Steven D. Bull

Redox processes in microdroplets studied by voltammetry, microscopy and ESR spectroscopy: oxidation ofN,N,N′,N′-tetrahexylphenylene diamine deposited on solid electrode surfaces and immersed in aqueous electrolyte solution

Electrochemical processes in nonhomogeneous media such as emulsions allow the electrolysis of materials of low solubility, which gives rise to new ways of controlling reaction environments, and/or simpler separation of products. However, the mechanistic details of these redox processes are not well understood. Two types of processes may be considered: (i) reactions in the two-phase solution environment coupled by simple mass transport inone phase to the interfacial electron transfer, and (ii) reactions involvingboth liquids in direct contact to the electrode surface (wetting). The latter type of process can be studied separately and is analyzed in this work for the one electron oxidation of the oilN,N,N′,N′-tetrahexylphenylene diamine deposited in the form of microdroplets on basal plane pyrolytic graphite, gold, or siliconized indium tin oxide (ITO) electrodes and immersed in aqueous electrolyte. The bulk conversion of the nonconducting oily material proceeds via the uptake of anions from the aqueous phase and the formation of a dark blue product monitored by voltammetry, optical microscopy, and ESR spectroscopy. The process is extremely sensitive to the type of anion which is electroinserted into the organic phase as is shown for ClO4−, PF6−, Cl−, I−, and OH−. In the case of electroinsertion of hydroxide, a coupled chemical process occurs that can be monitored voltammetrically.

Anion Detection by Electro‐Insertion into N, N, N′, N′‐Tetrahexyl‐Phenylenediamine (THPD) Microdroplets Studied by Voltammetry, EQCM, and SEM Techniques

The electrochemical oxidation and re-reduction of N′,N′,N,N-tetrahexylphenylene diamine (THPD) deposited in form of microdroplets on a basal plane pyrolytic graphite or gold electrode is shown to be a chemically reversible process in the presence of aqueous electrolyte media containing NO3−, SCN−, ClO4−, or PF6−. Sharp voltammetric responses with a mid point potential, Emid, characteristic of the type of anion and its concentration are observed. The oxidation product, an ionic liquid, undergoes rapid ion exchange when the anion of the aqueous electrolyte is exchanged with anions of lower Emid replacing anions of higher Emid. Although the effect of the supporting electrolyte cation in the case of the alkali metals K+, Na+, and Li+ is not significant, a considerable change in the voltammetric peak shape and Emid occurs in the presence of protons. This effect, attributed to the protonation of THPD, is also sensitive to the type of anion present with anions of lower Emid causing more facile protonation. After protonation of THPD, oxidation and re-reduction can be shown to be associated with H+ expulsion and uptake. Deposited onto the rough surface of a gold coated planoconvex quartz crystal oscillator the [THPD+ClO4−]oil deposit can be observed in form of micron-sized droplets in SEM images. A strong frequency response of the crystal oscillator in an electrochemical quartz crystal microbalance experiment associated with the oxidation and re-reduction of THPD can be detected but is not related to changes in mass. Rather, this frequency response may be attributed to changes in the viscosity and/or coverage of the oily deposit.

Chiral relay auxiliaries

Chiral auxiliaries and templates are effective tools for the asymmetric synthesis of homochiral molecules.1 Most chiral auxiliaries are small heterocyclic compounds which rely on sterically demanding functional groups to control the conformation of their ring systems. Under ideal circumstances, the conformation of an auxiliary should be constrained to ensure that its prochiral centre reacts with a reagent via diastereoisomeric transition states which are sufficiently different in energy to ensure that only a single diastereoisomer is formed as product. In order to maximise the diastereoselectivity observed for an auxiliary, it would appear reasonable that the Stereocontrolling functional group adopts a position in space as close as possible to the newly forming stereogenic centre. Structural considerations dictate that realisation of this ideal is not always attainable and numerous examples of chiral auxiliaries that rely on relatively remote stereogenic centres to control diastereoselectivity are known. Alkylation of the enolates of Seebach’s imidazolidinone (1),2 or Schollkopf’s bis-lactim ether auxiliary (2),3 for example, are controlled via 1,3- and 1,4- asymmetric induction respectively (scheme 1).

Emulsion electrosynthesis in the presence of power ultrasound Biphasic Kolbe coupling processes at platinum and boron-doped diamond electrodes

Abstract The electrochemical oxidation of aliphatic carboxylic acids, hexanoic, heptanoic, and lauric acid, under biphasic conditions is studied as a model system for ultrasound enhanced Kolbe electrosynthesis processes. Power ultrasound is used to generate an in situ emulsified medium and to remove reaction products continuously from the electrode surface. A clean and highly efficient process at platinum electrodes with formation of ‘one electron’ products only occurs in marked contrast to processes in monophasic media. For hexanoic acid the Kolbe dimer product R–R is formed in up to 75% yield with 45% current efficiency at 0.18 A cm −2 current density and in the presence of 190 W cm −2 ultrasound. The mechanism is explained in terms of a dynamically modified electrode surface, at which hydrophobic products are immediately ‘trapped’ via partitioning into a non-polar organic phase and transported away into the emulsion system. Kolbe electrosynthesis is undertaken both at platinum electrodes and at free-standing polycrystalline boron-doped diamond electrodes, in order to minimize the surface erosion effect induced by power ultrasound. The type and yield of products obtained from the biphasic Kolbe electrolysis process at diamond electrodes are essentially identical to those observed at platinum and based on this observation, the presence of a biphasic reaction layer at the electrode surface is postulated to govern the process.

