Conformational Space of (1R,2S)-Dimethyl-Ephedrinium and (1S,2S)-Dimethyl-Pseudoephedrinium in the Synthesis of Nanoporous Aluminophosphates

Abstract

In this work, we report a comparative study about the structure-directing effect of two chiral diastereoisomers, (1R,2S)-dimethyl-ephedrinium and (1S,2S)-dimethyl-pseudoephedrinium, for the synthesis of nanoporous aluminophosphates. Both isomers direct the crystallization of the one-dimensional channels composing the AFI framework in the presence of divalent dopants when the crystallization temperature is moderate (140 °C). An increase of the crystallization temperature (to 180 °C) severely limits the structure-directing ability toward the AFI framework because of a poor hydrothermal stability of the organic cations, especially of the (1S,2S)-isomer. 13C NMR and molecular simulations studies of these organic cations in solution and confined within nanoporous zeolitic materials allow us to clearly identify 13C resonance signals of particular methyl groups which vary as a function of the conformational space and enable us to monitor the occurrence of the different conformers in diverse environments. In vacuo and in solution, both organic cations display a conformation with an open-configuration, in which the bulky phenyl and trimethylammonium groups site in “anti” configuration. Such conformational space is altered when the cations are confined within the limited space of the one-dimensional channels of the AFI structure. In this case, despite folded-conformations being intrinsically less stable, lateral packing interactions within the channels drive an incorporation of the cations as a mixture of conformers (with open- and folded-configurations) for the (1R,2S)-isomer or as folded conformers for the (1S,2S)-isomer, showing the importance of the conformational space of organic cations during the structure-directing phenomenon.