-modified graphite felt electrode by use of chiral base (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yloxyl)

The S-isomers of four racemic sec-alcohols, which possess a chiral centre at the α-position to the hydroxy group, were oxidized to the corresponding ketones whereas the R-isomers remained unreacted on a TEMPO-modified graphite felt electrode in the presence of (–)-sparteine, where the enantiopurity of the remaining R-isomers was > 99% and the current efficiency for the produced ketones was > 90%.

Oxidation of alcohols with nitroxyl radical under polymer-supported two-phase conditions

The oxidation of alcohols to carbonyl compounds was studied using potassium hexacyanoferrate(III) mediated by nitroxyl radical as the catalyst under polymer-supported organic–aqueous two-phase conditions. Primary alcohols are readily oxidized to the corresponding aldehydes in excellent yield with no over-oxidation to carboxylic acids. Secondary alcohols are converted to the corresponding ketones with a much lower efficiency. The oxidation reactions of primary alcohols in the presence of secondary alcohols is strongly favored. Primary-secondary diols are selectively transformed into hydroxy aldehydes with small amounts of ketoaldehydes, the amount of isomeric keto alcohols remaining is less than 1%.

Electrocatalytic reduction of aldehydes and ketones on nickel(II) tetraazamacrocyclic complex-modified graphite felt electrode

Abstract Electrocatalytic reduction of aldehydes and ketones was studied using a nickel(II) tetraazamacrocyclic complex-modified graphite felt (GF) electrode. The nickel(II) tetraazamacrocyclic complex-modified GF electrode was prepared by attaching nickel(II) (6-(2′-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane)perchlorate chemically to the carboxyl groups of a thin poly(acrylic acid) (PAA) layer coated on the GF. The modified electrode gave a reversible electron transfer for the nickel(II)–nickel(I) redox couple in cyclic voltammetry at −0.95 V versus Ag/AgCl. A preparative electrocatalytic reduction of aldehydes and ketones to the corresponding alcohol and pinacol compounds was successfully achieved on the modified electrode with an adequate current efficiency (46.9–75.9%), conversion (48.1–84.6%) and turnover number of the nickel catalyst (1053–2267).

Enantioselective electrocatalytic oxidation of racemic amines using a chiral 1-azaspiro[5.5]undecane N -oxyl radical

A preparative electrocatalytic oxidation of racemic amines, which contain a chiral centre α to the amino group, on (6S, 7R, 10R)-4-acetylamino-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]undecane N-oxyl yielded mixtures of carbonyl compounds (54.3–66.1%) and amines (33.9–45.7%) after 5 h of electrolysis, in which the current efficiency, turnover number, enantiopurity of the remaining (R)-isomers and S values were 90.7–94.8%, 21.7–26.5, 62–78% and 4.7–5.8, respectively.

Asymmetric electrochemical lactonization of diols on a chiral 1-azaspiro[5.5]undecane N-oxyl radical mediator-modified graphite felt electrode.

A graphite felt electrode modified with (6S,7R,10R)-4-amino-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]undecane N-oxyl was prepared for electrocatalytic oxidation of diols; electrolysis of diols on the modified electrode yielded optically active lactones (92.0-96.4%), with an enantiopurity of 82-99% ee.