Heterogeneous, Supported and Biocatalytic Hydrogenations

Figu re 4.7 The molecular structure of complex [lrCl(cod) l,3-bis[(R)-l-ferrocenylethyl]benzymidazol-2-ylidene ] (115). 

Catalyst Substrate Pressure (bar) Time (h) Conversion [%) Chemoselectivity (To)  

Conditions substrate, 50 mg solvent, PrOH 6.5 g catalyst 25 mg (calcined at 300°C and reduced at450°C). 

Iridium nanoparticles generated in l-n-butyl-3-methylimidazolium (BMI)-based ionic liquids were found to be excellent recyclable catalytic systems for the hydrogenation of a variety of substrates, including ketones such as simple ketones. The Ir nanoparticles were prepared by simple reduchon of [Ir(cod)Cl]2 dispersed in BMI-PFis at 75 °C under 4 atm of H2. Benzaldehyde, cyclopentanone, methyl butanone and derivatives were hydrogenated with almost complete conversion, with TOFs ranging from 17 to 96h under solventless conditions (substrate Ir ratio = 250, 75 °C, 4 atm FI2) [102]. 

Rhodium and iridium nanoparhcles entrapped in aluminum oxyhydroxide nanofibers were shown by Park et al. to be suitable catalysts for the hydrogenation of arenes and ketones at room temperature, with hydrogen at ambient pressure [103]. Rhodium in aluminum oxyhydroxide [Rh/A10(0H)] and iridium in aluminum oxyhydroxide [Ir/A10(0H)], were simply prepared from readily available reagents such as RhCls and IrCls hydrates, 2-butanol and Al(O-sec-Bu) at 100°C. Substrates such as cyclopentanone, 2-heptanone, ethyl pyruvate, acetone and 2,6-dimethyl-4-heptanone were reduced to the corresponding alcohols either in n-hexane at room temperature (maximum TOF 99 h for ethyl pyruvate) or in solventless conditions at 75 °C using 4 atm of H2 (maximum TOF 660h for acetone, 330 for 2-heptanone). 

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