Highly Efficient Cyclic Dinucleotide Based Artificial Metalloribozymes for Enantioselective Friedel-Crafts Reactions in Water

Au(III)/TPPMS-catalyzed Friedel-Crafts benzylation of deactivated N-alkylanilines in water

The Friedel-Crafts reaction of electronically deactivated anilines including those with strong electron-withdrawing NO2, CN or CO2H groups is challenging due to the reduced electron density of the aromatic ring. Here, we demonstrate the Au(III)/TPPMS-catalyzed Friedel-Crafts benzylation of deactivated anilines in water. This reaction exhibits operational simplicity and a broad substrate scope with high regioselectivity, enabling rapid access to 2-benzylanilines.

Uridine triphosphate hybrid catalyst for carbon‑carbon bond formation reactions with enhanced enantioselectivity by mercury(II) ions

DNA hybrid catalysts are constructed by embedding active metal species into the chiral scaffolds of DNA, which have been successfully applied to some important aqueous-phase enantioselective transformations. Owing to simple components and inherent chirality, nucleotide hybrid catalysts are emerging in response to soving the unclear locations of catalytic centers and the plausible catalytic mechanisms in DNA-based asymmetric catalysis. However, the tertiary structure of nucleotides lacks tunability, severely impeding further design of nucleotide hybrid catalysts for potential applications. To this end, a design strategy for tunable nucleotide hybrid catalysts is put forward by introducing metal-mediated base pairs. Herein, we found that the formation of uracil‑mercury(II)-uracil (U-Hg2+-U) base pairs could enhance the enantioselectivity in uracil-containing nucleotide-based asymmetric reactions. Compared with uracil triphosphate (UTP) complexing with Cu2+ ions (UTP∙Cu2+), the presence of Hg2+ ions gave rise to an increased enantiomeric excess (ee) of 38 % in Diels-Alder reactions and 22 % ee in Michael reactions. The Hg2+-tuning behaviors of UTP hybrid catalyst have been demonstrated to largely depend on nucleotides, Hg2+ concentrations, metal cofactors, additives and reaction types. Based on ultraviolet-visible, circular dichroism and nuclear magnetic resonance spectroscopic techniques, the chiral enhancement of Hg2+-containing UTP hybrid catalyst is proved to largely depend on the formation of U-Hg2+-U base pairs and the plausible cross-linked structure of UTP-Hg2+-UTP/Cu2+ assembly. This work provides a tunable strategy based on the concept of metal-mediated base pairs, allowing further design of potent oligonucleotide-based catalysts for other enantioselective reactions.

Efficient Silver(I)-Containing I-Motif DNA Hybrid Catalyst for Enantioselective Diels-Alder Reactions

The inherent chiral structures of DNA serve as attractive scaffolds to construct DNA hybrid catalysts for valuable enantioselective transformations. Duplex and G-quadruplex DNA-based enantioselective catalysis has made great progress, yet novel design strategies of DNA hybrid catalysts are highly demanding and atomistic analysis of active centers is still challenging. DNA i-motif structures could be finely tuned by different cytosine-cytosine base pairs, providing a new platform to design DNA catalysts. Herein, we found that a human telomeric i-motif DNA containing cytosine-silver(I)-cytosine (C-Ag+-C) base pairs interacting with Cu(II) ions (i-motif DNA(Ag+)/Cu2+) could catalyze Diels-Alder reactions with full conversions and up to 95 % enantiomeric excess. As characterized by various physicochemical techniques, the presence of Ag+ is proved to replace the protons in hemiprotonated cytosine-cytosine (C : C+) base pairs and stabilize the DNA i-motif to allow the acceptance of Cu(II) ions. The i-motif DNA(Ag+)/Cu2+ catalyst shows about 8-fold rate acceleration compared with DNA and Cu2+. Based on DNA mutation experiments, thermodynamic studies and density function theory calculations, the catalytic center of Cu(II) ion is proposed to be located in a specific loop region via binding to one nitrogen-7 atom of an unpaired adenine and two phosphate-oxygen atoms from nearby deoxythymidine monophosphate and deoxyadenosine monophosphate, respectively.

