Olefin metathesis in supercritical carbon dioxide

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction me dium for ring-opening metathesis polymerization (ROMP) and ring-closing ole fin metathesis (RCM) reactions using well-defined metal catalysts. The moly bdenum 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 c ompound 1 is readily soluble in scCO(2), complexes 2 and 3 behave liked het erogeneous catalysts in this reaction medium. Importantly, however, the uni que properties Of scCO(2) provide significant advantages beyond simple solv ent 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 metathesi s polymerization), and to applications Of scCO(2) as a protective medium fo r basic amine functions. The latter phenomenon is explained by the reversib le formation of the corresponding carbamic acid as evidenced by H-1 NMR dat a obtained in compressed CO2. Together with its environmentally and toxicol ogically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthes is and production.