Highly Selective Macrocycle Formations by Metathesis Catalysts Fixated in Nanopores

Abstract

Ruthenium-based metathesis catalysts immobilized on mesocellular siliceous foam (MCF) bearing large nanopores proved highly efficient and selective for macrocyclic ring-closing metathesis (RCM). Kinetic studies revealed that the homogeneous counterpart exhibited far higher activity that accounted for more oligomerization pathways and resulted in less macrocyclization products. Meanwhile, the immobilized catalysts showed lower conversion rates leading to higher yields of macrocyclic products in a given reaction time, with conversion rates and yields dependent upon pore size, catalyst loading density, and linker length. The macrocycle formations via RCM were accelerated by increasing the pore size and decreasing the catalyst loading density while retaining the comparably high yield. The catalysts immobilized on MCF, of which silica surface is rigid and pores are relatively large, showed high conversion rates and yields compared with an analogue immobilized on TentaGel resins, of which backbone becomes flexible upon swelling in the reaction medium. It is noteworthy that the selectivity for the macrocyclic RCM can be significantly improved by tuning the catalyst initiation rates via immobilization onto the support materials in which well-defined three-dimentional network of large nanopores are deployed.

Macrocyclization Reactions: The Importance of Conformational, Configurational, and Template-Induced Preorganization

“…Although the activity of the catalyst 433a can be reduced, the macrocyclization process of 432 can be significantly more selective toward the formation of the corresponding macrocyclic structure 434 via intramolecular processes. 435 As observed for many other supported systems, the results are greatly affected by the chemical and morphological nature of the support (433b and 433c) ( Figure 132) and, as could be expected, the yields for the macrocyclization increase with the reduction in the loading of the polymer chains in the solid support. 127,144 The work of Shu et al is illustrative of the involvement of different factors, including the role of the metal center, for the success of a cyclization by RCM.…”

“…Use of supported Ru catalysts for RCM 435. Chemical Reviews Review DOI:10.1021/acs.chemrev.5b00056Chem.…”

Kinetically E-selective macrocyclic ring-closing metathesis

“…Some products can be accessed with low E selectivities through RCM reactions with bis(α-olefin) compounds catalyzed by Ru complexes 45–47 .…”

Opportunities of Immobilized Homogeneous Metathesis Complexes as Prominent Heterogeneous Catalysts

“…who immobilized HG2 on MCF (Figure , 21 ) and performed macrocyclic RCM reactions (ring sizes of C14–C27; 5 m m ; 2 wt % Ru). For ring sizes larger than C20, the supported reaction led to a much higher selectivity of the corresponding cyclic structures by RCM, whereas homogeneously a substantial higher fraction of linear oligomers was formed through ADMET . Moreover, the heterogeneous catalyst was prone to diffusion limitation, which seems reasonable because of the high catalyst loading of 2 wt % ruthenium (high internal pore activity) and the substrate bulkiness (slow intraporous molecular diffusion).…”