On the Origin of Enantioselectivity in Chiral Zeolite Asymmetric Catalyst GTM-3: Host-Guest Transfer of Chirality.

Knowledge of how extra-large-pore chiral zeolite asymmetric catalysts based on the -ITV framework imprint their chirality during a catalytic reaction is crucial in order to spread the scope for the catalytic enantioselective production of chiral compounds of interest. In this work, we have carried out a combined experimental and computational study on the catalytic activity of antipode GTM-3 catalysts during the ring-opening of -stilbene oxide with 1-butanol. Identification of the enantiomers of all the chiral species unraveled a surprising catalytic behavior: these chiral catalysts promote the transformation of one enantiomer of -stilbene oxide in the corresponding product (with inversion of configuration of the attacked C) via an S2 mechanism, and at the same time, the transformation of the other enantiomer of -stilbene oxide via an S1-like mechanism into the (with retention of configuration) and secondary products (diphenylacetaldehyde via rearrangement and derived products). A computational study based on DFT + D methods suggested a potential explanation for this catalytic behavior, associated with a different orientation of -stilbene oxide enantiomers bound on the Ge(T7) positions in units, which is stabilized by the development of intraframework H-bonds between the interrupted T7OH adjacent positions characteristic of this framework. Calculations suggest that each enantiomer of -stilbene oxide follows a different reaction pathway, one favoring the S2 route by addition of butanol from the opposite side to form -products, while the different orientation of the antipode enantiomer disfavors such S2 route mainly by steric repulsions and at the same time favors the reaction toward the S1 mechanism to give and secondary-products. Our study suggests that the strong enantioselectivity of GTM-3 catalysts for this reaction is associated with the particular orientation adopted by the chiral reactants within the chiral nanospace provided by the -ITV framework, similarly to what occurs with enzymes, and such preferential orientation is directly controlled by the asymmetric cavities where the reaction takes place, by the particular features of the Ge active sites in adjacent interrupted positions and by the presence of several framework OH groups in the nearby nanospace that interact with guest species. The experimental observations and the reaction mechanism proposed suggest that GTM-3 catalysts prepared from the (1,2) enantiomer of the ,-ethyl-methyl-pseudoephedrinium organic agent should be enriched in the 432 enantiomorphic space group of the -ITV framework and GTM-3 prepared from the (1,2) enantiomer in the antipode 432. Interestingly, resolution of the absolute configuration of GTM-3 materials from 3D-electron diffraction data has been accomplished and confirms such an assignment, giving an average 82% enantio-enrichment in the corresponding chiral polymorph. Structure-solution of the location of the chiral structure-directing agents indicates that the transfer of chirality from the molecular component to the zeolite polymorph is governed by the development of strong H-bonds between the molecular hydroxyl group and the interrupted T(7)OH framework positions.