| In 2005,Roelfes first proposed the concept of DNA-based hybrid catalyst.This novel catalytic system combines ligand-metal complexes with DNA by supramolecular self-assembly or covalently anchoring.Subsequently,DNA-based hybrid catalyst are widely used in asymmetric synthesis reactions of carbon-carbon bonds and carbon-heteroatom bonds.DNA-based hybrid catalysts not only have high catalytic ability,but also have some enantioselectivity to the reactions.In this paper,a new type of DNA-based hybrid catalyst achiral ligand was designed and synthesized.This organic small molecule binds pyrrole tetraamide(Py4)with 4,4'-dimethyl-2,2'-bipyridine(dmbpy)by covalent bonding to form a polyamide-bipyridine compound.Subsequently,a new DNA-based hybrid catalytic system was constructed by using this ligand and st-DNA.The hybrid catalyst successfully catalyzes the Friedel-Crafts alkylation reactions between a series of ?,/?-unsaturated-2-acetylimidazoles containing different substituents and indole derivatives.Although this ligand is bound to DNA by supramolecular self-assembly mode,the specific recognition ability of Py4 to the AT-rich region of DNA minor groove allows the binding sites of the catalytic center to DNA to be precisely controlled.So that the direction of the ee value of the reactions is kept consistent.In order to further study the factors influencing the ee value,a series of oligonucleotides were designed and synthesized.The effect of DNA sequence on the catalytic performance was investigated by using the oligonucleotides-based hybrid catalytic system.Py4-dmbpy and DNA binding patterns were also discussed by circular dichroism and UV absorption spectroscopy.The use of supramolecular self-assembly strategy to achieve accurate positioning of the catalytic center on the DNA backbone not only avoids the operational inconvenience of covalent anchoring strategy,and can keep the chiral microenvironment in which the reaction occurs consistent.This is of great significance for the further study of the chiral introduction mechanism of DNA-based hybrid catalyst,and provides a new idea for the design of new achiral ligands based on supramolecular self-assembly strategy.In view of the high catalytic ability and enantioselectivity of DNA-based hybrid catalyst with bipyridine as ligand,the bi-planar compounds of N,N-(benzimidazolylidene)-amine(IDB)and N,N-(benzimidazolylidene)-acetamide(IDB(O-acetyl))were synthesized.The corresponding Diels-Alder reactions and Friedel-Crafts alkylation reactions were catalyzed using the DNA-based hybrid catalysts constructed by this two componds,and the results were found to be very different.In order to understand the causes of the results,the binding mode between IDB and IDB(O-acetyl)with DNA were investigated by circular dichroism and UV absorption spectroscopy respectively.The experimental results show that the little change of ligand structure can change its binding mode with DNA,which makes the constructed DNA-based hybrid catalyst have completely different catalytic effect.This provides a further help for us to study the relationship between the structure of the ligand and the enantioselectivity,and it is of significance to the development of new ligands based on bisbenzimidazole and the optimization of ligands.Three novel achiral ligands of DNA-based hybrid catalyst,Py4-dmbpy,IDB and IDB(O-acetyl),were designed and synthesized.On the basis of the successful application of Py4-dmbpy and IDB(O-acetyl)to the asymmetric catalytic reactions,the precise location of the catalytic center on the DNA was realized by using the former and the effects of ligand structure on the catalytic performance of DNA-based hybrid catalysts were explored by using the latter.The experimental results are of great significance for studying the chiral introduction mechanism of DNA-based hybrid catalyst in asymmetric reactions,and also provide a theoretical basis for the design and synthesis of efficient and controlled DNA-based hybrid catalyst. |
PDF Full Text Request