A γ-cyclodextrin thiazolium salt holoenzyme mimic for the benzoin condensation

Synthesis of benzoin under supramolecular catalysis involving cyclodextrins in water: application for the preparation of the antiepileptic drug phenytoin

Among the cyclodextrins screened for the synthesis of 2-hydroxy-1,2-diphenylethanone (benzoin) in water, 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) exhibited the highest yield in the benzoin condensation reactions, and HP-β-CD can be recycled several times with little loss of activity through the addition of fresh VB₁. As an example of supramolecular catalysis, the methodology was applied to the “green” synthesis of the antiepileptic drug phenytoin through benzoin condensation, oxidation, and cyclization reactions in the presence of HP-β-CD, without the use of any harmful organic solvent. Moreover, the complexation behaviors of HP-β-CD with benzaldehyde and intermediates were studied by UV-vis and 2D-ROESY NMR spectroscopies to reveal the plausible mechanisms of the reactions, and HP-β-CD did not act as a simple phase transfer agent.

Benzoin was synthesized using cyclodextrins as phase transfer catalysts. As an example of supramolecular catalysis, the methodology was applied to the “green” synthesis of the antiepileptic drug Phenytoin in the presence of HP-β-CD.

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

Microwave Assisted Benzoin Condensation using Thiamine as Catalyst

α-Hydroxyketones were prepared in appreciable yields at a very high speed, (by the benzoin condensation) under microwave irradiation using a catalytic amount of thiamine hydrochloride, from various aromatic as well as heteroaromatic aldehydes. They can be used as intermediates in NCE synthesis.

Supramolecular Chemistry and Self-Organization: A Veritable Playground for Catalysis

The directed assembly of molecular building blocks into discrete supermolecules or extended supramolecular networks through noncovalent intermolecular interactions is an ongoing challenge in chemistry. This challenge may be overcome by establishing a hierarchy of intermolecular interactions that, in turn, may facilitate the edification of supramolecular assemblies. As noncovalent interactions can be used to accelerate the reaction rates and/or to increase their selectivity, the development of efficient and practical catalytic systems, using supramolecular chemistry, has been achieved during the last few decades. However, between discrete and extended supramolecular assemblies, the newly developed “colloidal tectonics” concept allows us to link the molecular and macroscopic scales through the structured engineering of colloidal structures that can be applied to the design of predictable, versatile, and switchable catalytic systems. The main cutting-edge strategies involving supramolecular chemistry and self-organization in catalysis will be discussed and compared in this review.

Kinetic studies of manganese(III) and cobalt(II) Schiff base complexes as synthetic hydrolases for p-nitrophenyl picolinate in CTAB surfactant micellar solution

Four symmetrical bis-Schiff bases with either benzo-10-aza-crown ether or morpholino pendants and their Mn(III) and Co(II) complexes have been employed as models for hydrolase enzymes by studying the kinetics of their hydrolysis reactions with p-nitrophenyl picolinate (PNPP) in buffered CTAB micellar solution. A kinetic model of PNPP cleavage catalysed by these complexes is proposed. The effects of complex structures and reaction temperature on the rate of PNPP hydrolysis have been examined. All four complexes exhibit higher catalytic activity in the buffered CTAB micellar solution and the rate increases with pH of the buffer at 25 °C. The complexes containing two crown ether group exhibit higher catalytic activities than their crown-free analogues, and the catalytic activity of the phenyl-bridged Schiff base complex is larger than that of the ethyl-bridged Schiff base complex for the same substituents and metal. The catalytic activity of the manganese(III) complex exceeds that of the cobalt(II) complex in catalysing hydrolysis of PNPP under the same ligand.

Theoretical investigations on the mechanism of benzoin condensation catalyzed by pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene

A new annulated N-heterocyclic carbene (NHC), pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene, has been synthesized and its good catalytic activity for benzoin condensation has been experimentally determined by You and co-workers recently [ Ma , Y. J. , Wei , S. P. , Lan , J. B. , Wang , J. Z. , Xie , R. G. , and You , J. S. J. Org. Chem. 2008 , 73 , 8256 ]. In this work, the mechanism of the title reaction has been intensively studied computationally by employing the density functional theory (B3LYP) method in conjunction with 6-31+G(d) and 6-311+G(2d,p) basis sets. Our results indicate that path A (in which a sequence of intermolecular proton transfers between two carbene/benzaldehyde coupling intermediates affords enamine) and path B (in which a t-BuOH assisted hydrogen transfer generates enamine) proposed on the basis of the Breslow mechanism are competitive for their similar barriers. In path A, the first intermolecular proton transfer between two N-heterocyclic carbene/benzaldehyde coupled intermediates to form tertiary alcohol and enolate anion is theoretically the rate-determining step with corresponding barrier (30.93 kcal/mol), while the t-BuOH assisted hydrogen transfer generating Breslow enamine is the rate-determining step with corresponding barrier (28.84 kcal/mol) in path B. The coupling of carbene and benzaldehyde, and the coupling of enamine and another benzaldehyde to form a C-C bond are partially rate-determining for their relatively significant barriers (24.06 and 26.95 kcal/mol, respectively), being the same in both paths A and B. Our results are in nice agreement with the experimental result in a kinetic investigation of thiazolium ion-catalyzed benzoin condensation performed by White and Leeper in 2001.

