Ans with 2-guanidinobenzimidazole in aprotic and bipolar aprotic solvents: hydrogen-bonded nucleophile effects

C.E.S. ALVARO; RAMOS DE DEBIAGGI S.; AYALA A.D.; NUDELMAN N.S.

Los Cocos, Córdoba

Congreso; 9th Latin American Conference on Physical Organic Chemistry. IUPAC Conference on Physical Organic Chemistry; 2007

IUPAC

The inter- and intramolecular hydrogen bonding are one of the principal types of weak non-covalent interactions that play a role in defining the physical properties and reactivity of a large variety of structures in chemical and biological systems. In a thorough examination of nucleophile structure effect regarding its self aggregation states, such as its capacity to form intra- or intermolecular hydrogen bond in ANS reactions, the present study describes the kinetics of the reactions between 1-halo-2,4-dinitrobenzene (X = F, Cl) with 2-guanidinobenzimidazole (2-GB). The reactions were studied at 40º ± 0.2 C under pseudo first-order conditions, in dimetylsulphoxide (DMSO), toluene, and in toluene-DMSO mixtures, using varying amounts of 2-GB. For the reactions carried out in pure DMSO, the second-order rate coefficients were independent on the amine concentration in the reactions of both substrates, 2,4-dinitrofluorobenzene (DNFB) and 2,4-dinitrochlorobenzene (DNClB). By the contrary, the reactions with DNFB in toluene, showed a kinetic behavior consistent with a base-catalyzed decomposition of the zwitterionic intermediate.   These results suggest an intramolecular hydrogen bonding of 2-GB in toluene that it is not developed in DMSO. To confirm this interpretation the reactions were then studied in binary mixtures DMSO-toluene, starting from small amounts of DMSO. The great increase in rate observed by small additions of DMSO, is surprising on the basis of the classical effect of a dipolar aprotic medium and it is consistent with a nucleophile:co-solvent mixed aggregate. To support the kinetic results, theoretical semi-empirical and ab-initio calculations were performed.