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

Abstract

Chiral salen complexes are privileged and versatile catalysts used in a wide variety of enantioselective transformations. Researches for their use in multicatalysis, including cooperative as well as tandem processes, are more recent, but many reports already highlight the important effect of the salen ligand structure on reaction rates and/or selectivities. This review article thus outlines the literature covering last five years, on the current developments of chiral polymetallic salen complexes used in asymmetric heterogeneous catalysis; their preparation, their efficiency as catalysts in various reactions and their recyclability are highlighted according to the immobilization procedures involved for their heterogenization. These methods include grafting on organic or inorganic supports insuring easy recovery by simple filtration. Polymerization processes are also part of recent research, leading to high densities of catalytic sites to favor their cooperativity. Combination of salen complexes via non covalent interactions or their introduction on MOFs and COFs networks is currently in full expansion, with examples of highly functionalized and enantioenriched products issued from tandem catalysis.

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes

“…Bridges such as propylidenediamines induce significant distortions in the coordination polyhedron, which may be unsuitable for Cu­(III). Surprisingly, the effect of the length of the bridge on both the electronic structure and the electrochemistry of sterically hindered salen complexes is yet limited to Pt, Pd, and Ni in noncoordinating solvents. , A greater knowledge of this specific effect would also be of great importance not only from a fundamental point of view, but also for expending the field of applications of redox-active salen compounds to electrocatalysis, sensing and heterogeneous catalysis since the diamine linker is an attractive site for anchoring the complexes onto various supports. , …”

Metal–Photoswitch Friendship: From Photochromic Complexes to Functional Materials

“…Photochromic bidentate ligands (e.g., functionalized azobenzene and diarylethene derivatives) can also serve to bridge two metal centers (M 1 –Φ–M 2 ), allowing the spatial orientation of the metal cations to be modulated through irradiation (Figure ). Inspired by natural catalysis occurring through enzymes, which could involve cooperation between two metal centers, an azobenzene moiety has been used to control the spatial arrangement of two catalytically active sites. − As shown in Figure , the azobenzene-based ligand was used to bridge two chiral titanium­(salen) complexes to catalyze asymmetric sulfoxidation reactions . Specifically, irradiation with 365-nm wavelength resulted in formation of the cis -isomer, which brought the titanium centers in close proximity and catalyzed sulfoxidation reactions.…”

Post-Modification of Copolymers Obtained by ATRP for an Application in Heterogeneous Asymmetric Salen Catalysis

“…), they promote the enantioselective catalytic formation of carbon–carbon or carbon–heteroatom, bonds with excellent efficiency both in terms of activity and selectivity [ 1 , 2 ]. The immobilization of such catalysts is currently particularly under study [ 3 , 4 , 5 ] to solve two problems; the anchoring of such complexes on a support allows, on the one hand, their easy separation at the end of the reaction, with minimal contamination of the target products in metallic traces. On the other hand, their recovery being greatly facilitated by simple filtration, these robust salen complexes can be engaged again in asymmetric catalytic transformations, leading to an important increase in turnover number (TON).…”