Olefin Metathesis in Supercritical Carbon Dioxide

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Abstract
Liquid or supercritical carbon dioxide (scCO2) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy3 or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO2, complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO2 provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO2 as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by 1H NMR data obtained in compressed CO2. Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.