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.

Effect of medium on reactivity for alkaline hydrolysis of p-nitrophenyl acetate and S-p-nitrophenyl thioacetate in DMSO–H2O mixtures of varying compositions: ground state and transition state contributions

Second-order rate constants (kN) for reactions of p-nitrophenyl acetate (1) and S-p-nitrophenyl thioacetate (2) with OH– have been measured spectrophotometrically in DMSO–H2O mixtures of varying compositions at 25.0 ± 0.1 °C. The kN value increases from 11.6 to 32 800 M–1 s–1 for the reactions of 1 and from 5.90 to 190 000 M–1 s–1 for those of 2 as the reaction medium changes from H2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H2O. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state, i.e., the reactions of 1 and 2 with OH– proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step. Addition of DMSO to H2O would destabilize OH– through electronic repulsion between the anion and the negative-dipole end in DMSO. However, destabilization of OH– in the ground state is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. The effect of medium on reactivity has been dissected into the ground state and transition state contributions through combination of the kinetic data with the transfer enthalpies (ΔΔHtr) from H2O to DMSO–H2O mixtures for OH– ion.

Influence of solvent mixture on nucleophilicity parameters: the case of pyrrolidine in methanol-acetonitrile

The course of organic chemical reactions is efficiently modelled through the concepts of “electrophiles” and “nucleophiles” (meaning electron-seeking and nucleus-seeking reactive species). On the one hand, an advanced approach of the correlation of the nucleophilicity parameters N and electrophilicity E has been delivered from the linear free energy relationship log k (20 °C) = s(N + E). On the other hand, the general influence of the solvent mixtures, which are very often employed in preparative synthetic chemistry, has been poorly explored theoretically and experimentally, to date. Herein, we combined experimental and theoretical studies of the solvent influence on pyrrolidine nucleophilicity. We determined the nucleophilicity parameters N and s of pyrrolidine at 20 °C in CH₃OH/CH₃CN mixtures containing 0, 20, 40, 60, 80 and 100% CH₃CN by kinetic investigations of their nucleophilic substitution reactions to a series of 2-methoxy-3-X-5-nitrothiophenes 1a–e (X = NO₂, CN, COCH₃, CO₂CH₃, CONH₂). Depending on the resulting solvation medium, the N parameters range from 15.72 to 18.32 on the empirical nucleophilicity scale of Mayr. The nucleophilicity parameters N first evolve linearly with the content of acetonitrile up to 60% CH₃CN by volume, but is non linear for higher amounts. We designed a general computation protocol to investigate the solvent effect at the atomistic scale. The nucleophilicity in solvent mixtures was evaluated by combining classical molecular dynamic (MD) simulations of solvated pyrrolidine and a few density functional theory (DFT) calculations of Parr nucleophilicity. The pyrrolidine theoretical nucleophilicity 1/ω obtained in various CH₃OH/CH₃CN mixtures are in excellent agreement with Mayr's nucleophilicity (N) parameters measured. Analyses of the molecular dynamic trajectories reveal that the decrease of the nucleophilicity in methanol rich mixtures arises predominantly from the solvation of the pyrrolidine by methanol molecules through strong hydrogen bonds. Last, we proposed a simple model to predict and accurately reproduce the experimentally obtained nucleophilicity values.

Combined experiments and modelling rationalize the large influence of solvent composition on pyrrolidine nucleophilicity.