Chiral Porous TADDOL-Embedded Organic Polymers for Asymmetric Diethylzinc Addition to Aldehydes

Abstract

Two chiral porous organic polymers (CPOPs) were synthesized by linking a TADDOL-embedded building block with arylethynylenes units. The CPOPs are highly stable to thermal treatment, moisture, acidity, and basicity. The dihydroxy groups of TADDOL inside the pores promise the CPOPs as good asymmetric catalysts after postsynthesis modification with a secondary metal center. After being treated with Ti(O i Pr) 4 , the CPOPs could be used as highly effective and reusable heterogeneous catalyst for asymmetric diethylzinc addition to aldehydes with up to 99% conversion and 95% ee. This work promises the potential of generating chiral solid catalysts with unique and practically useful enantioselective functions via a modular approach.As one of the most important porous materials, porous organic polymers (POPs) have attracted considerable attention during the past decades, owing to their wide range of potential applications in heterogeneous catalysis, 15 gas adsorption 610 and molecular separations. 1114 In comparison with metalorganic frameworks (MOFs) 1518 and crystalline covalent-organic frameworks (COFs), 1922 these amorphous porous materials show high stability to thermal treatment, moisture, and acidity/ basicity conditions, thus presenting an excellent platform for engineering recyclable and reusable heterogeneous catalysts. POPs can also be constructed from catalytic active molecular building blocks and then employed for chemical catalysis with high activity, whose catalytic performance can be rationally tuned by adjusting the monomeric units. 2325 While an appreciable amount of work has been carried out on the use of POPs in gas storage and separations, only a small number of reports have explored their use in catalysis, especially in asymmetric catalysis, 2630 mainly because of the difficulty in their controllable synthesis and intricate behaviors in catalysis progress.TADDOL (α,α,α¤,α¤-tetraaryl-1,3-dioxolane-4,5-dimethanol) derivatives with intrinsic C 2 symmetry have been established as one of the best known privileged ligands in the field of asymmetric synthesis.3135 Heterogeneous TADDOL catalysts can be prepared by immobilizing TADDOL to resins, 33 highly porous silica gel, 34 or by copolymerizing TADDOL derivatives with styrene, 35 but only a few of them exhibit satisfactory enantioselectivity. Asymmetric diethylzinc addition to aldehydes is important for preparing chiral alcohols. 3638 In this work, we describe the synthesis of two chiral porous organic polymers (CPOPs) from a TADDOL based building unit and demonstrate their heterogeneous catalytic activities in asymmetric diethylzinc addition to aldehydes after post-modified with Ti(O i Pr) 4 , with up to 95% ee. Results and DiscussionTADDOL tetrabromo starting material A (4R,5R)-2,2-dimethyl-α,α,α¤,α¤-tetra-P-bromophenyl-1,3-dioxolane-4,5-dimethanol was prepared in two steps from readily available L-tartaric acid in moderate overall yield (39%). As shown in Scheme 1, chiral porous organic polymers (CPOPs 1 and 2) were synthesized in good yields (9...

Homochiral Porous Metal–Organic Polyhedra with Multiple Kinds of Vertices

“…Optically pure tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL) with C 2 -symmetry has exhibited a promising platform for chemical conversions owing to its easy availability and facile modification. We have previously demonstrated the successful construction of chiral COF and POP by introducing -Br and −CHO into TADDOL backbones. − To this end, we started with using TADDOL-derived tetrapyridine ( L ) as an organic linker (Scheme ). Considering the semirigidity of TADDOL skeleton, ,, the overall geometry of the L ligand can be a distorted square pyramid, as estimated by the arrangement of the four 3-pyridyl N atoms within L .…”

Homochiral 2D Porous Covalent Organic Frameworks for Heterogeneous Asymmetric Catalysis

“…As shown in Table , CCOF- 1 displayed enantioselectivities comparable to those of its homogeneous control for the examined substrates as well. Notably, TADDOLs have been immobilized on diverse supports such as resin, dendrimers, amorphous porous organic polymers, and mesoporous silica, but these catalysts in general suffer from the disadvantages of fewer and uneven catalytic sites. − For the diethylzinc addition reaction, the enantioselectivities observed for CCOF- 1 are comparable to or higher than those of reported TADDOL-POP-derived catalysts , and are higher than those of chiral BINOL–MOF-derived catalysts (Table S1). …”

Main-Chain Organic Frameworks with Advanced Catalytic Functionalities

“…In such a case, it would be more difficult to improve the activity and selectivity via framework structure modification. As shown in the two examples below, although the activities were greater for the heterogeneous systems ( 20 and 22 ), the selectivities were still slightly lower than the homogeneous analogs. − …”