Asymmetric construction of tetrahedral chiral zinc with high configurational stability and catalytic activity

Abstract

Chiral metal complexes show promise as asymmetric catalysts and optical materials. Chiral-at-metal complexes composed of achiral ligands have expanded the versatility and applicability of chiral metal complexes, especially for octahedral and half-sandwich complexes. However, Werner-type tetrahedral complexes with a stereogenic metal centre are rarely used as chiral-at-metal complexes because they are too labile to ensure the absolute configuration of the metal centre. Here we report the asymmetric construction of a tetrahedral chiral-at-zinc complex with high configurational stability, using an unsymmetric tridentate ligand. Coordination/substitution of a chiral auxiliary ligand on zinc followed by crystallisation yields an enantiopure chiral-only-at-zinc complex (> 99% ee). The enantiomer excess remains very high at 99% ee even after heating at 70 °C in benzene for one week. With this configurationally stable zinc complex of the tridentate ligand, the remaining one labile site on the zinc can be used for a highly selective asymmetric oxa-Diels-Alder reaction (98% yield, 87% ee) without substantial racemisation.

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

“…Chiral Cu(I) complexes have also been used as asymmetric photocatalysts; however, tetrahedral copper complexes readily undergo fast ligand exchange, which prevents the exploitation of any innate metal-centered chirality. 294 Thus, enantiocontrol requires the use of chiral ligands. Ohkubo achieved an enantiodiscriminating photoreduction of a racemic mixture of Λand Δ-Co(edta) − using the photocatalyst Cu(dmp)(diop) + (267), yielding an enantiomer ratio of 1.17:1 (Scheme 96).…”

“…Chiral Cu­(I) complexes have also been used as asymmetric photocatalysts; however, tetrahedral copper complexes readily undergo fast ligand exchange, which prevents the exploitation of any innate metal-centered chirality . Thus, enantiocontrol requires the use of chiral ligands.…”

Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels–Alder and 1,3-dipolar cycloadditions

“…When enantiopure S-I was applied to the asymmetric oxa-Diels-Alder reaction of Danishefsky's diene with 1-naphthaldehyde the Diels-Alder Radduct was obtained in 98% yield with 93.5/6.5 er. 12 We have recently reported the application of achiral tripodal tetradentate ligands to the synthesis of octahedral chiral-atmetal rhodium, iridium and ruthenium complexes. 13 In particular, enantiopure rhodium complexes efficiently catalyze the enantioselectively Diels-Alder reaction between methacrolein and cyclopentadiene with enantiomeric ratios of up to >99/1 (ref.…”

“…When enantiopure S -I was applied to the asymmetric oxa-Diels–Alder reaction of Danishefsky's diene with 1-naphthaldehyde the Diels–Alder R -adduct was obtained in 98% yield with 93.5/6.5 er. 12 …”

Synthesis and a Catalytic Study of Diastereomeric Cationic Chiral-at-Cobalt Complexes Based on (R,R)-1,2-Diphenylethylenediamine

“…Generally, the chiral centers in molecules are presented by carbon, nitrogen, silicon, phosphorus, and sulfur atoms, but there is also a plethora of examples with a chirality at metal centers (metal centrochirality) in the literature. , Metal complexes may exhibit a chirality owing to the different arrangement of (a)­chiral ligands around the metal center, and these are called “ chiral-at-metal ” or “ stereogenic-at-metal ” complexes. , In case the metal complexes have an octahedral geometry, they are designated as Λ (a left-handed propeller) and Δ (a right-handed propeller) configurations. , It was demonstrated that the chiral-at-metal complexes (mostly based on cobalt, iridium, rhodium, ruthenium, iron, etc.) are an efficient and promising class of chiral catalysts for different enantioselective reactions. ,− …”