Cyclic Dinucleotide-Based Enantioselective Fluorination in Water

The diverse structures of DNA serve as potent chiral scaffolds for DNA-based asymmetric catalysis, yet in most cases tens to hundreds of nucleotides in DNA hybrid catalysts hinder the deep insight into their structure-activity relationship. Owing to the structural simplicity and design flexibility of nucleotides, nucleotide-based catalysts have been emerging as a promising way to obtain fine structural information and understand the catalytic mechanisms. Herein, we found that a cyclic dinucleotide of cyclic di-AMP (c-di-AMP) and 1,10-phenanthroline copper(II) nitrate (Cu(phen)(NO₃)₂) are assembled to a c-di-AMP-based catalyst (c-di-AMP/Cu(phen)(NO₃)₂), which could fast achieve enantioselective fluorination in water with 90-99% yields and up to 90% enantiomeric excess (ee). The host-guest interaction between c-di-AMP and Cu(phen)(NO₃)₂ has been proposed mainly in a supramolecular interaction mode as evidenced by spectroscopic techniques of ultraviolet-visible, fluorescence, circular dichroism, and nuclear magnetic resonance. Cu(phen)(NO₃)₂ tightly binds to c-di-AMP with a binding constant of 1.7 ± 0.3 × 10⁵ M^-1, and the assembly of c-di-AMP/Cu(phen)(NO₃)₂ shows a modest rate enhancement to carbon-fluorine bond formations as supported by kinetic studies.

A Localized Enantioselective Catalytic Site on Short DNA Sequences and Their Amphiphiles

A DNA-based artificial metalloenzyme (ArM) consisting of a copper(II) complex of 4,4'-dimethyl-2,2'-bipyridine (dmbipy-Cu) bound to double-stranded DNA (dsDNA) as short as 8 base pairs with only 2 contiguous central pairs (G for guanine and C for cytosine) catalyzes the highly enantioselective Diels-Alder reaction, Michael addition, and Friedel-Crafts alkylation in water. Molecular simulations indicate that these minimal sequences provide a single site where dmbipy-Cu is groove-bound and able to function as an enantioselective catalyst. Enantioselective preference inverts when d-DNA is replaced with l-DNA. When the DNA is conjugated to a hydrophobic tail, the obtained ArMs exhibit enantioselective performance in a methanol-water mixture superior to that of non-amphiphilic dsDNA, and dsDNA-amphiphiles with more complex G•C-rich sequences.

Guanosine Borate Hydrogel and Self-Assembled Nanostructures Capable of Enantioselective Aldol Reaction in Water

A guanosine-based hydrogel formed by the self-assembly of guanosine and 4-((l-prolinamide)methyl)phenylboronic acid was constructed. The G quartets were selectively stabilized by K⁺ ions to form a self-supporting transparent hydrogel. These guanosine-derived assemblies were used to catalyze the aldol reaction in water without any additives, affording desirable conversion and enantioselectivity of the product. The controlled assays of small-molecule components indicated that the stable assemblies were the definite species that achieved high enantioselective catalysis. The current catalytic system can be readily recovered by simple extraction and still acquired good performance of the reaction after four cycles.

A Cu(II)-ATP complex efficiently catalyses enantioselective Diels-Alder reactions

Natural biomolecules have been used extensively as chiral scaffolds that bind/surround metal complexes to achieve stereoselectivity in catalytic reactions. ATP is ubiquitously found in nature as an energy-storing molecule and can complex diverse metal cations. However, in biotic reactions ATP-metal complexes are thought to function mostly as co-substrates undergoing phosphoanhydride bond cleavage reactions rather than participating in catalytic mechanisms. Here, we report that a specific Cu(II)-ATP complex (Cu²⁺·ATP) efficiently catalyses Diels-Alder reactions with high reactivity and enantioselectivity. We investigate the substrates and stereoselectivity of the reaction, characterise the catalyst by a range of physicochemical experiments and propose the reaction mechanism based on density functional theory (DFT) calculations. It is found that three key residues (N7, β-phosphate and γ-phosphate) in ATP are important for the efficient catalytic activity and stereocontrol via complexation of the Cu(II) ion. In addition to the potential technological uses, these findings could have general implications for the chemical selection of complex mixtures in prebiotic scenarios.

Covalently Functionalized DNA Duplexes and Quadruplexes as Hybrid Catalysts in an Enantioselective Friedel-Crafts Reaction

The precise site-specific positioning of metal-ligand complexes on various DNA structures through covalent linkages has gained importance in the development of hybrid catalysts for aqueous-phase homogeneous catalysis. Covalently modified double-stranded and G-quadruplex DNA-based hybrid catalysts have been investigated separately. To understand the role of different DNA secondary structures in enantioselective Friedel-Crafts alkylation, a well-known G-quadruplex-forming sequence was covalently modified at different positions. The catalytic performance of this modified DNA strand was studied in the presence and absence of a complementary DNA sequence, resulting in the formation of two different secondary structures, namely duplex and G-quadruplex. Indeed, the secondary structures had a tremendous effect on both the yield and stereoselectivity of the catalyzed reaction. In addition, the position of the modification, the topology of the DNA, the nature of the ligand, and the length of the linker between ligand and DNA were found to modulate the catalytic performance of the hybrid catalysts. Using the optimal linker length, the quadruplexes formed the (-)-enantiomer with up to 65% ee, while the duplex yielded the (+)-enantiomer with up to 62% ee. This study unveils a new and simple way to control the stereochemical outcome of a Friedel-Crafts reaction.