Browsing by Author "Castro, Enrique"
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Item Hydrogen bond contribution to preferential solvation in SNAr reactions(ACS Publications, 2013) Ormazábal-Toledo, Rodrigo; Santos, Jose; Ríos, Paulina; Castro, Enrique; Campodónico, Paola; Contreras, RenatoPreferential solvation in aromatic nucleophilic substitution reactions is discussed using a kinetic study complemented with quantum chemical calculations. The model system is the reaction of a series of secondary alicyclic amines toward phenyl 2,4,6-trinitrophenyl ether in aqueous ethanol mixtures of different compositions. From solvent effect studies, it is found that only piperidine is sensitive to solvation effects, a result that may be traced to the polarity of the solvent composition in the ethanol/water mixture, which points to a specific electrophilic solvation in the aqueous phase.Item Kinetic and theoretical study on nucleofugality in the phenolysis of 3-nitrophenyl and 4-nitrophenyl 4-cyanophenyl thionocarbonates(Elsevier, 2013) Castro, Enrique; Cañete, Alvaro; Campodónico, Paola; Cepeda, Marjorie; Pavez, Paulina; Contreras, Renato; Santos, JoseThe phenolysis of 3-nitrophenyl 4-cyanophenyl thionocarbonate (1) and 4-nitrophenyl 4-cyanophenyl thionocarbonate (2) are subjected to a kinetic investigation in order to evaluate the nucleofugality of the corresponding leaving groups. For the reaction of 2 only 4-nitrophenoxide is obtained as leaving group. For the reaction of 1 the nucleofugality ratio 3-nitrophenoxide/4-cyanophenoxide is 1/3 from the corresponding T− intermediate. Theoretical calculations confirm the experimental results. From these results it can be concluded that the non-leaving group affects the nucleofugality ratio.Item Predicting the reaction mechanism of nucleophilic substitutions at carbonyl and thiocarbonyl centres of esters and thioesters(John Wiley & Sons, 2012) Ormazábal-Toledo, Rodrigo; Castro, Enrique; Santos, Jose; Millan, Daniela; Cañete, Alvaro; Contreras, Renato; Campodónico, PaolaIn nucleophilic substitution reactions at carbonyl centres, there are two possible channels. The first one occurs when the attack of nucleophilic agents takes place simultaneously with the departure of the nucleofuge. This process is named as concerted. The second possibility is the formation of a reaction intermediate, typically a tetrahedral intermediate from which the nucleofuge departs after passing through a second transition state. This second mechanism is defined as stepwise. Whether a concerted or stepwise mechanism is to be expected for a given reaction depends on several factors. Among these determinants are the nucleophilicity of the attacking group, the leaving group ability of the nucleofuge, and the solvent, which affects both the stability of the intermediate or the transition states involved. The role of the electrophilic centre can however become an important factor that can determine the reaction mechanism. In this work we show that the group nucleophilic Fukui function model may be used to rationalize and to predict the reaction mechanism of the title compounds towards alicyclic amines. In general, when the electrophilic carbon centre is attached to the soft sulfur atom, the reaction mechanism is predicted to follow a stepwise route. When the electrophilic carbon atom is attached to a harder oxygen centre, the reaction mechanism is determined by chemical substitution at the nucleofuge moiety. Experimental verification for a set of four substrates is presented.