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

Abstract

The development of chiral zeolitic catalysts possessing extra-large pores and endowed with the capability of enantioselectively processing bulky products represents one of the greatest challenges in chemistry. Here, we report the discovery of GTM-3, an enantio-enriched extra-large pore chiral zeolite material with -ITV framework structure, obtained using a simple enantiopure organic cation derived from the chiral pool, N,N-ethyl-methyl-pseudoephedrinium, as the chiral-inductor agent. We demonstrate the enantio-enrichment of GTM-3 in one of the two enantiomorphic polymorphs using the two enantiomers of the organic cation. Interestingly, we prove the ability of this zeolitic material to perform enantioselective catalytic operations with very large substrates, here exemplified by the catalytic epoxide aperture of the bulky trans-stilbene oxide with alcohols, yielding unprecedented product enantiomeric excesses up to 30%. Our discovery opens the way for the use of accessible chiral zeolitic materials for the catalytic asymmetric synthesis of chiral pharmaceutical compounds.

Scheme 1. Molecular Structure of the Two Enantiomers of N,N-Ethyl-methyl-pseudoephedrinium

Figure 1

Chiral -ITV framework with P4₁32 (top) or P4₃32 (bottom) enantiomorphic space groups, showing the helicoidal chain units (in the three directions, like spiral staircases, dashed rectangles) with right-hand and left-hand helices, respectively, turning 90° in clockwise (+90°) or anticlockwise (−90°) directions (viewed from top to bottom).

Figure 2

Cs-corrected STEM-ADF analysis: (a) along the [001] projection with the FD inset (a magnified image is presented on the top right corner with the model superimposed); (b) [111] projection with the FD and a closer observation, with the model superimposed (Si in blue and O in red) shown in the inset.

Figure 3

Enantioselective crystallization: XRD patterns of samples prepared with SS-EMPS (left) or RR-EMPS (right) as SDA in the gel, using 5% (top) or 10% (bottom) of GTM-3 seeds previously prepared with SS-EMPS (red lines) or RR-EMPS (blue lines). Crystallization was carried out at 100 °C for 72 h.

Figure 4

Enantiomeric excesses (%) of reactants (top), u-products (middle), and l-products (bottom) as a function of conversion, using Al-free GTM-3 catalysts obtained with SS-EMPS (red) or RR-EMPS (blue), for the trans-stilbene epoxide aperture with 1-hexanol.

Scheme 2. Ring Aperture of Chiral trans-Stilbene Oxide with 1-Hexanol Giving Inversion u-Products (R,S + S,R) or Retention l-Products (R,R + S,S). Species in Columns Are Enantiomers