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 ºC). An increase of the crystallization temperature (to 180 ºC) severely limits the structure-directing ability towards the AFI framework because of a poor hydrothermal stability of the organic cations, especially of the (1S,2S)-isomer. 13 C NMR and molecular simulations studies of these organic cations in solution and confined within nanoporous zeolitic materials allow us to clearly identify 13 C 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.

Synthesis of large-pore zeolites from chiral structure-directing agents with two l-prolinol units

“…Calculation of the NMR chemical shielding of the different isomers was carried out with the gauge-including projector augmented-wave method (GIPAW) developed by Pickard and Mauri, 43 as implemented in the CASTEP code, using a σ ref value of 176 ppm, the same as in our previous works. 29,32,44 The conformational behaviour of the SDA cations in water was studied by NVT Molecular Dynamics simulations, in the same way as reported in our previous work. 35 8 SDA cations, 16 Cl − anions (for charge-compensation) and 160 water molecules were included in the simulation cell, and 10 ns of MD simulations in NVT ensemble were run at 423 K.…”

“…In recent years, we have successfully studied several organic SDAs prepared from chiral precursors derived from the chiral pool, on the one hand from chiral alkaloids (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine, [27][28][29][30][31][32] and on the other from L-prolinol (derived from L-proline amino acid). [33][34][35][36][37][38] L-Prolinol is a useful chiral precursor since it provides a rigid ring with an N atom that can be quaternized with two different alkyl substituents, providing an additional stereogenic centre.…”

Conformational sieving effect of organic structure-directing agents during the synthesis of zeolitic materials

“…In order to properly understand the structure-directing behavior of chiral SDAs, in our group we have been working for some time with derivatives of chiral alkaloids (1R,2S)ephedrine and (1S,2S)-pseudoephedrine during the synthesis of zeolitic nanoporous materials, especially in AlPO4 composition using secondary or tertiary amines [31][32][33][34][35][36][37][38].…”

“…These molecules were rationally selected as chiral SDAs since they contain two stereogenic centers, they are commercially available, they resist hydrothermal treatments, and they contain aromatic rings and H-bond forming and accepting groups that impart a very rich supramolecular chemistry to these species. In particular, we have found that the presence of such H-bond donor and acceptor groups within the molecule provides a very rich conformational space, which in turn alters the resulting supramolecular chemistry and the structure-directing behavior of these chiral SDAs [35,37,[39][40][41][42]. Moreover, we have shown that 13 C NMR spectroscopy can be very useful for monitoring the conformational space of these organic species upon confinement in nanoporous spaces [35], revealing stabilization of particular conformations under confinement that are unstable in open spaces (in solution).…”

“…In particular, we have found that the presence of such H-bond donor and acceptor groups within the molecule provides a very rich conformational space, which in turn alters the resulting supramolecular chemistry and the structure-directing behavior of these chiral SDAs [35,37,[39][40][41][42]. Moreover, we have shown that 13 C NMR spectroscopy can be very useful for monitoring the conformational space of these organic species upon confinement in nanoporous spaces [35], revealing stabilization of particular conformations under confinement that are unstable in open spaces (in solution). On the other hand, the occurrence of diastereoisomerism can also notably alter the outcome of a crystallization route [43][44][45].…”