Ruthenium-Based Olefin Metathesis Catalysts Coordinated with NHeterocyclic Carbene Ligands: Synthesis and Applications

The improved synthesis and olefin metathesis activity of N-heterocyclic carbene (NHC)-coordinated ruthenium alkylidenes of the form (NHC)(L)x(Cl)2Ru=CHR (x = 1 or 2) are reported. In order to circumvent the handling of highly sensitive free carbenes, N-heterocyclic carbene "adducts" were prepared in high yields by the reaction of nucleophilic bases with N,N'-diarylimidazolium salts. Most notably, the addition of trichloromethyl anion to N,N'-dimesityl-4,5-dihydroimidazolium chloride produced an air-, moisture-, and temperature-stable crystalline adduct, 2-trichloromethyl-4,5-dihydro-imidazolidine. When this species is heated above the critical temperature of 55 degrees C in the presence of (PCy3)2(Cl)2Ru=CHPh, a single, clean phosphine substitution reaction occurs to form the NHC-coordinated benzylidene (NHC)(PCy)3(Cl)2Ru=CHPh in 84% isolated yield. This procedure has been successfully scaled up to industrial production and remains the most effective catalyst synthesis to date. The NHC-coordinated catalysts show dramatically expanded activity relative to their bis-phosphine counterparts. The high yielding, trans-stereoselective cross metathesis of various acroyl substrates is the first example of the ruthenium-catalyzed metathesis of olefins directly substituted with electron-withdrawing functionality. Ring-opening cross metathesis of acroyl species with relatively high ring strain cyclooctadiene and norbornene monomers has also been achieved in good yields and perfect regioselectivity when the norbornene is asymmetrically substituted with a bridgehead methyl group. Further expansion of the substrate scope was achieved when the catalyst's phosphine ligand was replaced with more weakly bound 3-bromopyridine (3-Br-pyr) ligands. The resulting catalyst (NHC)(3-Br-pyr)2(Cl)2Ru=CHPh produced synthetically useful yields (>= 67%) in the cross metathesis of acrylonitrile and terminal olefins (as opposed to less than 30% yield with the phosphine-coordinated catalyst). NHC-coordinated catalysts therefore allow both electron-rich and electron-poor olefins to undergo metathesis in the same pot, potentially leading to synthetically valuable products containing electronically differentiated olefins. The lower activity of phosphine-coordinated catalysts relative to those coordinated with 3-bromopyridine can be addressed by the addition of "phosphine scavengers" to the former. Higher pKa carboxylic acids (such as acetic and benzoic acids) are capable of accelerating catalysis as effectively as the much stronger hydrochloric acid, without concomitant catalyst decomposition. These properties make carboxylic acids the optimal choice for use with sensitive organic substrates.