Binuclear Double-Stranded Helicates and Their Catalytic Applications in Desymmetrization of Mesodiols

Abstract

The ligand L1 of 4-methyl-2,6-diformylphenol and L2 of 4-tert-butyl-2,6-diformylphenol are synthesized through Schiff base condensation with rac-, (R)-(+), or (S)-(−)-1,1′-binaphthyl-2,2′-diamine (BNDA). As a result, the racemic L1 rac, L2 rac, and enantiopure L1 RR, L1 SS, L2 RR, and L2 SS ligands are obtained incorporating Cu­(II) and Zn­(II) salts by a simple one-pot metal template method. The series of dinuclear complexes of [M2LX2] (here, M = Cu2+, Zn2+; X = acetate ion, chloride ion; L = L1 RR, L1 SS, L1 rac, L2 RR, L2 SS, L2 rac) formulas are obtained in common. Among them, the single crystal X-ray structures for [Zn2L1 rac(OAc)2] and [Zn2L1 SSCl2] complexes are obtained. The detailed crystal structure and the chiroptical studies performed on these complexes dictates a self-sorting behavior in their self-assembly process and illustrate a chirality transfer from the ligand to the metal center on the complexes. The enantiopure dinuclear complexes [M2LRRX2] and [M2LSSX2] generate enantiopure ΛΛ and ΔΔ isomers, respectively, but the racemic complexes produce only homochiral ΛΛ and ΔΔ assemblies. The detailed studies based on UFLC (Ultra Fast Liquid Chromatography), CD, and single crystal X-ray structure together show the absence of heterochiral ΛΔ mesocate. All these complexes are adapted as catalysts for desymmetrization of various mesodiols, and the enantiopure complexes are found to give efficient enantioselectivity in desymmetrization of mesodiols with benzoyl chloride to monobenzoylated ester providing 98% yield and 92% ee.

Enantioselective Michael Addition Reaction Catalysed by Enantiopure Binuclear Nickel(II) Close‐Ended Helicates

“…As a result of screening different Michael acceptors, the 4‐flouro substitued β‐nitrostyrene ( 11 a ) produced 11 b in good yield (98%) and ee (94%) suggesting that electron withdrawing group in the phenyl ring of niitrostyrene supporting the formation of carbocation which further facilitates the interaction of carbanion of diethylamalonate. However, the aromatic Michael acceptors are found to give better results than the aliphatics suggesting the possible strong π‐π interaction between the aromatic nitrostyrenes and catalyst matching to our earlier observation derived in our earlier work …”

“…We report here the synthesis of series of binuclear double‐stranded helicates 1 , 1′ , 2 , and 2′ using nickel(II) chloride and enantiopure ligands L 1 and L 2 . These ligands were obtained using ( R )‐(+)‐1,1′‐BNDA or ( S )‐(−)‐1,1′‐BNDA with 4‐methyl‐2,6‐diformylphenol or 4‐ tert ‐butyl‐2,6‐diformylphenol following our recent report . As a result we produced four different hexadentate helical ligands L 1 RR , L 1 SS , L 2 RR & L 2 SS as shown in scheme .…”

“…The combined effect of helical chirality and metal centred chirality on enantioselective catalysis has been of keen interest . In this series recently, we have reported a series of binuclear double‐stranded Cu 2 and Zn 2 helicates and their catalysis in asymmetric reactions . With that encouragements, we here report the synthesis of few important close‐ended nickel(II) helicates and their catalytic application in Michael addition reaction.…”

One‐pot synthesis of [2+2]‐helicate‐like macrocycle and 2+4‐μ4‐oxo tetranuclear open frame complexes: Chiroptical properties and asymmetric oxidative coupling of 2‐naphthols

“…The crystal structures of 1 0 , 3 and 4, suggests that the BNDA in them exhibit a self-sorting [18] behaviour; A similar complex reported [11] by us recently have been illustrated to follow a similar self-sorting behaviour based on their chirality. Possessing BNDA a similar situation in the present complex [Cu 2 L a RR .Cl 2 ], 1 0 inspired us to investigate its role of chirality followed in the selfassembly process.…”

“…In our recent report [ 11 ] described the successful synthesis of such BNDA based [ 2+2 ]‐helicates by one‐pot metal (copper(II), zinc(II)) template techniques and the failure of Gao et al in such attempts we inspired to investigate the mechanism behind the synthetic strategy by playing with the subcomponents. Hence, in the present study demonstrate (Scheme 1) the synthetic viability for the formation of [ 2+2 ]‐terminally closed double‐stranded helicate‐like macrocycle using subcomponents self‐assembly techniques.…”