The Metathesis of alpha-Olefins over Supported Re-Catalysts in Supercritical CO2

At 35 ◦C, in the presence of supercritical carbon dioxide (80–150 bar) as a solvent, a-olefins (RCH=CH2, R = C4–C6) undergo highly selective self-metathesis catalyzed by supported Re-oxide (7%). To the best of our knowledge, this is the first procedure for the metathesis of alkenes, in which heterogeneous catalysts are combined with the use of dense CO2. The intrinsic eco-compatibility and the unique physicochemical properties of this medium offer both environmental and synthetic advantages: not only conventional toxic solvents (e.g. n-heptane and toluene) can be replaced, but the reaction is faster. For instance, after 2 h, the average conversion of 1-octene is 67% and 40% in scCO2 and n-heptane, respectively. The product of self-methatesis, 7-tetradecene, can be isolated in yields up to 68%. At 90 bar, the reaction is rather sensitive to the mole fraction of the olefin (in scCO2); though, the enhancement of the pressure (and the density) of the supercritical medium does not induce significant effects on either the rate or the selectivity of the process. The nature of the catalytic support also greatly affects the reaction outcome: Re-oxide shows good activity if dispersed over g-Al2O3, while silica-based systems are ineffective.