Kinetics and mechanism of base-catalysed aldolization of 1,3-dimethoxy-2-propanone

1,3-Dimethoxy-2-propanone (HA) in an aqueous alkaline solution undergoes specifically base-catalysed aldolization with the formation of a dimer, 4-hydroxy-1,3,5-trimethoxy-4-methoxymethyl-2-pentanone (H2A2). The reaction is reversible and involves the formation or decomposition of an intermediate anion, HA2-, as the rate-determining step. The formation of a carbanion ion, A-, of the starting compound and of the HA2- anion are rapid, preceding, and generally base-catalysed reaction steps. The activation and thermodynamic parameters of the reversible reaction were determined from the dependences of the rate and equilibrium constants on the temperature.

Electrochemical evaluation of electron acceptor materials

The electrochemical half-wave reduction potential (E1/2) of 33 electron acceptor molecules in dry methylene chloride has been measured by cyclic voltammetry (CV) at a mercury electrode. The differences in E1/2 values for the reversible reduction to the corresponding anion radicals reflect the electronic influence of the substituent at different ring positions.The measurements of the half-wave reduction potentials provided a convenient method for determining the conducting energy levels of the electron relay molecules in condensed media. Repetitive potential cycle measurements provided information on the chemical stability and cyclability of the intermediate anion radicals formed during electron transport processes.

2,4,4-Trimethylpentenyl anions

The allylic anions derived by proton abstraction from 2,4,4-trimethyl-1-pentene 1 and 2,4,4-trimethyl-2-pentene 2 have been studied as short-lived intermediates. The anions were generated in t-butyl alcohol-O-d-potassium t-butoxide at 215 °C and relative rates of hydrogen–deuterium exchange and isomerization were compared. The isomerization of the thermodynamically less stable 2 to 1 occurs through the cis-allylic anion with a low 10% intramolecular component. The intermediate anion preferentially collapses to give 2 (3 to 1). Treatment of 1 leads more frequently into the less substituted allylic anion.

A generalized mechanism for superexchange

The concepts of bond orbitals and unrestricted Hartree-Fock orbitals are used in an attempt to explain the mechanism of superexchange between magnetic cations via an intermediate anion. The resulting theory predicts the same results as the standard theories when the cation-anion-cation angle is 180°. For other values of this angle, however, the nature and strength of the interaction depends on the distribution of bonds round the anion and is not solely a function of angle. The theory is applied to compounds having the corundum-ilmenite and spinel structures, where it correlates the magnetic structures and transition temperatures for a number of compounds and enables these properties to be predicted for others.