Santiago F. Yunes

Reações intramoleculares como modelos não miméticos de catálise enzimática

This review gives a critical idea on the importance of intramolecular reactions as models for enzymatic catalysis. Intramolecular lactonizations, ester and amide hydrolysis studies result in theories which try to explain the difference between intermolecular, intramolecular and enzyme reactions and rationalize the enhancement promoted by these biological catalyst.

Hydrolysis of 1,8- and 2,3-naphthalic anhydrides and the mechanism of cyclization of 1,8-naphthalic acid in aqueous solutions

Naphthalene-1,8-dicarboxylic acid, 1,8-Acid, cyclizes spontaneously in acidic aqueous solution to naphthalene-1,8-dicarboxylic anhydride, 1,8-An, and here we present an ab initio study of the reaction pathway. The effect of pH on the hydrolysis of 1,8-An was analysed and compared with the hydrolysis of naphthalene-2, 3-dicarboxylic anhydride, 2,3-An, to naphthalene-2,3-dicarboxylic acid, 2,3-Acid. The values of the pKas of 1,8-Acid and 2,3-Acid were ca. 3.5 and 3.0, for monoanion formation, pKa1, and 5.5 and 5.0 for dianion formation, pKa2, respectively. Fluorimetric titration demonstrated that the diprotonated 2,3-Acid, AH2, was further protonated to yield AH3+. The pH–rate constant profile for 2,3-An hydrolysis showed a water reaction between pHs 1.0 and 6.0 and a base catalysed hydrolysis above pH 7.0. Under no condition was 2,3-An formed from 2,3-Acid. The pH dependent decomposition kinetics of 1,8-An is complex and, below pH 6.0, the pH–rate constant profile was fitted by assuming that both AH2 and AH3+ are in equilibrium with 1,8-An. The values of the equilibrium constants for 1,8-An formation from AH2 and AH3+ were ca. 4 and 100 in dilute and concentrated acid, respectively. Ab initio calculations for a possible reaction pathway connecting the undissociated 1,8-Acid to 1,8-An show a transition state where an intramolecular proton transfer is concerted with oxygen alignment towards the carbonyl centre. The planar intermediate is then dehydrated yielding a complex between water and 1,8-An.

The reaction of 1,2-Dichloro-4,5-dinitrobenzene with hydroxide ion: roles of meisenheimer complexes and radical pairs

The reaction of 1,2-dichloro-4,5-dinitrobenzene (DCDNB) with aqueous OH- produces (after acidification) 2-nitro-4,5-dichlorophenol with loss of NO2. Nevertheless, with > 2 mol L-1 OH-, only DCDNB was recovered due to the formation of the long-lived 3,6-dihydroxy Meisenheimer complex (M2-), and that in acid, reverted to the starting material. Fast formation of monohydroxy Meisenheimer complex (M1-) can be followed in DMSO:H2O 7:3 v/v and rate constants for its interconversion with DCDNB and for formation and return with M2- complex were estimated, with evidence for these reactions in DMSO:H2O 1:1 v/v and H2O. The rapid hydrogen exchange in OD- /D2O limits the use of1 H nuclear magnetic resonance (NMR) spectroscopy in identifying intermediates. 1 H and13 C NMR signals of M2- complex were observed in DMSO-H2O-KOH. There is evidence for the formation of free radicals in DMSO:H2O 4:1 v/v, and overall kinetics in more aqueous medium were treated in terms of the transient existence of anionic radical pairs.