In Water or in the Presence of Water?

Triggering activity of catalytic rod-like supramolecular polymers

Supramolecular polymers based on benzene-1,3,5-tricarboxamides (BTAs) functionalized with an L- or D-proline moiety display high catalytic activity towards aldol reactions in water. High turnover frequencies (TOF) of up to 27×10(-4) s(-1) and excellent stereoselectivities (up to 96% de, up to 99% ee) were observed. In addition, the catalyst could be reused and remained active at catalyst loadings and substrate concentrations as low as 0.1 mol % and 50 mM, respectively. A temperature-induced conformational change in the supramolecular polymer triggers the high activity of the catalyst. The supramolecular polymer's helical sense in combination with the configuration of the proline (L- or D-) is responsible for the observed selectivity.

On-water, catalyst-free and room-temperature construction of 2-aryl-1,3,4-oxadiazole derivatives from 1,1-dichloro-2-nitroethene and hydrazides

The image of construction of 2-aryl-1,3,4-oxadiazole derivatives from 1,1-dichloro-2-nitroethene and hydrazides.

Highly efficient click reaction on water catalyzed by a ruthenium complex

Reactivity of ruthenium-catalyzed click reaction has been enhanced greatly by using H₂O as the solvent.

Intramolecular hydrogen bonding guides a cationic amphiphilic organocatalyst to highly stereoselective aldol reactions in water

A novel amphiphilic acylguanidine organocatalyst using intramolecular hydrogen bonding as a key structural motif is efficient for asymmetric aldol reactions of ketones in water at neutral pH.

Quantum and Molecular Mechanical (QM/MM) Monte Carlo Techniques for Modeling Condensed-Phase Reactions

A recent review (Acc. Chem. Res. 2010, 43:142-151) examined our use and development of a combined quantum and molecular mechanical (QM/MM) technique for modelling organic and enzymatic reactions. Advances included the PDDG/PM3 semiempirical QM (SQM) method, computation of multi-dimensional potentials of mean force (PMF), incorporation of on-the-fly QM in Monte Carlo simulations, and a polynomial quadrature method for rapidly treating proton-transfer reactions. The current article serves as a follow up on our progress. Highlights include new reactions, alternative SQM methods, a polarizable OPLS force field, and novel solvent environments, e.g., "on water" and room temperature ionic liquids. The methodology is strikingly accurate across a wide range of condensed-phase and antibody-catalyzed reactions including substitution, decarboxylation, elimination, isomerization, and pericyclic classes. Comparisons are made to systems treated with continuum-based solvents and ab initio or density functional theory (DFT) methods. Overall, the QM/MM methodology provides detailed characterization of reaction paths, proper configurational sampling, several advantages over implicit solvent models, and a reasonable computational cost.

The smallest organocatalyst in highly enantioselective direct aldol reaction in wet solvent-free conditions

The catalytic efficacy of the smallest organocatalyst, l-proline hydrazide, prepared from a cheaply available natural amino acid, such as l-proline, was studied for the direct asymmetric aldol reaction of various ketones with aromatic aldehydes at room temperature in the presence of several acid additives.

One-pot three-component Mannich reaction in water catalyzed by eco-friendly, hydrophobic and recyclable sulfonic acid based nanosilica (SBA-15-Ph-PrSO3H)

A mild, effective and green method for the Mannich reaction of aromatic aldehydes, aromatic amines and ketones has been accomplished in good to excellent yields using hydrophobic SBA-15-Ph-PrSO₃H in water at room temperature.

Water-promoted ortho-selective monohydroxymethylation of phenols in the NaBO2 system

Water-promoted ortho-selective monohydroxymethylation of phenols in the NaBO₂ system generates salicyl alcohols in excellent yields.

