Asymmetric Alkene Epoxidation with Chromium Oxo Salen Complexes. A Systematic Study of Salen Ligand Substituents

Abstract

[structure: see text]. In the stoichiometric asymmetric epoxidation of E-beta-methylstyrene with cationic chromium-salen oxo complexes, enantioselectivity is increased by halo-substitution at the 3,3'- and 6,6'-positions and decreased at the 4,4'- and 5,5'-positions on the salen rings. Addition of triphenylphosphine oxide significantly increases selection with 3,3'- or 5,5'-substituents but not with 4,4'- or 6,6'-substituents. Use of nitrate counterion is beneficial in most cases. The results are discussed with respect to the mode of stereoselection in metal-salen epoxidations.

Three‐Membered Heterocycles. Structure and Reactivity

“…Gilheany and coworkers [419] have observed high (>90%) enantioselectivities for trans-alkenes using stoichiometric chromium complexes, albeit with modest chemical yields, presumably resulting from the formation of a m-oxo Cr(IV) dimer in situ. A systematic study of the effect of aromatic substituents on enantioselectivities [420] is consistent with an oblique approach of the substrate to a nonplanar (stepped) oxidized catalyst (Figure 2.14). An interesting reversal of chiral induction in chromium(III)-salen complexes using a tartaric derived alicyclic diamine moiety (i.e., 361) has been observed by Mosset, Saalfrank and coworkers [421].…”

Substituent electronic effects in chiral ligands for asymmetric catalysis

“…Gilheany and co-workers have carried out extensive investigations into the asymmetric epoxidation of trans-bmethylstyrene using oxo-chromium(V)-salen complexes 2, Scheme 2 [3,4].…”

Heterogeneous enantioselective epoxidation catalyzed by Mn(salen) complexes grafted onto mesoporous materials by phenoxy group

“…Another opinion considers that pathway b is reasonable and pathway c is also not excluded due to the assumed folded Mn(salen) structure [19]. The different pathway will produce different enantioselectivity for asymmetric epoxidation [18][19][20]. For the heterogeneous catalysts, one approach pathway of the substrate to the active center may be restricted or enhanced by the grafting mode and the support.…”