Artificial enzymes with thiazolium and imidazolium coenzyme mimics

Hydrophobic thiazolium and imidazolium coenzyme mimics in the presence of modified-polyethylenimine enzyme mimics catalyze the benzoin condensation 2300-3300 times faster than the coenzyme mimics alone. Polycationic enzyme mimics provide not only a hydrophobic binding domain for coenzyme and substrate, but also electrostatic stabilization of anionic species that arise along the reaction pathway of the benzoin condensation.

N-Heterocyclic Carbenes as Organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

Kinetics and Mechanism of Hydrolysis of P-Nitrophenyl Picolinate by Transition-Metal Hydroxamate Containing Benzo-15-Crown-5

Two hydroxamic acids HL1 and HL2 containing benzo-15-crown-5 have been synthesised and their transition metal (Cu2+, Co2+, Zn2+, Mn2+) complexes have been employed as models to mimic hydrolase in catalytic hydrolysis of p-nitrophenyl picolinate(PNPP). The kinetics and mechanism of PNPP hydrolysis have been investigated. A kinetic mathematical model of PNPP cleavage catalysed by these complexes is proposed. The effects of reactive temperature and metal ion in complexes on the rate of catalytic PNPP hydrolysis are discussed. The results show that the transition-metal hydroxamates (ML12, ML22, M= Cu2+, Co2+, Zn2+, Mn2+) containing benzo-15-crown-5 exhibit high activity in catalytic PNPP hydrolysis. The rate of PNPP hydrolysis catalysed by these complexes increased along with the increase of pH values of the buffer solution; the catalytic activity of different metal ions in complexes of the same ligand deceased in the order Cu2+> Co2+ >Zn2+ > Mn2+; the pseudo-first-order-rate constants of PNPP hydrolysis catalysed by the complexes are over 1000 times more than that of spontaneous hydrolysis of PNPP.

Potential of enzyme mimics in biomimetic sensors: a modified-cyclodextrin as a dehydrogenase enzyme mimic

This paper reports the application of a dehydrogenase enzyme mimic as a biomimetic sensor. The model compound investigated was a beta-cyclodextrin (beta-CD) derivative with a nicotinamide group attached to the secondary face of a beta-CD (Fig. 1g). It was envisaged that the nicotinamide group would act as the electron transfer agent and that the cyclodextrin would provide a suitable hydrophobic cavity for the reaction to take place in. Ethanol, propranalol, dopamine and acetone were used as substrates in backgrounds of hydrophilic and hydrophobic anions. Electrochemical and fluorescence techniques were used to study the catalytic effects in solution. It was found that the size of the analyte and the hydrophobicity of the anion affected the catalytic activity of the dehydrogenase mimic. Catalytic effects were most enhanced with ethanol and dopamine in presence of larger and more strongly solvated anions, SO4(2-) and H2PO4- which are excluded from the cavity. The molecule was also immobilised in a sol-gel matrix and investigated as a sol-gel electrochemical biomimetic sensor. Concentration dependence with increasing aliquots of ethanol was observed. These results indicated that a re-usable biomimetic sensor is indeed feasible.

Kinetics of the thiazolium ion-catalyzed benzoin condensation

The formation of benzoin (Ph-CHOH-CO-Ph) from two molecules of benzaldehyde, catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide in methanol buffered with Et(3)N/Et(3)NH(+)Cl(-) has been studied. Initial-rate studies at various concentrations of PhCHO (0.1-1.7 M) showed that the reaction is close to being first order in PhCHO. Following the reaction in deuteriomethanol, (1)H NMR spectroscopy allowed rate constants for all three kinetically significant steps to be determined. These show that all three steps are partially rate-determining. A normal deuterium kinetic isotope effect for the overall reaction (k(H)/k(D) approximately 3.4) is observed using PhCDO, and a large inverse solvent isotope effect (k(D)/k(H) approximately 5.9) is observed using deuteriomethanol, consistent with the kinetic scheme presented here.