Enantioselective Hydrolysis of Amino Acid Esters Promoted by Bis(β-cyclodextrin) Copper Complexes

Monomeric, Oligomeric, Polymeric, and Supramolecular Cyclodextrins as Catalysts for Green Chemistry

This review comprehensively covers recent developments of cyclodextrin-mediated chemical transformations for green chemistry. These cyclic oligomers of glucose are nontoxic, eco-friendly, and recyclable to accomplish eminent functions in water. Their most important feature is to form inclusion complexes with reactants, intermediates, and/or catalysts. As a result, their cavities serve as sterically restricted and apolar reaction fields to promote the efficiency and selectivity of reactions. Furthermore, unstable reagents and intermediates are protected from undesired side reactions. The scope of their applications has been further widened through covalent or noncovalent modifications. Combinations of them with metal catalysis are especially successful. In terms of these effects, various chemical reactions are achieved with high selectivity and yield so that valuable chemicals are synthesized from multiple components in one-pot reactions. Furthermore, cyclodextrin units are orderly assembled in oligomers and polymers to show their cooperation for advanced properties. Recently, cyclodextrin-based metal-organic frameworks and polyoxometalate-cyclodextrin frameworks have been fabricated and employed for unique applications. Cyclodextrins fulfill many requirements for green chemistry and should make enormous contributions to this growing field.

Artificial Metalloenzymes: Recent Developments and Innovations in Bioinorganic Catalysis

Cellular life is orchestrated by the biochemical components of cells that include nucleic acids, lipids, carbohydrates, proteins, and cofactors such as metabolites and metals, all of which coalesce and function synchronously within the cell. Metalloenzymes allow for such complex chemical processes, as they catalyze a myriad of biochemical reactions both efficiently and selectively, where the metal cofactor provides additional functionality to promote reactivity not readily achieved in their absence. While the past 60 years have yielded considerable insight on how enzymes catalyze these reactions, a need to engineer and develop artificial metalloenzymes has been driven not only by industrial and therapeutic needs, but also by innate human curiosity. The design of miniature enzymes, both rationally and through serendipity, using both organic and inorganic building blocks has been explored by many scientists over the years and significant progress has been made. Herein, recent developments over the past 5 years in areas that have not been recently reviewed are summarized, and prospects for future research in these areas are addressed.

pH-Controlled Chirality Inversion in Enantiodifferentiating Photocyclodimerization of 2-Antharacenecarboxylic Acid Mediated by γ-Cyclodextrin Derivatives

Several γ-cyclodextrin (γ-CDx) derivatives were used as chiral hosts for the photocyclodimerization of 2-anthracenecarboxylic acid (AC). The effect of pH on photoreactivity and stereochemical outcome of photoproducts was investigated. Upon changing the solution pH, the stereochemical outcome of HH cyclodimer 3 was inverted from 25.2% to -64.4% and 41.2% to -76.2%, respectively, in the photocyclodimerization of AC mediated by bis-quinoline-modified γ-CDx 7 and its N-methylated derivative 8.

Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Complexes Formed by the Self-Assembly of a Hydrophobic Bis(Zn2+-cyclen) Complex, Copper, and Barbital Units That Are Functionalized with Amino Acids in a Two-Phase Solvent System

We previously reported on the preparation of supramolecular complexes by the 2:2:2 assembly of a dinuclear Zn²⁺-cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) complex having a 2,2′-bipyridyl linker equipped with 0~2 long alkyl chains (Zn₂L¹~Zn₂L³), 5,5-diethylbarbituric acid (Bar) derivatives, and a copper(II) ion (Cu²⁺) in aqueous solution and two-phase solvent systems and their phosphatase activities for the hydrolysis of mono(4-nitrophenyl) phosphate (MNP). These supermolecules contain Cu₂(μ-OH)₂ core that mimics the active site of alkaline phosphatase (AP), and one of the ethyl groups of the barbital moiety is located in close proximity to the Cu₂(μ-OH)₂ core. The generally accepted knowledge that the amino acids around the metal center in the active site of AP play important roles in its hydrolytic activity inspired us to modify the side chain of Bar with various functional groups in an attempt to mimic the active site of AP in the artificial system, especially in two-phase solvent system. In this paper, we report on the design and synthesis of new supramolecular complexes that are prepared by the combined use of bis(Zn²⁺-cyclen) complexes (Zn₂L¹, Zn₂L², and Zn₂L³), Cu²⁺, and Bar derivatives containing amino acid residues. We present successful formation of these artificial AP mimics with respect to the kinetics of the MNP hydrolysis obeying Michaelis–Menten scheme in aqueous solution and a two-phase solvent system and to the mode of the product inhibition by inorganic phosphate.

Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Phosphatases Formed from a Monoalkylated Dizinc(II) Complex, Cyclic Diimide Units, and Copper(II) in Two-Phase Solvent System

Design and synthesis of enzyme mimic with programmed molecular interaction among several building blocks including metal complexes and metal chelators is of intellectual and practical significance. The preparation of artificial enzymes that mimic the natural enzymes such as hydrolases, phosphatases, etc. remains a great challenge in the field of supramolecular chemistry. Herein we report on the design and synthesis of asymmetric (nonsymmetric) supermolecules by the 2:2:2 self-assembly of an amphiphilic zinc(II)-cyclen complex containing a 2,2'-bipyridyl linker and one long alkyl chain (Zn₂L³), barbital analogues, and Cu²⁺ as model compounds of an enzyme alkaline phosphatase that catalyzes the hydrolysis of phosphate monoesters such as mono(4-nitrophenyl)phosphate at neutral pH in two-phase solvent system (H₂O/CHCl₃) in pH 7.4 and 37 °C. Hydrolytic activity of these complexes was found to be catalytic, and their catalytic turnover numbers are 3-4. The mechanistic studies based on the UV/vis and emission spectra of the H₂O and CHCl₃ phases of the reaction mixtures suggest that the hydrophilicity/hydrophobicity balance of the supramolecular catalysts is an important factor for catalytic activity.

A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions

From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [Ni^II₂(L)(H₂O)₂(CH₃OH)][ClO₄]·CH₃OH (1) (H₃L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N',N″-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH₃CN/CH₃OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [Ni^II₅(H₂L)₂(μ₃-OH)₂(μ-O₂CCH₃)₄][ClO₄]₂·CH₃CO₂C₂H₅ (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₃)(H₂O)₂][ClO₄]·4H₂O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H₃L with 2 equiv of Ni^II(O₂CCH₃)₂·4H₂O, followed by the usual workup, and recrystallization from CH₂Cl₂ led to the isolation of [Ni^II₂(H₂L)(μ-O₂CCH₃)₂][ClO₄]·CH₂Cl₂·2H₂O (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni^II(O₂CCH₃)₂·4H₂O with H₃L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni^II-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₂CH₃)(H₂O)₂][ClO₄] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = -25.0(1) cm^-1 for 1, J = -20.0(1) cm^-1 for 4, and J = -18.80(8) cm^-1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (J_a = +16.02 cm^-1, and J_b = +9.02 cm^-1) and an antiferromagnetic (J_c = -49.7 cm^-1), have been identified.