Zinc Complexes of Bipyrrolidine‐Based Diamine‐Diphenolato and Diamine‐Diolato Ligands: Predetermination of Helical Chirality Around Tetrahedral Centres

Abstract

The coordination chemistry of chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families having the (R,R)-2,2Ј-bipyrrolidine core around Zn II was investigated. Reactions with diethylzinc led to complexes of the type [{ONNO}Zn] for most ligands and to bridging dinuclear complexes of the type [μ-Lig 4 (ZnEt) 2 ] for one of the diamine-diolato ligands. Reactions with bis(hexamethyldisilazide)zinc led to complete conversions. Most complexes were obtained as mononuclear complexes. The least bulky Salan ligand, Lig 3 , led to an equilibrium between mononuclear and dinuclear complexes in noncoordinating solvents. All ligands [a]

Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis‐Dioxidomolybdenum(VI) Complexes

“…We observed the same diastereoselectivity with an ( S , S )‐configured bis(pyrolidine) salan ligand, that resulted exclusively in the Λ‐configured Mo complex . However, in some cases the formation of both diastereomers of Zn and Ti complexes were observed, even if enantiopure salan ligands were used , , . Furthermore, a molybdenum(VI) complex was reported, that gave a 1:1 mixture of diastereomers when a chiral salalen ligand (only one imine moiety reduced to the corresponding amine) was used …”

“…Such high diastereoselectivities were also observed for other ( R , R )‐enantiopure salan ligands like H 4 L 2 , leading to the Δ‐configuration at the molybdenum atom . Kol and co‐workers reported similar behavior for titanium(IV) complexes, and could also show that the ( S , S )‐configured cyclohexyldiamine ligand gives the opposite Λ‐configuration at the metal atom , , …”

“…This chiral induction, which is also known as predetermination of chirality at the metal center, has been previously reported for various transition metal complexes, underlining the potential of such complexes for the application in asymmetric catalysis. Kol and co‐workers showed that predetermination of chirality around a metal center strongly depends on the design of the chiral ligand . Thus, there is a great interest in investigating new combinations of chiral ligands and metal cores in order to prepare enantiopure chiral‐at‐metal complexes.…”

The Dual‐Stereocontrol Mechanism: Heteroselective Polymerization of rac‐Lactide and Syndioselective Polymerization of meso‐Lactide by Chiral Aluminum Salan Catalysts

“…We and others have previously demonstrated that {ONNO}type tetradentate-dianionic ligands assembled around the chiral-2,2'-bipyrrolidine core tend to wrap diastereoselectively around different metals. [19][20][21][22][23] In contrast, aluminum complexes of salan ligands assembled around the chiral diaminocyclohexane core were obtained as mixtures of diastereomers which led to low levels of stereocontrol in rac-a nd meso-lactide polymerization. [24] Aiming at higher stereoselectivity while not retarding reactivity, [25,26] we targeted two new ligand precursors:t he C 2 -symmetric Lig 1 H 2 bearing ortho-phenyl substituents on the two phenolates,and the hybrid C 1 -symmetric Lig 2 H 2 bearing an ortho-phenyl phenolate and a2 ,4-dichlorophenolate (Scheme 3).…”

Mechanistic Insight into the Stereochemical Control of Lactide Polymerization by Salan–Aluminum Catalysts

“…hexane core,t hey led to single diastereomers of predetermined chirality of the Ndonors when wrapping around octahedral Group 4t ransition-metal centers as well as around tetrahedral zinc centers. [17,18] We hypothesized that such ahigh diastereoselectivity would be attained around the pentacoordinate aluminum as well.…”