A. M. Freitas

Electroreductive cyclisation of unsaturated halides catalysed by nickel macrocyclic complexes

Abstract The indirect electroreduction of unsaturated aliphatic halides using Ni(II) complexes as electron-transfer mediators is achieved in N,N′ -dimethylformamide by constant current electrolysis between a cathode and a sacrificial anode in a diaphragmless cell. Cyclic compounds are obtained in low to high yields in conditions that avoid the usual preparation using organotin reagents.

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+.

Investigation of the Wittig reaction with electrogenerated bases: Influence of experimental conditions on the yield and stereochemistry

Abstract The influence of experimental parameters on the yields and stereochemistry of the Wittig reaction was studied using the synthesis of stilbene initiated by different electrogenerated bases as a model reaction. Electrolyses were carried out in batch cells under different conditions, including constant potential or constant current with different electrode materials and using solvents of different polarity. It is shown that proper selection of experimental conditions in Wittig reactions allows a choice of stereoselectivity in the reactions and the optimization of yields.

Synthesis and allylic reactivity of α-bromomethyl β-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)oxy α,β-unsaturated carbonyl compounds

Abstract Under radical conditions, N -bromosuccinimide converts α-methyl β-(2,3,4,6-tetra- O -acetyl-β- d -glucopyranosyl)oxy α,β-unsaturated carbonyl compounds into their α-bromomethyl derivatives. The bromides undergo nucleophilic displacement reactions without rearrangement with azide, O -ethyl dithiocarbonate and thiocyanate anions; with acetate anion, there is a preference for the formation of rearranged acetates with reasonable stereoselection.

Preparation and reactions of novel cyclic β-oxosulphonium salts obtained by the acid-induced cyclisation of 1-diazo-ω-phenylthio-2-alkanones

With perchloric acid, the diazo-ketones PhS[CH2]nCOCHN2(1a)–(1d) give the corresponding cyclic β-oxosulphonium salts (2a)–(2d); the p-chlorophenyl analogue of (2d) was similarly prepared. The salts (2a), (2c), and (2d) react with triphenylphosphine at C(2) to give the acyclic phosphonium salts (3a), (3c), and (3d) and, analogously, with potassium O-ethyl dithiocarbonate to give the corresponding acyclic O-ethyl dithiocarbonates (3f), (3g), and (3h). All the reactions of the salt (2b) with nucleophiles gave either 1-phenylthiobut-3-en-2-one (11) or products of its Michael addition. Salts (2c) and (2d) with sodium methoxide in methanol provide, respectively, methyl ω-phenylthio-butanoate and -pentanoate in a process that involves the loss of a methylene group. These and other reactions are considered to proceed via ylide intermediates; the intermediate derived by the deprotonation of (2d) being isolated, whereas that from (2c) rearranged to give 3-phenylthiocyclopentanone.