Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity

Abstract

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as homogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in diene ring-closing metathesis reactions, leading to increased macrocylization selectivity. COF pore-wall modification by immobilization of a Grubbs-Hoveyda-type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF-catalyst (MMC:O = 1.30) of up to 44% compared to the homogeneous catalyst (MMC:O = 0.90) was observed along with a substrate-size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

Threefold reactivity of a COF-embedded rhenium catalyst: reductive etherification, oxidative esterification or transfer hydrogenation

“…S7, ESI †). These features may originate from structural distortions of the COF crystal in the presence of [Re( C12 Anth-py 2 )(CO) 3 Br], which supports the hypothesis of the catalyst occupying the COF pores. Such shifts in the positions of the Raman features have been reported for metal-organic frameworks (MOF, e.g.…”

“…1 The high surface area of a COF and its customizable pore size, shape, and environment render this class of materials promising candidates for heterogeneous catalysis. 2,3 Several strategies have been reported to develop COF materials for catalysis, including (i) incorporation of catalytic sites into the COF building blocks, [4][5][6] (ii) post-synthetic metal coordination by the pore wall, 7,8 (iii) integration of a pre-assembled molecular catalyst via postsynthetic reactions, 9 (iv) confinement of metal nanoparticles [10][11][12] or complexes 13 within the pores, and (v) insertion of polymers to provide cooperative catalytic sites. 14,15 One well-known metal-catalyzed reaction of recent importance is transfer hydrogenation (TH) in both biomimetic and synthetic schemes.…”

“…1b) with a sharp reflection at 2y = 3.51 was preserved. FTIR spectroscopy evidenced both the preservation of the pristine TFB-BD spectrum and the appearance of new features at 2028, 1926, and 1902 cm À1 -corresponding to the Re(CRO) 3 moiety of [Re( C12 Anth-py 2 )(CO) 3 Br] (Fig. 1c).…”

Schrock molybdenum alkylidene catalyst enables selective formation of macrocyclic unsaturated lactones by ring-closing metathesis at high-concentration

“…by using specially designed homogeneous catalysts 21 or tailored ruthenium complexes covalently bound to mesoporous silica with defined pore diameters. [22][23][24][25] Another approach was related to the use of cavitands. 26 In contrast, we decided to make use of the intriguing observation by Fogg et al, that macrocyclisation via RCM always involves oligomerisation being in equilibrium with the backbiting and RCM processes.…”

Hydrosilylation of Alkynes Under Continuous Flow Using Polyurethane‐Based Monolithic Supports with Tailored Mesoporosity

“…[21][22][23] Compared to poly(norbornene) or poly(styrene) based monoliths, polyurethane-based monoliths are more polar and offer the possibility of surface functionalization via excess hydroxyl or isocyanate groups. In course of our activities in the area of molecular heterogeneous catalysis in confined geometries [24][25][26][27][28][29][30] we were interested in synthesizing polyurethane-based monolithic supports that fulfill the general requirements for polymeric monoliths such as unitary structure, incompressibility, transport pores in the micrometer range, high linear flow (up to 20 mm s −1 ) at low back pressure (<10 bar), while having a tailored mesoporosity in the 2-10 nm range. These mesoporous monoliths were then surface functionalized with quaternary ammonium groups followed by immobilization of an ionic Rh-catalyst-containig IL for use in heterogeneous, biphasic, continuous catalysis using the mesopores as confinements [31][32][33] that ultimately govern the reactivity of the Rh-catalyst.…”