Synthesis of Metal–(Pentadentate‐Salen) Complexes: Asymmetric Epoxidation with Aqueous Hydrogen Peroxide and Asymmetric Cyclopropanation (salenH₂: N,N′‐bis(salicylidene)ethylene‐1,2‐diamine)

Abstract

It is known that the rates and stereochemical outcomes of epoxidations and cyclopropanations using a metallosalen (salenH(2): N,N'-bis(salicylidene)ethylene-1,2-diamine) complex as catalyst are affected by a trans effect of the apical ligand of the complex. By taking into consideration this trans effect, we have synthesized optically active pentadentate salen ligands bearing an imidazole or pyridine derivative as the fifth coordinating group, and have prepared the corresponding manganese(III) and cobalt(II) complexes, in which the fifth ligand is expected to intramolecularly coordinate to the metal center and exert a trans effect. Indeed, high enantioselectivity has been achieved in epoxidations using aqueous hydrogen peroxide as the terminal oxidant and in cyclopropanations with these complexes as catalysts. In general, metallosalen-catalyzed reactions have been carried out in the presence of an excess of a donor ligand; however, the present reactions do not need the addition of any extra donor ligand.

Cyclopropanation Reactions Mediated by Group 9 Metal Porphyrin Complexes

“…[51,54,55] For example, the reaction of styrene with α-diazoacetate gave, in the presence of different cobalt catalysts, the corresponding cis and trans isomers with very high diastereo-and enantioselectivity. The cis-cyclopropane was obtained with 99 % dr and 98 % ee, [56] whereas the trans isomer was formed with 82 % dr and 86 % ee. [57] Yamada et al developed a catalytic system based on chiral ketoiminatocobalt(II) complexes to attain very good diastereo-and enantiomeric excesses.…”

Synthesis and characterization of copper(II), cobalt(II), nickel(II), and iron(III) complexes with two diamine Schiff bases and catalytic reactivity of a chiral diamine cobalt(II) complex

“…The maximum level of diastereoselectivity reached, however, is not very satisfactory (maximum ratio trans:cis = 74:26) if compared to the results reported some years ago by the group of Katsuki, using Co(II) Schiff bases complexes[54,55]. That is the reason why we did not try to investigate further the optimization of the catalyst loading; our[Co]/EDA ratio of 1:30 is still far from the one reported[Co]/EDA ratio of 1:200[56][57][58][59] by employing chiral Co(II)-porphyrin complexes.…”

Asymmetric Epoxidation of Chromenes Using Manganese(III) Complexes with Novel Chiral Salen-like Schiff Base Ligands

“…It is known that some 2,2-disubstituted-3,4-epoxychromene derivatives show unique physiological activity and have wide range of applications [36]. The asymmetric catalytic activity of the Mn(III) complexes 4a-4c are systematically investigated with the substrates of 2,2-dimethylchromene (A), 2,2,6-trimethylchromene (B), 6-tert-butyl-2,2-dimethylchromene (C), 6-chloro-2,2-dimethylchromene (D), 6-nitro-2,2-dimethylchromene (E), and 6-methoxy-2,2-dimethylchromene (F).…”