Synthesis of Cr(III) Salen Complexes as Supramolecular Catalytic Systems for Ring‐Opening Reactions of Epoxides

Abstract

The synthesis of two conformationally restricted Cr(III) salen complexes, 2 and 3, is described. Together, they constitute a supramolecular hydrogen‐bonding catalytic system for the recently reported asymmetric ring‐opening reactions of epoxides by a dynamic supramolecular catalyst. The synthesis involves state‐of‐the art transformations in frontline synthetic chemistry applied to heterocyclic chemistry. Hence, palladium‐catalyzed reactions were employed, including carbonylative annelation and Suzuki cross‐coupling reactions, for the formation of one of the heterocyclic rings (quinolone) and the functionalization of the formed rings. For the construction of the second heterocyclic ring (isoquinolone), a Curtius rearrangement was employed. The corresponding salen ligands were then prepared by Schiff‐base reactions, yielding the final complexes after metal insertion. For reference purposes the less conformationally restricted Cr(III) complexes 4 and 5 were also synthesized.

Mimicking Enzymes: Taking Advantage of the Substrate-Recognition Properties of Metalloporphyrins in Supramolecular Catalysis

“…[84] A chromium-based version at the place of manganese has been reported although no catalytic activity is known at this stage. [85] Scheme 5. A zinc(II)-porphyrin-based supramolecular manganese catalyst applied in substrate-selective epoxidation exploiting remote Zn … N interactions.…”

“…84 A chromium-based version in place of manganese has been reported; however, no catalytic activity has been described at this stage. 85 In 2017, our group reported carbon-carbon bond-forming reactions in which halopyridine substrates were selectively bound to zinc(II)-containing scaffolds via Zn … N interactions, whilst a trivial palladium/triphenylphosphine catalytic system was operating (Figure 4). 86 Zinc(II)-porphyrin A and zinc(II)-salphen B were employed in this study as they…”

“…84 A chromium-based version in place of manganese has been reported; however, no catalytic activity has been described at this stage. 85…”

Asymmetric Ring-Opening of Epoxides Catalyzed by Metal–Salen Complexes

“…Wärnmark and coworkers [35,36] explored hydrogen-bonded supramolecular catalysts, designing Cr-salen complexes capable of forming heterodimers through complementary hydrogen-bonding motifs (complexes 17 and 18 in Figure 5). In addition, complexes containing an alkyl strap were also synthesized (complexes 19 and 20 in Figure 5) in order to force the hydrogen-bonding moieties of the monomers into a favorable conformation to assist the aggregation of the desired cyclic heterodimer.…”

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co–salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides

“…Chiral metal–salen complexes were designed for catalyzing reaction processes that resulted in good yield, high regioselective and enantioselective control for the asymmetric ring opening of terminal epoxides. Various metals have been explored to optimize the catalytic properties of chiral metal–salens, such as Cr [ 7 ], Co [ 8 ], Fe [ 9 ], Ti [ 10 ], Al [ 11 ], Y [ 12 ], and Mn [ 13 ] and investigated with numerous nucleophiles to afford chiral molecules. In addition to the variation of metals, salen ligands have also been studied with regard to conformational differences, for instance, oligosalen [ 14 ], macrocyclic oligosalen [ 15 ], and polymeric salen [ 16 ].…”