Polymer resin hydrogenation methods

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

The nitropyridines can be reduced to the corresponding aminopyridine and several new methods have been developed to achieve this transformation. 3-Nitropyridine is reduced in excellent yield by the mixed borohydride prepared from 1 equiv of ZrCU and 4 equiv of NaBFLi in THF under reflux <2000SL683>. 3-Nitropyridines may be reduced to the aminopyridine using sodium hydrosulfite in THF/H20 at room temperature <2005JME5104>. 2-Nitropyridine can be reduced to the amine in near quantitative yield by transfer hydrogenation in the presence of 10% Pd/C and recyclable polymer-supported formate, prepared from aminomethylpolystyrene resin and ammonium formate <2005SC223> (Equation 64). The resin is easily recovered by filtration and may be used up to 10 times. 

U sing polymers was one of the very first methods in order to heterogenize the catalyst in a homogeneous catalytic reaction. Thus, thanks to these supports, the catalyst acquires the property of insolubility while maintaining its catalytic performance [39-42]. Some authors synthesized phosphonated resins, such as polystyrene, and used them as a ligand in several rhodium and platinum complexes. Thus, hydrogenation [43, 44], hydrosilylation [45], and hydroformylation of olefins were catalyzed. 

Some solution resins are made via solvent or solution polymerization. This technique of polymerization has been performed in many media that are solvents for monomer but not polymer. Solvents may include methyl alcohol, diethyl ether, dioxane, toluene, benzene, acetone, etc., which may be used singly or as blends. A suitable catalyst for use with methyl alcohol, for example, would be benzoyl peroxide. Hydrogen peroxide may be used with acetone. The resin is separated as a solid by filtering and/or water quenching. This method is usually used for copolymers because of the poor solubility of poly(vinyl chloride). 

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