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

Macrocyclic oligomeric Co(III)-salen complexes derived from cyclooctene salen monomers are among the most active catalysts for asymmetric epoxide ring-opening reactions. Due to the uncontrollable feature of the ring-expanding olefin metathesis step during catalyst synthesis, the macrocyclic oligomeric Co(III)-salen complexes are produced as mixtures of oligomers with different ring sizes. We rationalize that the ring size of the Co(III)-salen oligomers might have a significant effect on the catalytic efficiency and selectivity and report here a purification protocol to isolate macrocyclic dimers, trimers, tetramers and oligomeric mixtures with larger size rings. Hydrolytic kinetic resolution (HKR) tests using allyl glycidyl ether as substrate show that the dimer is inactive at a catalyst loading of 0.01 mol%. Increasing ring size shows a remarkable effect on reaction rates with the largest ring-size species exhibiting superior selectivities and activities. NMR studies reveal that the dimeric catalyst is strained which is not observed for the larger ring-size catalysts. Computational modeling studies indicate that the dimer is lacking the flexibility to allow adjacent Co(III)-salen groups to form a bimetallic complex. Further catalytic tests of larger ring-size Co(III)-salen complexes (tetramer to hexamer mixture) by investigating the HKR of various racemic terminal epoxides and the asymmetric epoxide ring-opening with different nucleophiles demonstrate the superior catalytic activity of large ring-size macrocyclic catalysts. Furthermore, this study demonstrates again the structural (or configurational) sensitivity of Co(III)-salen catalyst towards the selectivity and efficiency of cooperative bimetallic reactions.