François Huet

Investigation of electrochemical processes by an electrochemical noise analysis. Theoretical and experimental aspects in potentiostatic regime

Abstract A general framework for a model of the stochastic behaviour of the electrochemical interface only governed by chemical or electrochemical reactions is presented. It leads to both the current-voltage curve, the electrochemical impedance and the electrochemical noise. An experimental arrangement and a special signal processing are described for analyzing the current fluctuations in potentiostatic regime: the power spectral density of the electrochemical noise is measured through the cross spectrum of the responses of two measurement channels. An application to the passivation of iron in acidic medium is presented both from the theoretical and experimental points of view. The agreement between the model predictions and the experimental data is good in the low current range while there is a discrepancy in the higher current range. Some interpretations are discussed.

Microbiological reduction of keto-sulfones. Application in a three-step synthesis of (S)-(+)-β-angelica lactone

Abstract Microbiological reductions of several keto-sulfones led to the corresponding hydroxy-sulfones in moderate to high enantiomeric excess. A three-step synthesis of (S)-(+)-β-angelica lactone from ethyl-4-oxo-3-(phenyl-sulfonyl)-pentanoate via the intermediate chiral alcohol is described.

Local electrochemical impedance measurement: scanning vibrating electrode technique in ac mode

The paper gives a new insight into the meaning of local impedances and the related measurement techniques. By numerical simulation it is shown that the only fully relevant quantity is obtained through local values of both the current density and potential. A recently developed equipment based on a commercial SVET device modified for ac polarization is applied to corrosion situations. Under oxygen reduction control of the corrosion rate, serious artifacts are found on the dc current density mapping and local impedance. The drawback is explained by the contribution of a chemical term (redox potential) in the probe potential. A model is elaborated accounting for the resulting side effects in both dc and ac measurements.

Rapid access to acyclic nucleosides via conjugate addition

The synthesis of several acyclic nucleosides 5 and 6, analogs of penciclovir, was achieved by Michael addition as the key step. This reaction worked not only for the protected natural bases but even for the less nucleophilic deaza purine and deaza pyrimidine.

Synthesis of Novel Nucleosides with a Fused Cyclopropane Ring Substituted by a Hydroxymethyl Group

Abstract The two diastereomers of a 3-oxabicyclo[3.1.0]hexan-2-ol 21a – b with the cyclopropane ring substituted by a hydroxymethyl group were synthesized from both products 17a and 18a obtained from the bromohydroxylation of epoxide 16a derived from 3-oxabicyclo[3.2.0]hept-6-en-2-one 15 . This preparation involved two stereospecific C 4 –C 3 ring contractions leading to cis , cis -trisubstituted cyclopropane compounds 18a and 12 . The hydroxylactols 21a – b thus obtained were diacetylated and the single product 22 was subjected to substitution at the anomeric position by protected or free bases, providing, in each case, only one of the possible products. Deprotection with ammonia yielded new bicyclic nucleosides 11a – d .

High selectivities in electrophilic additions to cyclobutene compounds

Abstract Epoxidation of 2, 3, 4 with m-CPBA mainly led to the cis-attack products whereas 1 and 6 led to the other selectivity. The result was reversed, from 4, with Paynes reagent. Bromohydroxylation of 4 involved an intermediate bromonium ion syn to the substituents. Haloselenylations occurred with the syn-selectivity from 1, 2, 3 and 4, to the anti-selectivity from 6, and without selectivity from 5. NOE enhancement measurements and several chemical correlations led to the stereochemical assignments. Formation of the intramolecular reaction products 24 and 25 was also pointed out.

New results concerning the oscillations observed for the system iron–sulphuric acid

The oscillation process observed on iron in sulphuric acid at the beginning of the dissolution plateau has been studied by measuring simultaneously the electrolyte resistance and the current vs time. The influence of the viscosity and mass transport has been investigated by using a rotating disc electrode (RDE), a submerged impinging jet (SIJ) cell and a rotating cylindrical electrode (RCE). The oscillation process occurring at the beginning of the plateau could be linked to the existence of a viscous layer of low conductivity corresponding to a colloidal dispersion of salt or/and hydroxide.

ASYMMETRIZATION OF MESO-1,3-DIOLS UTILISING PSEUDOMONAS FLUORESCENS LIPASE

Abstract A convenient and enantioselective synthesis of monoacetates of meso-1,3-diols 2-substituted with an alkoxymethyl or a thiophenylmethyl group, by enzyme catalyzed acylation, is described. The absolute stereochemistries of two monoacetates were assigned by chemical correlation.

Preparation of lactones with several ring sizes via the same intermediate

Abstract Several lactones were prepared from (S)-1-(phenylsulfonyl)-3-butanol 2 by cyclization of alkylation products of its silyl derivative 3 with electrophilic reagents bearing a functional group.

Electrochemical investigation of locally depassivated iron. A comparison of various techniques

Abstract Electrochemical aspects of the damage caused in situ to the passive film on iron have been investigated by means of ac impedance and spectral analysis of the current fluctuations. These techniques can provide useful information on the individual depassivation-repassivation transient and its interaction with the surrounding passive surface. It has been shown that it is rather difficult to define an intrinsic rate of repassivation which could be independent of the depassivation technique. Firstly the repassivation has been studied by potential jump experiments. Secondly depassivation by abrasion due to the projection of a suspension of particles or focused laser pulse have been developed. The results are discussed in terms of the classical model involving a sequence of depassivation-repassivation events induced by local breakdown of the film.