Construction of Monophosphine–Metal Complexes in Privileged Diphosphine-Based Covalent Organic Frameworks for Catalytic Asymmetric Hydrogenation

Abstract

Privileged diphosphine ligands that chelate many transition metals to form stable chelation complexes are essential in a variety of catalytic processes. However, the exact identity of the catalytically active moieties remains ambiguous because the chelated metal catalysts may undergo rearrangement during catalysis to produce monophosphine–metal complexes, which are hard to isolate and evaluate the activities. By taking advantage of the isolation of two phosphorus atoms, we demonstrate here the successful construction of chiral monophosphine–Ir/Ru complexes of diphosphine ligands in covalent organic frameworks (COFs) for enantioselective hydrogenation. By condensation of the tetraaldehyde of enantiopure MeO-BIPHEP and linear aromatic diamines, we prepare two homochiral two-dimensional COFs with ABC stacking, in which the two P atoms of each diphosphine are separated and fixed far apart. Post-synthetic metalations of the COFs thus afford the single-site Ir/Ru–monophosphine catalysts, in contrast to the homogeneous chelated analogues, that demonstrated excellent catalytic and recyclable performance in the asymmetric hydrogenation of quinolines and β-ketoesters, affording up to 99.9% enantiomeric excess. Owing to the fact that the porous catalyst is capable of adsorbing and concentrating hydrogen, the catalytic reactions are promoted under ambient/medium pressure, which are typically performed under high pressure for homogeneous catalysis. This work not only shows that monophosphine–metal complexes of diphosphines can be catalytically active centers for asymmetric hydrogenation reactions but also provides a new strategy to prepare new types of privileged phosphine-based heterogeneous catalysts.

Covalent organic frameworks in heterogeneous catalysis: recent advances and future perspective

“…The metaled COF-OFe was used to catalyze the alcoholysis of epoxides, showing a high 6 and 3D imine COF (COF-300) 88 were employed as scaffolds to host palladium acetate for the Suzuki-Miyaura coupling reaction, with the imine linkages serving as anchoring sites for the immobilization of Pd(II) species. The adjustable skeletons and open pores of COFs facilitate the incorporation of various noble metal complexes, such as Ir, 89 Ru, Pd, 90,91 and Au, 92,93 which demonstrate efficient and recyclable catalytic performance in several reactions including hydrogenation, oxidation, Suzuki-Miyaura cross-coupling, carboxylation, etc. (Table 1).…”

Post-synthetic modifications of covalent organic frameworks (COFs) for diverse applications

“…Therefore, hydrogenation can be accomplished under ambient pressure. 64 Also, Ni can be anchored to COFs via post-synthetic metalation. For the first time in 2016, McGrier and coworkers provided a three-dimensional boronic acid-based COF with conjugated dehydrobenzoannulene units for Ni binding through the strong p-metal complex formation with Ni.…”

Construction of Crystalline Nitrone-Linked Covalent Organic Frameworks Via Kröhnke Oxidation

“…Since Yaghi et al reported the first covalent organic frameworks (COFs) in 2005, COFs have caused many scientists’ interest in both fundamental research (synthesis, properties, theoretical simulation, etc.) and possible applications (gas absorption/separation, catalysis, energy-related devices, sensing, imaging, and so on). − During the advancement of COFs, synthetic chemistry has been demonstrated to display their critical role in the evolution of structures, connectivity, and functionalities of COFs. − To date, a majority of the reported methodologies, including boronate ester formation reactions, − Schiff-base chemistry, − and Knoevenagel reactions, − have been used to construct corresponding boronate-, imine-, or vinylene-type two-dimensional (2D) or three-dimensional (3D) COFs. Besides, other chemical reactions such as Debus–Radziszewski reaction, Povarov reaction, − Pictet–Spengler reaction, Doebner reaction, etc. − have also successfully been employed to prepare a series of fused-ring-linked COFs with exceptional properties.…”