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The mechanism proposed by Evans for the dialkylaluminum chloride promoted Diels-Alder reaction of cyclopentadiene with alpha,beta-unsaturated N-acyloxazolidinones has been widely used as a basis for the rationalization of the experimental selectivities observed in many different types of reactions in which oxazolidinones or imidazolidinones are used as chiral auxiliaries. In this manuscript we introduce a new and more general model based on molecular modeling and NMR spectroscopy data that avoids several ambiguous concepts raised by the Evans model and fully explains all available experimental data. While the Evans proposal relies on the formation of high-energetic ionic chelates that promote the rotation of the amide bond in the N-acyloxazolidinone molecule, our model is based on the catalysis by means of low-energetic mono- or bicomplexes at the chain and the ring carbonyl groups that are easily observed by NMR spectroscopy measurements. The observed selectivities are explained by a chirality-transfer concept, in which an achiral Lewis acid works as a bridge for the transfer of chirality between a chiral auxiliary and a prochiral reactive center. Different to the Evans proposal, this mechanism fully explains the experimental selectivities for low Lewis acid concentrations, based on the catalysis by means of concurrent monocomplexes at the chain or the ring carbonyl groups, as well as the increased reaction rates and selectivities experimentally observed for high Lewis acid concentrations. The model can be extrapolated to nonchelating and other chelating Lewis acids, thereby allowing for the rationalization of much experimental data that were never explained by the Evans proposal.

The carbon and fluorine chemical shifts of mixtures of carbon dioxide and Krytox, a carboxylic acid end-capped perfluorinated polyether used as stabilizer for the dispersion polymerization of methyl methacrylate, have been studied using high-pressure, high-resolution nuclear magnetic resonance. 13C and 19F spectra were measured in the density region between 0.54 and 0.73 g.cm(-3) at 334 K for different solutions of Krytox in scCO2 (0.22, 1.13 and 1.72 w/w %). An in-house developed high-pressure apparatus with the capability to change in situ the sample composition was used for this purpose using a 10 mm polyether ketone NMR tube. The nature of CO2-Krytox interaction was assessed both by comparing the CO2 deltaC variation of neat CO2 with that of mixtures with increasing surfactant composition and by the analysis of Krytox 19F corrected chemical shifts in terms of medium magnetic susceptibility. Ab initio calculations, at the second-order Møller-Plesset level of theory to include the effects of electron correlation, were performed to access and compare the nature of the interactions between CO2 and perfluorinated and nonfluorinated analogue model molecules. Both experimental 13C and 19F HP-NMR results and molecular modeling studies support a F...CO2 site-specific Lewis acid-Lewis base interaction model. A positive entropic variation for the formation of CO2-fluorinated solute complex is advanced as an explanation for the higher solubility of perfluorinated molecules when compared to the nonfluorinated analogues.