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[(NHC)(NHC ewg )RuCl 2 (CHPh)] Complexes with Modified NHC ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins
Author(s) -
Sashuk Volodymyr,
Peeck Lars H.,
Plenio Herbert
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200903275
Subject(s) - chemistry , ligand (biochemistry) , catalysis , metathesis , stereochemistry , ruthenium , medicinal chemistry , salt metathesis reaction , organic chemistry , receptor , biochemistry , polymerization , polymer
Imidazolium salts (NHC ewg ⋅ HCl) with electronically variable substituents in the 4,5‐position (H,H or Cl,Cl or H,NO 2 or CN,CN) and sterically variable substituents in the 1,3‐position (Me,Me or Et,Et or i Pr, i Pr or Me, i Pr) were synthesized and converted into the respective [AgI(NHC) ewg ] complexes. The reactions of [(NHC)RuCl 2 (CHPh)(py) 2 ] with the [AgI(NHC ewg )] complexes provide the respective [(NHC)(NHC ewg )RuCl 2 (CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring‐closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2–0.5 mol % at 80 °C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC ewg group with 1,3‐Me, i Pr and 4,5‐Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL‐NHC)(NHC ewg )RuCl 2 (CHPh)] and [(FL‐NHC ewg )(NHC)RuCl 2 (CHPh)], with a dansyl fluorophore (FL)‐tagged electron‐rich NHC ligand (FL‐NHC) and an electron‐deficient NHC ligand (FL‐NHC ewg ) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore‐tagged ligand. In this manner, it was shown for ring‐opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC ewg ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.