No abstract

Controlled catalysis: The combination of magnetically and gravimetrically recoverable catalysts such as the nanoparticle‐bound base (see picture; blue) and the resin‐bound acid (green), respectively, allow the application of non‐complementary catalysts to multistep, one‐pot reaction cascades (e.g. A→B→C). The catalysts are recovered after reaction and reused in subsequent, unrelated reactions.

The Co(III)--salen-catalyzed (salen=N,N'-bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C(3) building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant Co(III)--salen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site-site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer-based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Co--salen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.

A new class of hybrid organic/inorganic molecular catalysts with high, local catalyst concentrations is demonstrated by supporting organic and organometallic catalysts on magnetic nanoparticle based polymer brushes (MPB). Poly(styrene) brushes containing Co(III)-salen or piperazine side chains are prepared via atom-transfer radical polymerization (ATRP) from Fe 3 O 4 nanoparticles modified with appropriate initiator molecules. The polymer brush architecture promotes the cooperative interactions required for Co-salen catalyzed ring-opening of epoxides as demonstrated in the hydrolytic kinetic resolution of racepichlorohydrin. In addition, the piperazine functionalized MPB catalyst contains the high catalyst concentration that is required for promoting the Knoevenagel condensation of benzaldehyde and malononitrile with this type of amine catalyst. All the MPB catalysts were easily removed from solution via application of a magnetic field, allowing straightforward recovery and reuse. The versatile MPB architecture can be used to create a variety of recoverable supported organic or organometallic catalysts.

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.