A broadly applicable and practical oligomeric (salen)Co catalyst for enantioselective epoxide ring-opening reactions

Abstract

The (salen) Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen) Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen) Co monomer 1 for each reaction class.

Calix[8]arene as New Platform for Cobalt‐Salen Complexes Immobilization and Use in Hydrolytic Kinetic Resolution of Epoxides

“…Jacobsen and his group described highly enhanced reactivity and enantioselectivity by employing cyclic multimeric cobalt‐salen complexes. While using their monomeric Co(OTs)‐complex at 4 °C, 80 % conversion could be reached after three days and the diol was isolated with 64 % ee; the oligomeric species allowed increasing both the activity (>99 % conversion after 11 h) and the selectivity (96 % ee) ,,. In our case, an adjustment of the reaction conditions was necessary to obtain a complete conversion into (1 R ,2 R )‐cyclohexane‐1,2‐diol.…”

Chiral Salen Complexes for Asymmetric Heterogeneous Catalysis: Recent Examples for Recycling and Cooperativity

“…Such a process was pioneered by Jacobsen et al . by preparing a cyclic oligomeric Co(OTf)‐salen complex as practical catalyst for enantioselective epoxide ring‐opening reactions with different nucleophiles . The catalyst, which was synthetized from commercially available precursors, delivered higher activity and ee values than the related monomer, by favoring the necessary cooperativity due to the established bimetallic mechanism .…”

Applications of Chiral Three-membered Rings for Total Synthesis: A Review

“…In 1990s, Jacobsen et al demonstrated the exceptional catalytic performance of Cr(III)-salen complexes in the kinetic resolution of racemic terminal monosubstituted and 2,2-disubstituted epoxides by TMSN 3 , which occurred with complete regioselectivity for the terminal position and very high stereoselectivity factors of up to 280 [107]. Later, these authors reported enhanced reactivity by using dimeric catalyst 124 in hydrolytic kinetic resolution of terminal epoxides (Scheme 50) [108]. A recent synthetic application of kinetic resolution using primary alcohols was illustrated by the same authors in the total synthesis of (+)-reserpine via intermediate enone 125 [109].…”