Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions.

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis-diiodo/sulfur-chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis-cyclooctene by the RCM reaction of 1,9-undecadiene. Moreover, cis-1,4-polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5-cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.

Influence of Anionic Ligand Exchange in Latent Sulfur-Chelated Ruthenium Precatalysts.

Four new cis-dianionic S-chelated ruthenium benzylidenes were synthesized by chloride ligand exchange. The special cis-dianionic conformation of these complexes contributed to a particularly facile anion exchange process, producing room-temperature-latent precatalysts. Their catalytic activity was strongly influenced by the solvent used. The latent iodide complex very efficiently promoted ring-closing metathesis by heating in toluene. Conversely, carboxylate ligands produced quite poor catalysts, but could abstract chlorides from chlorinated solvents to transform into active precatalysts. In tetrahydrofuran (THF), the S-chelated dichloro complex was shown to promote cycloisomerization instead of metathesis; however, the metathesis activity in THF could be recovered in the presence of phenylacetylene as a cocatalyst. Under the same conditions, all the other complexes required addition of LiCl to mimic this dichotomous behavior.

Amino acids as chiral anionic ligands for ruthenium based asymmetric olefin metathesis.

Several amino acid ligands were introduced into the Hoveyda-Grubbs 2nd generation complex by a facile anionic ligand exchange. The chiral pre-catalysts obtained displayed enantioselectivity in asymmetric ring-closing and ring-opening cross-metathesis reactions. Reduction of the lability of the carboxylate ligands was found to be cardinal for improving the observed enantiomeric product enrichment.