A Tripodal Peptidic Titanium Phosphonate as a Homochiral Porous Solid Medium for the Heterogeneous Enantioselective Hydration of Epoxides

Abstract

A porous, homochiral titanium-phosphonate material based on a tripodal peptide scaffold was used as a heterogeneous reaction medium for the enantioselective hydration (> 99%) of styrene oxide. This titanium-phosphonate material, which was shown to contain confined chiral spaces, was prepared by polymerization of l-leucine onto a tris(2-aminoethyl)amine initiator, followed by capping with phosphonate groups and completed by non-aqueous condensation with titanium isopropoxide. Circular dichroism confirmed that the peptide tethers yielded a secondary structure. X-ray powder diffraction and transmission electron microscopy supported by a semi-empirical model showed the likely formation of a porous, lamellar material that was quantified by nitrogen adsorption.Keywords: catalyst design; enantioselectivity; hydrolysis; organic-inorganic composites; peptidesThe common strategy to carry out asymmetric catalysis for the preparation of enantiomerically enriched products, a cornerstone of modern synthetic chemistry, is to design homogeneous (soluble) metal-based and more recently also organic catalysts comprised of chiral ligands. The advantages of using insoluble catalyst systems, especially related to the practicality of recovering expensive catalysts, have also led to considerable efforts in the area of asymmetric heterogeneous catalysis. The typical approach utilized in this area is the covalent or non-covalent immobilization of known homogeneous catalysts onto supports.[1] More recently, we have also seen the incorporation of chiral ligands and chiral complexes into metal-organic framework materials, that is chiral MOFs, to yield what has been termed self-supported catalysts. Here, polytopic chiral ligands having both catalytically active and cross-linking sites are used for preparing homochiral materials that can be used in self-supported heterogeneous catalysis. [2] Another observation related to asymmetric catalysis is that, in principle, by carrying out reactions in chiral reaction media, enantiomerically enriched products can be attained. Historically, in homogeneous chiral media only low levels of enrichment have been observed, [3] although recently chiral ionic liquids were used as solvents for reactions that yielded high enantioselectivities, presumably due to their ionic nature.[4] Quite surprisingly there have been no reports on the use of porous, heterogeneous and homochiral media for asymmetric transformations. A quite singular example of asymmetric catalysis using insoluble peptidic, but non-porous media, is the epoxidation of a,b-unsaturated ketones on polypeptides (JuliaColonna reaction). [5] Based on the assumption that a more confined chiral environment may induce improved enantioselectivity, in this research we were interested in preparing porous, homochiral materials that can be used as heterogeneous media for enantioselective transformations. This would be the first step in the development of a general methodology for the preparation of chiral products from racemic or prochiral substrat...

An achiral manganese salen catalyst encapsulated in a peptidic phosphonate homochiral solid for the enantioselective formation of diols by consecutive epoxidation and hydration reactions

“…This material catalyzed the enantioselective ring opening hydrolysis of styrene oxides to the corresponding chiral vicinal diols by a mechanism that differs from kinetic resolution. 11 We now demonstrate that the inclusion of a Mn III -salen achiral catalyst into a porous, homochiral and insoluble titanium-phosphonate, thus confining the catalyst in a homochiral space, can be used for consecutive epoxidation and hydration reactions where styrene derivatives yield the corresponding vicinal diols with very high enantioselectivity.…”

“…The Mn-salen/TiP-PhGlyz material was tested for a consecutive reaction sequence where Mn-salen catalyzes the epoxidation of a styrene derivative with NaOCl 14 and the homochiral TiP-PhGly scaffold catalyzes the highly enantioselective ring opening of the intermediate epoxide to the corresponding diol. 11 The results are shown in Table 1.…”

Advances and Challenges in the Creation of Porous Metal Phosphonates

“…Moreover, other epoxides such as 4-methyl styrene oxide and α-4-methyl styrene oxide reacted in an identical manner, with a yield of~20% having~98% and~95% EE, respectively. The authors postulated that the epoxide was activated by moieties having a partial positive charge in the titanium phosphonate framework such as the phosphonate, or Ti(IV) centers, followed by the attack of water directed by the homochiral leucine peptide to yield S-styrene glycol [110]. Similarly, the enantioselective addition reaction to obtain optically active phenylpropanol has also been catalyzed by hybrid titanium phosphonates [111].…”

Organic–Inorganic Hybrid Metal Phosphonates as Recyclable Heterogeneous Catalysts

“…This can be overcome by making the hybrid metal phosphonate network by using bridging organophosphorous coupling agents. Further, porous titanium(IV) phosphonate materials of different structures have been utilized over the years in heterogeneous catalysis, adsorption of metal ions, proton conduction, photodecomposition of organic dyes, etc. Recently, titanium phosphate material MTiP‐1 has been synthesized through the evaporation‐induced self‐assembly method .…”