Ring-closing metathesis asymmetric, using chiral catalyst

Asymmetric Synthesis Using a Chiral Molybdenum Catalyst In olefin metathesis, a double bond is cleaved and a double bond is formed. Thus, a chiral carbon center is not constructed in the reaction. To realize the asymmetric induction by ring-closing metathesis, there are two procedures a kinetic resolution and desym-metrization of symmetric prochiral triene. Various molybdenum complexes are synthesized in order to explore the viabihty of these approaches (Figure 6.2). 

Ring-closing metathesis has been used successfully in asymmetric synthesis (Scheme 10). Chiral catalysts such as 11 can be employed for the kinetic resolution of racemates [21] (58 —> 59 -t 60). Scheme 10 demonstrates the formation of only one enantiomer 59 with a high level of optical purity. Enantiomer 60 does not undergo... 

Metal carbene complexes are also involved in metathesis (described in chapter 15). Exchange of carbene complexes with alkenes via a metallacyclobutane releases volatile alkenes such as ethylene with the formation of new alkenes. Ring closing metathesis is particularly favoured but normally leads to no new chiral centres. The simple Mo and Ru carbene catalysts described in chapter 15 cannot of course be used to induce asymmetry but a new generation of asymmetric Schrock 152 and Grubbs 153 catalysts can create asymmetry if a choice between two enantiotopic alkenes is offered.36... 

---