Electrocatalytic Oxidations Mediated By Nitroxyl Radicals

Nitroxyl radicals have been known since 20^th century.The unpaired electron in these radicals is delocalized over the nitrogenoxygen bond and this accounts for their wide use in synthetic and mechanistic reactions.2,2,6,6-Tetramethylpiperidinyl-l-oxyl?TEMPO?,as a kind of stable nitroxyl radicals,is widely employed as a mediator in electrochemical synthesis.In this paper,a novel electrochemical route to generate nitriles from aldehydes under mild conditions using a catalytic amount of TEMPO as the mediator and hexamethyldisilazane as the nitrogen source in the presence of acetic acid has been developed.A variety of aromatic,heteroaromatic and aliphatic aldehydes have been converted to their corresponding nitriles in good to excellent yields.The electrochemical reaction was investigated by cyclic voltammetry and the consumption of TEMPO⁺as the electrochemical redox mediator on Pt electrode was confirmed.In an effort to gain insight into the plausible reaction mechanism,reactions were studied by in-situ FTIR and the intermediates of the reactions were obtained through the formation of N,N'-disubstituted methanediamine between HMDS and aldehydes.A plausible reaction mechanism is proposed based on the evidence.Benzaldehyde can react with HMDS to give N,N'-dibenzylidene-1-phenylmethanediamine,which is oxidized by TEMPO⁺ions to yield benzaldehyde and benzonitrile.At the same time TEMPO⁺is reduced to TEMPOH,which can be regenerated by electro-oxidation.The kinetic resolution of racemic secondary alcohols is an important method to obtain optically active secondary alcohols.Chiral nitroxyl radicals are often used in the oxidative kinetic resolution of secondary alcohols.In this paper,4-?3-?pyrrol-1-yl?propionyloxy?-2,2,7-trimethyl-10-isopropyl-1-azaspiro-[5.5]undeca ne N-oxyl?Py-SPIROXYL?was obtained through reaction between pyrrole carboxylic and 4-Hydroxy-2,2,7-trimethyl-10-isopropyl-1-azaspiro-[5.5]undecane N-oxyl?4-Hydroxy-SPIROXYL?.The preparation of PPy-SPIROXYL/Pt electrode was carried out on Pt electrode in NaClO₄/CH₃CN solution by electrochemical polymerization.The electrocatalytic activity of PPy-SPIROXYL/Pt electrode was investigated by cyclic voltammetry.When employed in the electrochemical oxidation for optically active 1-phenylethanol,the PPy-SPIROXYL/Pt electrode showed higher electrocatalytic performance in the oxidation of?R?-1-phenylethanol.