Highly reactive chiral Ru-based catalysts possessing C(1)-symmetric N-heterocyclic carbene ligands adorned with one N-alkyl group and one N-aryl group were evaluated in asymmetric desymmetrizations to form cyclic products possessing a tetrasubstituted olefin.

In asymmetric olefin metathesis reactions, the addition of halide additives is often required to augment enantioselectivities, despite the fact that the additives result in catalysts with diminished reactivities. The preparation of new chiral Ru-based catalysts was accomplished by exploiting previously reported mechanistic studies. The catalysts possess a high level of reactivity and successfully induce high levels of asymmetry in desymmetrization reactions without the use of halide additives.

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Four new ruthenium-based olefin metathesis catalysts that possess an N-heterocyclic carbene (NHC) ligand with benzyl (Bn) or or n-propyl (n-Pr) N-alkyl groups have been prepared. The synthetic routes developed for the synthesis of the required dihydroimidazolium salts are general. Catalysts bearing larger NHC ligands with larger N-alkyl groups displayed improved thermal and solution state stability up to 80 8C. The reactivity of the new catalysts in ring-closing metathesis is directly related to the increased steric bulk of the NHC ligand. The new catalysts have been evaluated in desymmetrization reactions and the nature of the N-alkyl group of the NHC ligands has been shown to have an important effect on the observed enantioselectivities.

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