Hydrogen Bond Contribution to Preferential Solvation in SNAr Reactions

Gutmann’s Donor and Acceptor Numbers for Ionic Liquids and Deep Eutectic Solvents

An experimental and computational methodology for the analysis of the Lewis acid/base responses of ionic liquids (ILs) and deep eutectic solvents (DES) is proposed. It is based on the donor and acceptor of the electronic charge ability of Lewis acid and bases concepts (donicity and acceptor numbers, DN and AN, respectively) proposed by Viktor Gutmann. The binding enthalpy between the IL/DES with the probe antimony pentachloride (SbCl₅) in dichloroethane displays good correlations with experimental data. This approach could serve as a first approximation to predict the responses to H-bonding abilities of new IL or DES. Although useful, the problems encountered to model the electron AN of these solvents limit the usefulness of the approach to completely describe their polarity properties. The experimental data were recorded using UV–Vis spectroscopy for a wide range of ILs and a couple of DES. Two reactions were used as benchmarks to test the reliability of the DN model to discuss the reactivity of real systems in these neoteric solvents.

How the Nature of an Alpha-Nucleophile Determines a Brønsted Type-Plot and Its Reaction Pathways. An Experimental Study

The reactions between 2-chloro-5-nitro pyrimidine with a serie of α-nucleophile derivatives were kinetically evaluated. The kinetic study was carried out in aqueous media and the data shown an unusual split on the Brønsted type-plot, opening a controversial discussion based on reactivities and possible reaction pathways. These split Brønsted type-plots are discussed over the hypothetical transition state (TS) structures associated to concerted or stepwise mechanisms with emphasis on hydrogen bond interactions between electrophile/nucleophile pair able to determine the reactivities and the plausible reaction routes.

Experimental Analyses Emphasize the Stability of the Meisenheimer Complex in a SNAr Reaction Toward Trends in Reaction Pathways

The mechanism of S_NAr reactions between 2-chloro-5-nitropyrimidine with primary and secondary alicyclic amines, respectively, have been studied by kinetic measurements. The kinetic data obtained in aqueous media opens a controversial discussion based on Brönsted-type plots analysis. The first approach based on the kinetic data reveals a non-catalyzed pathway. Then, the subtlety of the mathematical treatment of the kinetic data is discussed over a concerted or stepwise mechanism, respectively.

High-Throughput Experimentation and Continuous Flow Evaluation of Nucleophilic Aromatic Substitution Reactions

Nucleophilic aromatic substitution (S_NAr) reactions were optimized using high-throughput experimentation techniques for execution under flow conditions. A total of 3072 unique reactions were evaluated with an analysis time of ∼3.5 s per reaction using a system that combines a liquid handling robot for reaction mixture preparation with desorption electrospray ionization (DESI) mass spectrometry (MS) for analysis. The reactions were performed in bulk microtiter arrays with and without incubation. In-house developed software was used to process the data and generate heat maps of the results. This information was then used to select the most promising conditions for continuous synthesis under microfluidic reactor conditions. Our results show that this HTE approach provides robust guidance for narrowing the range of conditions needed for optimization of S_NAr reactions.

Microwave-assisted simple synthesis of 2-anilinopyrimidines by the reaction of 2-chloro-4,6-dimethylpyrimidine with aniline derivatives

A series of 2-anilinopyrimidines including novel derivatives has been obtained from 2-chloro-4,6-dimethylpyrimidine by aromatic nucleophilic substitution with differently substituted anilines under microwave conditions. The substituents had a significant impact on the course and efficiency of the reaction. The results reported herein demonstrate the efficacy of microwaves in the synthesis of the title heterocyclic compounds as compared to the results obtained with conventional heating. The 2-anilinopyrimidines described are of potential bioactivity.

A rapid, one pot, green approach by microwave heating in ethanol provides a new method to obtain active and new potentially active compounds in high yields.

σ-Holes promote the concertedness in nucleophilic aromatic substitution reactions of nitroarenes

Concertedness in SNAr may be modulated by the presence of a σ-hole.

