No abstract

The ruthenium carbene complexes 1 and 2 (0.05-5 mol%) catalyse highly efficient macrocyclization reactions of 1,ω-dienes by ring-closing metathesis (RCM). Key parameters for successful RCM are (i) the presence of a functional group which serves as a relay entity that assembles the reacting sites, (ii) an appropriate distance between this polar group and the alkenes to be metathesized, and (iii) low steric congestion near the double bonds. Contrary to previous assumptions, however, the ring size formed and the conformational predisposition of the substrates for ring closure turned out to be of minor importance. These aspects are illustrated by some straightforward syntheses of macrocyclic lactones, lactams, ethers and ketones, including the musk odored perfume ingredients Exaltolide, Exaltone and Arova 16, of the macrolide recifeiolide (24), as well as of the alkaloids epilachnene (40) and its homologue 9-propyl-10-azacyclododecan-12-olide (39), which are active principles of the defense secretions of the pupae of the mexican beetle Epilachnar varivestis

Liquid or supercritical carbon dioxide (scCO(2)) 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 PCy(3) 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 scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) 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 scCO(2) 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 (1)H NMR data obtained in compressed CO(2). 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.

General. All reactions were carried out under Ar using Schlenk techniques. Chemical shifts are listed downfield in ppm relative to tetramethylsilane. Coupling