Generation and reactions of novel oxiranyl ‘Remote’ anions

Deprotonation of an oxiranyl β-proton takes place in a stereoselective manner providing the corresponding oxiranyl ‘remote’ anion. The anion is stabilized by chelation between the lithium and the carbonyl moiety of an ester, lactone, imide, or keto-group in the form of a five-membered cyclic intermediate. Certain ester-stabilized oxiranyl anions are stable and can be left in THF solution at −78°C for several hours. The generated anions undergo a stereoselective alkylation reaction to provide products, which could be useful intermediates in the synthesis of bioactive naturally occurring α-methylene bis-γ-butyrolactones.     

N-Arylation of nitrogen heterocycles with 2,4-difluoroiodobenzene

Arylation reactions of NH-heterocycles [specifically, pyrazole, 3-(trifluoromethyl)pyrazole, imidazole and pyrrole] with 2,4-difluoroiodobenzene in the absence and presence of copper catalysis are described. The combination of fluoro and iodo substituents in the same aryl substrate has facilitated both S_NAr reactions at the C-F bonds and copper-catalysed Ullmann-type coupling reactions at the C-I bond. Products arising from regioselective reactions and multiple substitutions have been isolated, providing a range of new N-arylated pyrazole, imidazole and pyrrole derivatives.     

Electrical conductance of conjugated oligomers at the single molecule level

We determine and compare, at the single molecule level and under identical environmental conditions, the electrical conductance of four conjugated phenylene oligomers comprising terminal sulfur anchor groups with simple structural and conjugation variations. The comparison shows that the conductance of oligo(phenylene vinylene) (OPV) is slightly higher than that of oligo(phenylene ethynylene) (OPE). We find that solubilizing side groups do neither prevent the molecules from being anchored within a break junction nor noticeably influence the conductance value.     

Comparative electrochemical and impedance studies of self-assembled rigid-rod molecular wires and alkanethiols on gold substrates

A study of the charge transfer and self-assembly characteristics of two new rigid-rod molecular wires 1 and 2 assembled on polycrystalline gold electrodes was carried out using electrochemical impedance spectroscopy and cyclic voltammetry. This class of wires have precisely controlled (ca. 1.5-2.5 nm) lengths of π-conjugation, with extended HOMO and LUMO wavefunctions. While rotations can occur around the C-C single bonds, the molecules cannot isomerise or fold due to their rigid backbone structures. The behaviour of these wires was compared with SAMs of heptanethiol (HPT) and dodecanethiol (DDT). It was found that SAMs of 1, which bears flexible hexyloxy sidechains, had randomly distributed pinholes which show microelectrode behaviour even when diluted with DDT. SAMs of 2, which do not have any sidechains, were well-organised at open-circuit potentials enabling evaluation of electron transfer kinetics assuming an average film thickness. However, impedance studies show that deviations from open circuit potentials resulted in an exponential decrease in charge transfer resistance, whereas capacitance remained constant, possibly attributable to conformational changes of the SAM. The syntheses and characterisation of the molecules is described.