Rational Approach to Polymer-Supported Catalysts: Synergy between Catalytic Reaction Mechanism and Polymer Design

Abstract

Supported catalysis is emerging as a cornerstone of transition metal catalysis, as environmental awareness necessitates "green" methodologies and transition metal resources become scarcer and more expensive. Although these supported systems are quite useful, especially in their capacity for transition metal catalyst recycling and recovery, higher activity and selectivity have been elusive compared with nonsupported catalysts. This Account describes recent developments in polymer-supported metal-salen complexes, which often surpass nonsupported analogues in catalytic activity and selectivity, demonstrating the effectiveness of a systematic, logical approach to designing supported catalysts from a detailed understanding of the catalytic reaction mechanism. Over the past few decades, a large number of transition metal complex catalysts have been supported on a variety of materials ranging from polymers to mesoporous silica. In particular, soluble polymer supports are advantageous because of the development of controlled and living polymerization methods that are tolerant to a wide variety of functional groups, including controlled radical polymerizations and ring-opening metathesis polymerization. These methods allow for tuning the density and structure of the catalyst sites along the polymer chain, thereby enabling the development of structure-property relationships between a catalyst and its polymer support. The fine-tuning of the catalyst-support interface, in combination with a detailed understanding of catalytic reaction mechanisms, not only permits the generation of reusable and recyclable polymer-supported catalysts but also facilitates the design and realization of supported catalysts that are significantly more active and selective than their nonsupported counterparts. These superior supported catalysts are accessible through the optimization of four basic variables in their design: (i) polymer backbone rigidity, (ii) the nature of the linker, (iii) catalyst site density, and (iv) the nature of the catalyst attachment. Herein, we describe the design of polymer supports tuned to enhance the catalytic activity or decrease, or even eliminate, decomposition pathways of salen-based transition metal catalysts that follow either a monometallic or a bimetallic reaction mechanism. These findings result in the creation of some of the most active and selective salen catalysts in the literature.

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

“…In the proposed bimetallic transition state of the Co-salencatalyzed hydrolytic kinetic resolution (HKR) mechanism, this flexibility greatly affected the catalyst performance. [14,21] In the monometallic mechanism of the Ru-salen cyclopropanation reaction, this flexibility could be less important. However, the linker does impact the steric hindrance of the polymer backbone near the catalytically active site, which can be important for polymer-supported catalysts in general.…”

Understanding the catalytic activity of single‐chain polymeric nanoparticles in water

“…This may result in undesired bimetallic interactions as a deactivating factor. 42 More hydrophobic substrates, 4-methyl-, 4-ethyl-, and 4-propyl-cyclohexanone, were also subjected to transfer hydrogenation using P3@Ru. All of the substrates were efficiently converted into the corresponding alcohols with TOF values comparably to those of cyclohexanone (see Supporting Information Table 1S).…”

Catalytic polymeric nanoreactors: more than a solid supported catalyst

“…Over the last decade, polymer-supported catalysts have been extensively used in organic synthesis. [2,3] By attaching the catalyst onto a polymer support it can be easily recovered after each reaction for recycling and not every site needs to be available for reaction. [4] However, these immobilized catalysts are sometimes less active than the corresponding original catalysts and the precursors can be expensive and difficult to synthesize.…”