Merete Folmer Nielsen

Stereoselectivity and mechanism in the electrohydrodimerisation of esters of cinnamic acid

Rate constants (kobs) and reaction orders have been determined for the cathodic reduction in DMF solution of 11 cinnamic acid esters including some derived from chiral alcohols and a dicinnamate derived from trans-cyclohexane-1,2-diol. The cinnamic acid esters typically reduce with high stereoselectivity to all-trans 3,4-diphenylcyclopentanone-2-carboxylates. The enhancement of rates of reaction by addition of water was studied for selected substrates and low energies of activation were found. Changes in the alkoxy or aryloxy groups also caused significant changes in rate and log kobs correlated linearly with E° values. The results from kinetic experiments were complemented by product studies of reactions aimed at probing reversibility of key reaction steps. The combined evidence is interpreted as unambiguous support for radical anion–radical anion coupling as the key step with complexation with water, prior to coupling, being crucial.The relative stereochemistry at C-3 and C-4 is fixed, irreversibly, at the coupling stage and there is strong evidence to suggest that templating in the complex between two radical anions and water determines the stereochemical outcome.

Synthesis and properties of redox-active dendrimers containing phenothiazines

A new family of redox-active dendrimers based on a poly(propylene imine) core with phenothiazines as the redox active unit have been prepared, and some of their properties have been investigated.

Kinetics of proton transfer from phosphonium ions to electrogenerated bases: polar, steric and structural influences on kinetic acidity and basicity

Derivative cyclic voltammetry (DCV) and linear sweep voltammetry (LSV) have been used to measure rates of proton transfer in DMSO solution between different types of electrogenerated base (EGB) and a series of phosphonium ions of relevance to ylide formation for synthetic reactions. Although the electrochemical methods are convenient for the measurement of rates of proton transfer in these systems a major conclusion of the study is that considerable care must be exercised in the application of the methods and in drawing general conclusions from the results. In particular, comparison of kinetic acidity with thermodynamic pK values in DMSO shows that a single Bronsted relationship does not hold for the series of phosphonium ions. The kinetic acidities are profoundly affected by: whether the EGB is a carbon or nitrogen base; the propensity of some of the phosphonium ions to enolise; and steric factors. Other measures of electron-demand at the acidic methylene groups (13C and 1H chemical shifts, reduction potentials) are consistent with the pK(DMSO) values. The kinetic results confirm that the Ph3P+ group is, for non-enolisable phosphonium salts, more activating than Bu3P+.

Synthesis, UV/vis spectra and electrochemical characterisation of arylthio and styryl substituted ferrocenes

AbstractTwo series of substituted ferrocenes were synthesised using either the Horner-Wadsworth-Emmons reaction or monolithiation of ferrocene. The series consist of arylthio- and styryl-ferrocenes with different substituents in the para position of the aryl rings of the systems. The electronic communication was investigated by comparing the substituent effects in absorption spectroscopy and in cyclic voltammetry. A small substituent effect was found in the electronic transitions of the styryl substituted ferrocenes. The oxidation of the ferrocene derivatives showed clear substituent effects as illustrated by the linear Hammett plots. The effect was shown to be an order of magnitude larger in the arylthio-systems than in the styryl systems. It is suggested that the reason behind the large effect is a direct sulfur-iron orbital overlap.

Electroorganic Reactions. Part 45. The Highly Stereoselective Electrochemical Hydrodimerisation of Methyl 4-tert-Butylcyclohex-1-enecarboxylate.

Abstract Methyl 4-tert-butylcyclohex-1-enecarboxylate undergoes 1 F reduction at a mercury cathode in DMF solution to give as the major single product a hydrodimer in which the cyclohexyl rings are joined axially and the methoxycarbonyl groups are also axial. The structure is confirmed by X-ray crystallography. Kinetic experiments, combined with force field calculations, indicate radicalanion/radical-anion coupling which is under stereoelectronic control and which dictates diaxial coupling of the cyclohexyl rings. Protonation of the resulting dimeric dianion is under thermodynamic control.

Enantioselective Cathodic Hydrogenation of Prostereogenic Activated Double Bonds

4-Methylcoumarine (1) is enantioselectively reduced in the presence of yohimbine at pH = 3 to afford 4-methyldihydrocoumarine (2) in 61% yield and nearly 70% ee. With 3-methylindenone and strychnine the dihydroproduct is formed with up to 36% ee. The role of the alkaloids is to catalyze the tautomerization of an intermediate enol radical and to enantioselectively protonate a ketoanion.

Electro-Organic Reactions. Part 48: Pathways for Carbon-Carbon Bond Formation: The Redox Chemistry of Quinodimethanes and of Alkenyl-Substituted Heteroaromatics

Carbon-carbon bond-forming reactions involving cathodic reduction of quinonemethides, electrogeneration of quinodimethanes and reduction of some vinylpyridines and vinylquinolines, are compared and contrasted. The quinonemethides undergo “conventional” electrohydrodimerisation via radical-anion coupling. Quinodimethanes (QDMs), formed by cathodic elimination from 1,4- and 1,2-di-(halomethyl)arenes, react usefully to give polymers or, for o-quinodimethanes, as dienes in Diels-Alder reactions. QDMs are electroactive and have been characterised electrochemically and spectroscopically. The vinylheteroaromatics give trans-1, 2-diheteroarylcyclobutanes as major products and these are shown to arise from an overall 0 F reaction (catalytic chain process) but with the key initial step being reaction between radical-anion and the neutral starting material.

The formation of relatively stable protonated anion-radicals in the cathodic hydrogenation of quinonemethides

Examination of the cathodic hydrogenation of two examples of quinonemethides reveals an unusual pathway in which the anion radicals of the quinonemethides fuchsone and its dimethyl analogue are protonated reversibly at oxygen by substituted phenols in DMSO to give relatively stable triarylmethyl radicals which are not reduced at the first reduction potential of the quinonemethides; this behaviour contrasts with that usually found, i.e. anion-radicals are usually formed at more cathodic potentials than reduction potentials of radicals derived from them by protonation.