Polymer-based hydrogenation catalyst

Whilst hydrogenation catalysts based on early transition metals are as active and selective as those based on late transition metals, they are usually not as compatible with functional groups, and this represents the major difficulty for their use in organic synthesis. Nonetheless, titanocene derivatives have been used in industry to hydrogenate unsaturated polymers. 

Numerous methods have been employed for hydrogenating conjugated diene-based polymers in the presence of suitable and effective hydrogenation catalysts. Typical hydrogenation catalysts can be classified into two types heterogeneous and homogeneous catalysts. 

The hydrogenation of unsaturated polymers like polyisoprene is based on the mobility of a soluble catalyst in the reaction medium. In the hydrogenation of such unsaturated polymers the soluble catalyst brings its active site to the C=C bonds in the polymer chain. In contrast, a heterogeneous catalyst requires that the polymer chain unfold to gain access to a catalytically active site on the surface of a metal particle. 

The homogeneous hydrogenation catalysts for polymer saturation can be classified into two types Ziegler-type (Ni, Co, Fe, Ti, Zr based) and noble metal (Rh, Ru, Pd) catalysts. 

A variety of Lewis bases have been used to control microstructure in anionic polymerization, the main requirement being that the Lewis base is sufficiently stable in the presence of the propagating anion to allow living polymerization. The most commonly used modifiers are ethers and tertiary amines. Since amines are poisons for many hydrogenation catalysts, ethers are used more frequently in the production of hydrogenated polymers. A further distinction can be made between monobasic species such as dialkyl ethers and bidentate species that have the potential to coordinate with lithium, such as glyme ethers and TMEDA (N, V, N V -tetramethylethylenediamine). The former must... 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

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