The significance of degenerate processes to enantioselective olefin metathesis reactions promoted by stereogenic-at-Mo complexes.
The present study provides spectroscopic and experimental evidence demonstrating that degenerate metathesis is critical to the effectiveness of this emerging class of chiral catalysts. Isolation and X-ray characterization of both diastereomeric complexes, as well as an examination of the reactivity and enantioselectivity patterns exhibited by such initiating neophylidenes in promoting ring-closing metathesis processes, are disclosed. Only when sufficient amounts of ethylene are generated and inversion at Mo through degenerate processes occurs at a sufficiently rapid rate is high enantioselectivity achieved, irrespective of the stereochemical identity of the initiating alkylidene (Curtin-Hammett kinetics). With diastereomeric metal complexes that undergo rapid interconversion, stereomutation at the metal center becomes inconsequential, and stereoselective synthesis of a chiral catalyst is not required.
Meek, SJ; Malcolmson, SJ; Li, B; Schrock, RR; Hoveyda, AH
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