Recyclable Polymer‐ and Silica‐Supported Ruthenium(II)‐Salen Bis‐pyridine Catalysts for the Asymmetric Cyclopropanation of Olefins

Abstract

Homogeneous ruthenium(II)-salen bis-pyridine complexes are known to be highly active and selective catalysts for the asymmetric cyclopropanation of terminal olefins. Here, new methods of heterogenization of these Ru-salen catalysts on polymer and porous silica supports are demonstrated for the facile recovery and recycle of these expensive catalysts. Activities, selectivities, and recyclabilities are investigated and compared to the analogous homogeneous and other supported catalysts for asymmetric cyclopropanation reactions. The catalysts are characterized with a variety of methods including solid state cross-polarization magic-angle spinning (CP MAS) 13 C and 29 Si NMR, FT-IR, elemental analysis, and thermogravimetric analysis. Initial investigations produced catalysts possessing high selectivities but decreasing activities upon reuse. Addition of excess pyridine during the washing steps between cycles was observed to maintain high catalytic activities over multiple cycles with no impact on selectivity. Polymer-supported catalysts showed superior activity and selectivity compared to the porous silicasupported catalyst. Additionally, a longer, flexible linker between the Ru-salen catalyst and support was observed to increase enantioselectivity and diastereoselectivity, but had no effect on activity of the resin catalysts. Furthermore, the polymer-supported Ru-salen-Py 2 catalysts were found to generate superior selectivities and yields compared to other leading heterogeneous asymmetric cyclopropanation catalysts.

On the Stability and Recyclability of Supported Metal–Ligand Complex Catalysts: Myths, Misconceptions and Critical Research Needs

“…The solid-supported ruthenium catalysts were easily recovered and recycled. Recycling catalysts 5a, 5b, and 6 demonstrated that the used catalysts could achieve very similar diastereoselectivities and enantioselectivities as in the initial runs, but decreased rates were observed, leading to longer reaction times [56]. For example, catalyst 5b required more than double the time of the original run to reach 95% yield in the third run (11 h vs. 5 h).…”

“…Supported catalyst 5b stands as the most selective supported asymmetric cyclopropanation catalyst known in the literature [56]. Interestingly, the silica supported catalyst was significantly less selective than the polymer-supported catalyst 5b, despite the seemingly similar metal coordination sphere.…”

“…7 Yield of cyclopropyl products versus time using catalyst 5b: (blue triangles) first cycle, (brown diamonds) third cycle, and (pink circles) third cycle with pyridine treatment between all previous cycles, adapted from ref. [56] these critical issues. Nonetheless, a complete understanding of the deactivation pathways in each case is not yet in hand and further studies are still needed.…”

“…Along this line, we developed poly(styrene) resin and mesoporous silica supported Ru(II)-salen bis pyridine complexes [56]. Two different salen-support linkers were used, an ethylene oxide based linker [55] and an phenylene linker [46] that gave the most active and least active Co-salen HKR catalysts, respectively, in previous studies.…”

“…Elemental analysis of catalyst 5a after 3 runs revealed significant ruthenium loss from the catalyst ( Table 6). It was hypothesized that this metal loss was associated with loss of protecting pyridine ligands during the catalyst recovery process, as the liquid media were removed and the solids were washed before reuse [56]. To this end, catalyst 5b was also recycled with addition of a small amount of pyridine to the wash solvent during catalyst recovery, with the goal of repopulating the empty coordination sites that lost pyridine during reaction and catalyst recovery.…”