Activation of Electrophile/Nucleophile Pair by a Nucleophilic and Electrophilic Solvation in a SNAr Reaction

Nucleophilic aromatic substitution reactions of 4-chloroquinazoline toward aniline and hydrazine were used as a model system to experimentally show that a substrate bearing heteroatoms on the aromatic ring as substituent is able to establish intramolecular hydrogen bond which may be activated by the reaction media and/or the nature of the nucleophile.

A Greener and Efficient Method for Nucleophilic Aromatic Substitution of Nitrogen-Containing Fused Heterocycles

A simple and efficient methodology for the nucleophilic aromatic substitution of nitrogen-containing fused heterocycles with interesting biological activities has been developed in an environmentally sound manner using polyethylene glycol (PEG-400) as the solvent, leading to the expected compounds in excellent yields in only five minutes.

Effect of the nature of the nucleophile and solvent on an SNAr reaction

The reaction of 2,4-dinitrobenzenesulfonyl chloride toward propylamine was kinetically evaluated in 19 organic solvents and 10 ionic liquids as reaction media.

Solvent effect on a model SNAr reaction in ionic liquid/water mixtures at different compositions

A biphasic behavior observed on the reaction rate of a model S_NAr reaction in ionic liquid/water mixtures was explained by preferential solvation effects in water rich mixtures, and an anion effect in mixtures with high ionic liquid content.

Iso-solvation effects in mixtures of ionic liquids on the kinetics of a model SNAr reaction

The S_NAr reaction between 1-chloro-2,4-dinitrobenzene and morpholine was used as a model system to study solvation effects in a series of mixtures involving imidazolium based ionic liquids.

Mechanism for the SNAr reaction of atrazine with endogenous thiols: experimental and theoretical study

The mechanism for the S_NAr reaction of atrazine with endogenous thiols: a stepwise or concerted process?

Nucleophilic substitution in ionizable Fischer thiocarbene complexes: steric effect of the alkyl substituent on the heteroatom

A detailed kinetic study has been carried out for the aminolysis of ionizable Fischer thiocarbene complexes (CO)5M[double bond, length as m-dash]C(SR)CH3 (M = Cr, W; R = iPr, nBu, cHex, tBu) with five primary amines and one secondary amine in aqueous acetonitrile solutions (50% MeCN-50% water (v/v)). The observed rate constants for the reaction with primary amines showed a first-order dependence on the amine concentration, while with morpholine, the rate constant has second-order dependence. The general base catalysis process was confirmed by the variation of the rate constants with the concentration of an external catalyst and the pH. The results agree with a stepwise mechanism where the nucleophilic addition to the carbene carbon to produce a tetrahedral intermediate (T±) is the first step, followed by a rapid deprotonation of to form the anion T- which leads to the products by general-acid catalysed leaving group (-SR) expulsion. In general, it was found that the chromium complexes are less reactive than the tungsten analogues. The obtained Brønsted parameters for the nucleophilic addition (βnuc) indicate that C-N bond formation has made little progress at the transition state. By using Charton's correlation, the role that the steric factor plays throughout the mechanism has been unraveled. The nucleophilic addition to the thiocarbenes is less sensitive to steric effects than the alkoxycarbenes regardless of the nature of the metal centre. Conversely, the steric effects on the general-base catalysis can be strong depending on the volume of the catalyst and the metal centre. On the basis of the structure-reactivity coefficients β and ψ and comparison with alkoxycarbene complexes, esters and thiolesters, insights into the main factors ruling the reactivity in terms of transition state imbalances are discussed.

Changes in the SNAr reaction mechanism brought about by preferential solvation

For model S_NAr reactions in mixtures of acetonitrile and water, we found preferential solvation in favor of the aqueous phase.

Mechanistic pathways of aromatic nucleophilic substitution in conventional solvents and ionic liquids

Solvation effects on the reaction mechanism for nucleophilic substitution reactions have been kinetically evaluated in conventional solvents and ionic liquids.