Catalytic asymmetric ring-opening metathesis/cross metathesis (AROM/CM) reactions. Mechanism and application to enantio selective synthesis of functionalized cyclopentanes

Studies regarding the first examples of catalytic asymmetric ring-opening m etathesis (AROM) reactions are detailed. This enantioselective cleavage of norbornyl alkenes is followed by an intermolecular cross metathesis with a terminal olefin partner; judicious selection of olefin is required so that oligomerization and dimerization side products are avoided. Results outline d herein suggest that the presence of suitably positioned heteroatom substi tuents may be critical to reaction efficiency. Mo-catalyzed tandem AROM/CM affords functionalized cyclopentyl dienes in > 98% ee and > 98% trans olefi n selectivity; both secondary and tertiary ether products can be obtained. The examples provided include the catalytic synthesis of an optically pure cyclopentyl epoxide and dimethyl acetal. Mechanistic studies suggest that i t is the more substituted benzylidene or silylated alkylidenes that are inv olved in the catalytic process (vs the corresponding Mo-methylidenes). Alth ough electron rich benzylidenes react more efficiently, the derived electro n poor Mo complexes promote AROM/CM transformations as well; alkylidenes th at bear a boron substituent are unreactive.