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.

Voltammetry of Electroactive Oil Droplets. Part II: Comparison of Experimental and Simulation Data for Coupled Ion and Electron Insertion Processes and Evidence for Microscale Convection

Modelling electrochemical processes at the three phase junction between electrode–aqueous electrolyte–oil droplet presents a considerable challenge due to the complexity of simultaneous electron transfer between electrode and droplet, ion uptake or expulsion between droplet and aqueous phase, the interaction of redox centers at high concentration, and transport processes accompanying the electrochemical process. For the case of oxidation of para-tetrahexylphenylenediamine (THPD) microdroplet deposits on basal plane pyrolytic graphite electrodes or random arrrays of microelectrodes (RAM) three models may be envisaged which proceed via A) exchange of ions between droplet and aqueous electrolyte with the electrochemical process commencing at the electrode–oil interface, B) rapid electron transport over the oil–aqueous electrolyte interface and the electrochemical process commencing from the oil–aqueous electrolyte interface inwards, and C) slow electron transport across the oil–aqueous electrolyte interface and the electrochemical process commencing solely from the triple interface. Numerical simulation procedures for these three models, which allow for interaction of redox centers via a regular solution theory approach, are compared with experimental data. A positive interaction parameter, Z=1.4, consistent with a dominant ionic liquid–ionic liquid and neutral oil–neutral oil type interaction is determined from experimental data recorded at sufficiently slow scan rates. The overall mechanism, which governs the voltammetric characteristics at higher scan rates, is shown to be apparently consistent with the triple interface model C). However, the rate of diffusional transport determined by comparison of experimental with simulation data is orders of magnitudes too high. Additional convection processes, possibly of the Marangoni type, appear to be responsible for the fast rate observed for the redox process. The significance of the models presented in the context of microdroplet deposits for other related electrochemical systems is discussed.

SuperQuat N-acyl-5,5-dimethyloxazolidin-2-ones for the asymmetric synthesis of α-alkyl and β-alkyl aldehydes

The proclivity of α-branched N-2′-benzyl-3′-phenylpropionyl derivatives of (S)-4-benzyl-5,5-dimethyl-, (S)-4-phenyl-5,5-dimethyl-, (S)-4-isopropyl-5,5-dimethyl-, (S)-4-benzyl- and (S)-4-benzyl-5,5-diphenyl-oxazolidin-2-ones to generate directly 2-benzyl-3-phenylpropionaldehyde upon hydride reduction with DIBAL is investigated. The (S)-4-benzyl-5,5-dimethyl-derivative proved optimal for inhibition of endocyclic nucleophilic attack, giving 2-benzyl-3-phenylpropionaldehyde in good yield upon reduction. Application of this methodology for the asymmetric synthesis of chiral aldehydes via diastereoselective enolate alkylation of a range of (S)-N-acyl-4-benzyl-5,5-dimethyloxazolidin-2-ones to afford an array of α-substituted-N-acyl-5,5-dimethyloxazolidin-2-ones (85–94% de) and subsequent reduction with DIBAL afforded directly non-racemic α-substituted aldehydes without loss of stereochemical integrity (87–94% ee). The extension of this protocol for the asymmetric synthesis of β-substituted aldehydes is demonstrated, via the diastereoselective conjugate addition of a range of organocuprates to (S)-N-acyl-4-phenyl-5,5-dimethyloxazolidin-2-ones which proceeds with high diastereoselectivity (generally >95% de). Reduction of the conjugate addition products with DIBAL gives non-racemic β-substituted aldehydes in high yields and in high ee (generally >95% ee). This methodology is exemplified by the asymmetric synthesis of (R)-3-isopropenylhept-6-enal, which has previously been used in the synthesis of (3Z,6R)-3-methyl-6-isopropenyl-3,9-decadien-1-yl acetate, a component of the sex pheromones of the California red scale.

SuperQuat, (S)-4-benzyl-5,5-dimethyl-oxazolidin-2-one for the asymmetric synthesis of α-substituted-aldehydes

Abstract Reduction of α-substituted-(S)-N-acyl-4-benzyl-5,5-dimethyl-oxazolidin-2-ones with DIBAL-H in CH2Cl2 affords α-substituted aldehydes with no loss of stereochemical integrity at their α-centre.

Practical synthesis of Schöllkopf's bis-lactim ether chiral auxiliary: (3S)-3,6-dihydro-2,5-dimethoxy-3-isopropyl-pyrazine

Abstract Practical methodology for the bis-O-methylation of (3 S )-isopropyl-piperazine-2,5-dione 4 on a 45 g scale to generate Schollkopfs bis-lactim ether chiral auxiliary (3 S )-3,6-dihydro-2,5-dimethoxy-3-isopropyl-pyrazine 1 has been developed. Monomethylated intermediates 5 and 6 are reported for the first time. The gelling effects of 4 in a range of common solvents are also described.

A practical asymmetric synthesis of homochiral α-arylglycines

Abstract Enantioselective reduction of a series of substituted aryl trichloromethyl ketones with the CBS-catecholborane reagent afforded homochiral aryl trichloromethyl carbinols which have been elaborated to give homochiral α-arylglycines in high e.e.