The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) is reported. These catalysts retain the high levels of reactivity found in the related achiral variants (>1a and >1b). Using the parent chiral catalysts >2a and >2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts >3->5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts >2a->4a (to form >2b->4bin situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst >5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of ≤1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.
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