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

Abstract

In this work we find a novel conformational sieving effect during the structuredirection of two chiral organic diastereoisomers, (1R,2S)-benzyl-ephedrine and (1S,2S)benzyl-pseudoephedrine, during the synthesis of zeolitic aluminophosphates. Protonation of each diastereoisomer can take place through two different stereochemical configurations, giving place to a new stereogenic N center that results in two stereoisomers for each molecule, with (1R,2S,NS) or (1R,2S,NR) configurations for benzyl-ephedrine or (1S,2S,NR) or (1S,2S,NS) configurations for benzylpseudoephedrine. Through a combination of 1 H and 13 C NMR spectroscopies and DFT+D computational calculations we are able to distinguish the two stereoisomers derived from benzyl-pseudoephedrine: results show a higher occurrence of the (1S,2S,NS) isomer in aqueous solution, displaying a folded molecular configuration, while the less abundant (1S,2S,NR) isomer displays instead a conformation with an elongated shape.Crystallization of microporous aluminophosphates in the presence of benzylpseudoephedrine results in materials with AFI structure based on one-dimensional channels with a surprising higher occurrence of the (1S,2S,NR) isomer, in sharp contrast with the behavior in aqueous solution. Such notable difference is ascribed to the elongated shape of the (1S,2S,NR) stereoisomer that can better fit within the AFI one-dimensional nano-channels. On the other hand, assignment of the 1 H and 13 C NMR bands to the different protonated stereoisomers of (1R,2S)-benzyl-ephedrine is not as clear due to the smaller splitting of the signals upon protonation through the two stereochemical configurations. This work demonstrates that confinement in nano-spaces can alter the relative stability of stereoisomers in open spaces, leading to the occurrence of species that would not be possible otherwise.

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.…”

GTM-3, an Extra-Large Pore Enantioselective Chiral Zeolitic Catalyst

“…Ideally, the SDA should be prepared from organic species available from the chiral pool, where nature readily provides enantiopure chiral precursors derived from natural products. In this line, we have systematically been exploring the use of alkaloids (1 R ,2 S )-ephedrine and (1 S ,2 S )-pseudoephedrine (Supporting Figure S1) as chiral blocks for building SDAs. − In the course of these investigations, we have recently discovered the crystallization of GTM-3 (GTM stands for Grupo de Tamices Moleculares ), a germanosilicate zeolite material with the -ITV framework, using (1 S ,2 S )- N , N -ethyl-methyl-pseudoephedrinium as SDA (hereafter referred to as EMPS) (Scheme ). -ITV, which was originally discovered by Corma and co-workers as ITQ-37, is one of the most interesting zeolite frameworks because of its extremely high porosity and, especially, of its chiral nature comprising a gyroidal channel system with 30-ring windows.…”

Development of mesoporous bioactive glass‐containing macroporous titanium for controlled release of antimicrobial drugs

“…[29][30][31][32][33] A common approach to introduce a highly ordered system of interconnected channel-like mesopores into the BAG phase, which is required for controlled drug delivery, is by the addition of a structuredirecting agent (SDA). [34][35][36][37] In this approach, surfactant molecules with a hydrophilic head and hydrophobic tail are added into the sol and organize themselves into micelles, spherical or cylindrical structures to form an ordered mesophase in the BAG sol, so that upon drying and calcination or extraction to remove the surfactants, a well-ordered mesoporous structure is obtained of which the final pore characteristics are highly dependent on the SDA. 20 SDAs that are commonly applied in the synthesis of mesoporous BAGs are cetyltrimethyl ammonium bromide (CTAB) and the triblock copolymers Pluronic P123 or F127.…”