Second generation supported catalysts

Alternatively, monolith-supported second generation Crubbs catalysts containing unsaturated (e.g. IMes) or saturated (e.g. SIMes) NHCs [5] can be prepared by a synthetic protocol summarized in Scheme 8.9. Surface-derivatization of a monolith was carried out with 7-oxanorborn-2-enedicarboxylic anhydride followed by conversion of the grafted poly(anhydride) into the corresponding poly-silver salt. 

Second generation Phillips catalysts involve use of titanium compounds that modify the surface chemistry of the support and enables production of polyethylene with higher MI (lower MW) (12). Titanium tetraisopropoxide, also known as tetraisopropyl titanate (TIPT), is the most commonly used modifier for these catalysts. Hexavalent chromium titanate species are probably formed on the surface as shown in Figure 5.3 (13). Catalyst surfaces contain a diversity of active sites and molecular weight distribution of the polymer is broader than that from generation catalysts. 

Scheme 11.12 Synthesis of a monolith-supported Grubbs second-generation ruthenium catalyst immobilized via the NHC ligand.

Treatment of Grubbs second generation metathesis catalyst with carbon monoxide resulted in a Buchner rearrangement in which the alkylidene inserted into one of the aryl substituents of the NHC (Scheme 2.29). It was proposed that CO, being a strong n-acceptor, decreased the electron density available for back donation and stabilisation of the alkylidene ligand. The alkylidene would therefore become more electrophilic, and was attacked by the adjacent aryl ring. These postulates were supported by a DFT study carried out by Cavallo and co-workers. ... 

Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NH3 with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 -105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. 

Scheme 8.9 Synthesis of a monolith-supported second-generation catalyst immobilized via a carboxylate ligand.

Second Generation Recyclable Boomerang Polymer Supported Catalysts for Olefin Metathesis Application of Ardu-engo Carbene Complexes, M. Ahmed, T. Arnauld, a. G.M. Barrett, et at, Synlett 2000, 1007-1009. 

The same authors recently described the synthesis of similar rhodium-complexed dendrimers supported on a resin having both interior and exterior functional groups. These were tested as catalysts for the hydroformylation of aryl alkenes and vinyl esters (52). The results show that the reactions proceeded with high selectivity for the branched aldehydes, with excellent yields, even up to the tenth cycle. The hydroformylation experiments were carried out with first- and a second-generation rhodium-complexed dendrimers as catalysts, with a mixture of 34.5 bar of CO and 34.5 bar of H2 in dichloromethane at room temperature. Each catalyst was easily recovered by simple filtration and was reusable for at least six more cycles without... 

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