QM/MM investigation on 1,3-dipolar cycloadditions of the phthalazinium dicyanomethanide with three different dipolarophiles on water and in solution

An "on water" environment, describing the reactions with insoluble reactants in water, has been reported to give high yields of products compared to organic solvents. The 1,3-dipolar cycloadditions of phthalazinium dicyanomethanide 1 with three different dipolarophiles, methyl vinyl ketone (MVK), methyl acrylate (MAC), and styrene (STY), have been investigated using QM/MM calculations in water, acetonitrile, and acetonitrile-water solvent mixtures, as well as at the vacuum-water interface. Monte Carlo statistical mechanics simulations utilizing the free-energy perturbation theory and PDDG/PM3 for the QM method have been used. The transition structures for all three reactions do not show large variations among different solvents. However, the calculated free energies of activation at the interface are found to be higher than those calculated in bulk water. Computed energy pair distributions and radial distribution functions reveal a uniform loss of hydrogen bonds for the reactants and transitions states in progressing from bulk water to the vacuum-water interface. The hydrophobic effects in the reactions of 1 with MVK and MAC are similar for both, and weaker than the effect in the reaction with STY. According to the results in water-acetonitrile mixtures at different molar ratios, it is clear that the special hydrogen bonding effects are the main reason which leads to the rapid rate enhancement in progressing from a water-acetonitrile molar ratio of 0.9 : 0.1 to pure water. New insights into solvent effects for 1,3-dipolar cycloadditions are presented herein.

Thermoresponsive Polymer-Supported l-Proline Micelle Catalysts for the Direct Asymmetric Aldol Reaction in Water

l-Proline moieties bound to a thermoresponsive polymer nanoreactor efficiently directed the asymmetric aldol reaction in water with excellent yields and enantioselectivity (ee). The reactions were efficient at higher temperatures in direct contrast to the low yields and ee values found when the reaction was carried out in a DMF/water mixture due to the location of the l-proline moieties within the hydrophobic pocket inside the core of the nanoreactors. This ideal environment formed for catalysis allows control over the water content as well as enhancing interactions between the carboxylic acid of l-proline and the aldehyde substrate. The nanoreactors were disassembled to fully water-soluble polymers by lowering the temperature to below the lower critical solution temperature (LCST) of the polymer, resulting in precipitation of the product in near pure form. The product was isolated by centrifugation and the polymer/water solution reused in additional catalytic cycles by heating the polymer above its LCST and thus reforming the nanoreactors. Although a small decrease in yield after five cycles was observed, the selectivity (anti/syn ratio and ee) remained high.

“On Water” Sonochemical Multicomponent Synthesis of Novel Bioactive 1,1′-(Aryl)bis(2-(cyclohexylamino)-2-oxoethane-1,1-diyl) Di(alkanoates and benzoates)

An expeditious, efficient, and ecofriendly ultrasound-promoted synthesis of a series of novel 1,1′-(aryl)bis(2-(cyclohexylamino)-2-oxoethane-1,1-diyl) di(alkanoate and benzoate) derivatives (6a–k), as useful building block candidates for natural products and drugs, is reported. The target compounds were prepared by the reaction of alkyl or aryl carboxylic acid, various bisaldehydes and isocyanocyclohexane in water using one-pot isocyanide-based multicomponent approach. This efficient protocol without using any catalyst furnished the products in short reaction times (4-5 min) and excellent yields (87–96%). The antibacterial activity of the selected products was also examined. Some products showed promising activities.

A Practical Catalyst-Free Synthesis of 6-Amino-4 Alkyl/Aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-carbonitrile in Aqueous Medium

A completely green and improved method for the synthesis of 6-amino-4-aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-carbonitriles by a four-component reaction of a mixture of ethyl acetoacetate, hydrazine hydrate, aldehyde, and malononitrile in boiling water is reported. Similar synthesis starting from aliphatic aldehydes was carried out in water: ethanol (1 : 1) at reflux temperature without using any catalyst.

D-Glucosamine in a chimeric prolinamide organocatalyst for direct asymmetric aldol addition

O-TBDPS D-glucosamine coupled with l-proline is reported to act as an efficient organocatalyst in the accomplishment of direct aldol reactions. Excellent results, in terms of chemical yields, as well as diastereomeric and enantiomeric ratios, are reported for the catalyzed additions of cyclohexanone and acetone to variously substituted benzaldehydes.

Organocatalysis in water at room temperature with in-flask catalyst recycling

A new designer surfactant is described containing a covalently bound organocatalyst, proline. This species is water-soluble and, via spontaneous nanomicelle formation, catalyzes aldol reactions on water-soluble or -insoluble substrates in water as the only medium. Recycling the catalyst is trivial, as the amphiphile/catalyst remains in the aqueous phase in the flask.