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Single (substitution) is better: New chiral ruthenium metathesis (pre)catalysts with a monosubstituted carbon atom in the N-heterocyclic carbene ligand are highly stable in solution, initiate easily, show very high enantioselectivity and excellent E selectivity in asymmetric ring-opening cross-metathesis (see scheme).
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Tiede et al. (2010) studied this question.
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