Understanding the effect of allylic methyls in olefin cross-metathesis

A series of NMR spectroscopy experiments have been conducted with both the model compound, 3-methyl-1-pentene and the corresponding ADMET monomer 3,6,9-trimethylundeca-1,10-diene (11) to better understand the effect of allylic methyls during olefin metathesis chemistry. Traditional ADMET catalysts such as Schrock’s molybdenum (1), and Grubbs’ ruthenium 1st and 2nd generation (2 and 3) were examined under cross-metathesis and ADMET conditions. Regardless of catalyst selection, 50% or less metathesis conversion was observed for all reactions, especially in the case of the more sterically encumbered diene. With Schrock’s molybdenum catalyst 1, the reaction leads to an accumulation of the non-productive metallacyclobutane, trapping the catalyst in an inactive form. With Grubbs’ ruthenium catalysts 2 and 3, the substrate coordinates to the metal center primarily to yield non-productive metathesis, which results in a build-up of the methylidene complex leading to catalyst decomposition. These results are directly correlated to the orientation of the substrate’s bulk during the metallacyclobutane formation, the alkyl branch being adjacent to the metal center in the case of the molybdenum catalyst 1, and opposite to it in the case of ruthenium catalyst 2 and 3.

The homometathesis of olefins containing allylic methyls is impeded by steric interactions within the ligand sphere of the catalytic intermediate, as proven by a series of NMR spectroscopy experiments using Schrock’s molybdenum, and Grubbs’ ruthenium catalysts.Figure optionsDownload as PowerPoint slide