J.E. Dubois

Dynamic stereochemistry of aldolization—XX ☆: Study of the stereochemical composition versus time; quantitative determination of kinetic and thermodynamic stereoselectivities

Abstract A kinetic study of a reversible diastereogenic aldol condensation is proposed in order to calculate unambiguously the kinetic and thermodynamic stereoselectivities (which correspond to the limiting compositions of the system). The diastereogenic step is found to be rate determining. Experimental and computer simulated concentration-time curves are in excellent agreement. Two examples of limiting stereochemical situations are considered, which demonstrate that the stereoselectivity may be, but not necessarily, rapidly changing during the reaction.

Dynamic stereochemistry of aldolization—XXI : Definition of the “restoring energy” of a system of reversible competitive reactions

Abstract A new quantity, called “restoring energy” ( E r ), has been defined in order to measure the distance between kinetic and thermodynamic stereoselectivity; the reverse reaction, whose rate constant ratio is related to the magnitude of E r restores the diastereomeric system to its equilibrium composition. Solvent and cation catalyst appear to have a marked influence on the “restoring energy” of the aldol condensation. Final products being taken as reference, these results are interpreted in terms of difference in type of interactions between two postulated transition state models (cyclic and open-chain).

Molecular interactions and reorientational motion of neat acetone in the liquid state. 17O NMR chemical shifts and linewidths at variable temperature

Abstract The structure and dynamics of neat liquid acetone have been studied in the 181–325 K temperature range by 17 O NMR. The chemical shift data are consistent with a monomer—dimer equilibrium; the standard enthalpy and entropy of association for the acetone dimer are −1.1 kcal/mol and −8.5 cal mol −1 K −1 , respectively. From the linewidth measurement, it seems that the dimer is only a short-lived electrostatic-collided complex which does not reorient as a whole: rotational motion of the oblate spheroid acetone is found to fit the hydrodynamic equation under “slip” boundary conditions.