The bigger, the better: Ring-size effects of macrocyclic oligomeric Co(iii)-salen catalysts

Poly(styrene) Resin‐Supported Cobalt(III) Salen Cyclic Oligomers as Active Heterogeneous HKR Catalysts

“…The resultant structures have the salen units freely dangling off the macrocyclic backbone 7b. We suggest that the high activity of this catalyst is due to the flexibility of these cyclic oligomeric supports 8. A drawback of these soluble supported catalysts is their removal from the reaction mixture 9.…”

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

“…It has been reported that the catalytic activity of macrocyclic oligomer supported Co(III)-salen changes dramatically with the macrocyclic ring size in the hydrolytic kinetic resolution of terminal epoxides. 25 The highest catalytic efficiency was achieved by larger ring size catalysts (pentamer to decamer). In contrast, the dimer has a poor catalytic activity under the same conditions followed by a trimer.…”

“…17 Weck et al developed a series of macrocyclic oligomeric cyclooctene supported salen complexes showing high activity in various asymmetric catalytic reactions. [23][24][25][26] Macrocyclic oligomeric cyclooctene supported Al(III)-salen with linkers of three different lengths (3-, 7-or 10-bond) were tested in the bimetallic mechanism based cyanide addition to unsaturated imides. The catalyst with the longest linker afforded both the highest yield and the highest enantioselectivity.…”

Chiral Cobalt‐Salen Complexes: Ubiquitous Species in Asymmetric Catalysis

“…The authors assumed that this resulted from the optimized cooperative reactions within the oligomers, due to their flexibility allowing an ideal placement of two active sites for bimetallic activation through an head‐to‐tail arrangement of the salen units. The Co(II)‐oligomers, obtained in an original way via ring‐opening metathesis polymerization of cyclooctene‐modified salen monomers and already presented as catalysts for the Henry reaction ( 9 in Figure 2) were tested as catalysts for the HKR of terminal epoxides, after activation under aerobic and acid conditions; [51] they also showed an enhanced activity due to efficient bimetallic activation, thanks to their flexibility and the high concentration of the cobalt active sites, provided the use of large ring‐size macrocyclic catalysts [52] …”