Benign Reaction Medium Research Articles

Solvatochromic Characterization of Near-Critical Water as a Benign Reaction Medium

Near-critical water (liquid water between 200 and 374 °C) offers an environmentally benign alternative for the replacement of undesirable solvents and catalysts. This work characterizes the solvent strength of liquid water at saturation pressure from ambient temperature to 275 °C in terms of its Kamlet−Taft dipolarity/polarizability, hydrogen-bond-donating acidity, and hydrogen-bond-accepting basicity using in situ UV−vis spectroscopy. The results suggest that near-critical water exhibits a wide range of polarity and hydrogen-bond-donor ability for tailoring chemical reactions and separations. These Kamlet−Taft solvent parameters can be used to correlate kinetic properties for reactions in water. As model reactions, the temperature-dependent kinetics of the hydrolyses of two nitroaromatic compounds, 4-nitroaniline and N,N-dimethyl nitroaniline, were determined in NCW in the temperature range of 200−275 °C. The hydronium ion dissociated from water promotes the initial hydrolysis reaction without the addition of any acid. Solvent effects on the rate constant were correlated with Kamlet−Taft solvent parameters based on a linear solvation energy relationship (LSER).

Carbon dioxide as an environmentally benign reaction medium for chemical synthesis

Carbon dioxide as an environmentally benign reaction medium for chemical synthesis

Carbon dioxide as an environmentally benign reaction medium for chemical synthesis

This contribution highlights recent developments from our research efforts at the Max-Planck-Institute for Coal Research concerning the use of compressed (liquid or supercritical) carbon dioxide as a solvent for environmentally friendly synthesis of fine chemicals. Representative examples discussed here include steel-promoted epoxidation using molecular oxygen as the primary oxidant and transition-metal-catalysed asymmetric C-C bond forming reactions.

Carbon dioxide as an environmentally benign reaction medium for chemical synthesis

Carbon dioxide as an environmentally benign reaction medium for chemical synthesis

Nickel-catalysed enantioselective hydrovinylation of styrenes in liquid or supercritical carbon dioxide

Compressed (liquid or supercritical) CO2 is an environmentally benign reaction medium for the highly efficient regio-, chemo- and enantio-selective nickel-catalysed hydrovinylation of styrenes and it allows for catalyst recycling and selective removal of the product from the reaction mixture.

Rhodium-Catalyzed Hydroformylation in Supercritical Carbon Dioxide

Supercritical carbon dioxide (scCO2) is an environmentally benign reaction medium for highly efficient rhodium-catalyzed hydroformylation reactions. Olefinic substrates can be hydroformylated in scCO2 at 40−65 °C to give the corresponding aldehydes in practically quantitative yields. The reaction course of the hydroformylation of 1-octene in scCO2 was analyzed in detail by online-GC monitoring. The influence of reaction parameters such as temperature, synthesis gas pressure, and [P]/Rh ratio on reaction rates and selectivities is grossly similar to the effects observed in conventional solvents. Maximum turnover frequencies of 1375, 500, and 115 h-1 were determined as lower limits for the catalytic activities under the present conditions for the unmodfied, phosphine-modified, and phosphite-modified systems, respectively. With unmodified catalysts, the hydroformylation rates are considerably higher in scCO2 than in organic solvents or liquid CO2 under otherwise identical conditions. Modified catalytic syste...