Steric and Electronic Effects of Carbene Substitution in Grubbs First-Generation Catalysts

The relative energies of the carbene species in a variety of metathesis reactions catalyzed by Grubbs first-generation catalyst (Cy3P)2Cl2RuCHC6H5 have been determined. High-quality equilibrium data are obtainable by 1H NMR methods. In the case of para-substituted aromatic derivatives (Cy3P)2Cl2RuCH(p-C6H4X), the carbenes are stabilized by donor substituents, and energies may be determined indirectly by use of either NMR data or Hammett correlations, because only electronic effects are operative. Four systems have been characterized by X-ray crystallography. The energies of alkyl-substituted carbenes (Cy3P)2Cl2RuCHR are almost entirely dependent on steric interactions.