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The SNAr mechanism invoked in this work concerned the substitution reaction of the methoxy group by a secondary amine on the 2-methoxy-5-nitrothiophene and 2-methoxy-3,5-dinitrothiophene molecules. We performed theoretical DFT and TD-DFT calculations. It was examined by theoretical DFT calculations. The hypothesis of a zwitterion formation during the kinetically determining step has been confirmed. However, our calculations have shown that the structure of this reaction intermediate is much more diffuse than commonly accepted. Its geometry can be described as one situation in which the nitrogen of the nucleophilic entity is not totally bound to the carbon, whereas the oxygen of the leaving group prepares to be separated from the substrate. The transition state corresponding to the formation of the zwitterion has a barrier of 17.78 kcal mol−1 in methanol. Three pathways have been studied to describe the second step, leading to the formation of methanol or methoxyammonium. The most probable pathway uses, in a first sequence, a second amine molecule to convert the zwitterion, a dipolar compound, into a hydrogen-bonding association of a carbanion located on the carbon C5 carrying the nitro group and an ammonium with a weak activation barrier of 2.25 kcal mol−1. The electron density supplement provided by the nucleophile is stored at the oxygen atoms of the 5-nitro group in position α of the sulfur atom. The presence of a second nitro group in position β modifies the structure of this carbanion by locating the negative charge on the carbon C3 carrying the second nitro. To visualize the reaction mechanism, we propose an innovative technique called reactive internal reaction coordinate that traces the reaction pathway in a 3D figure as a variation of the energy according to the most active internal coordinates in the transition-state zone of influence. [ABSTRACT FROM AUTHOR]

The binding of lanthanide(III) cations with p-isopropylcalix[n]arenes (n = 4, 6, 8) after partial deprotonation has been investigated using UV-Vis absorption spectrophotometry. The ease and degree of deprotonation of the calixarenes closely parallels the behaviour of their t-butyl analogues in the same solvents. The conditional stability constants of the praseodymium, europium, ytterbium and yttrium complexes were determined in DMF and in acetonitrile. The effect of water on the stability of lanthanide complexes in DMF was also treated. The results show that the cation binding selectivity, stoichiometry and stability of the complexes depend on various factors such as the degree of condensation (n) and the consequent number of O-donor atoms as well as the nature of the cation solvation. The stability of the Y(III), Pr(III), Eu(III) and Yb(III) complexes in DMF increased in all cases with increase of the ring size. DFT calculations were used to estimate the influence of the solvent on both the acidity and the conformations of the calixarene anions. [ABSTRACT FROM AUTHOR]

Allylation reaction of active methylene compounds with cyclic Baylis–Hillman (BH) alcohol catalyzed by 4-dimethyamino-pyridine (DMAP) has been investigated by means of density functional theory with B3LYP/6-311++G(d,p). The first step on the chemical path is considered as an acid–base reaction. It is then followed by allylation of active methylene compound with cyclic BH alcohol. Calculated gas-phase pKa values illustrate that active methylene compounds have higher acidity than the considered cyclic BH alcohol. The DMAP catalytic activity may be interpreted as a proton transfer bridge between the active methylene compounds and the cyclic BH alcohol. Two alternative competing reactivity sites are present. Regioselectivity has been carried out on the base of natural atomic charge, Fukui index. The computations help rationalizing the fact that the direct allylation is the favored reaction and leads to the end-product. [ABSTRACT FROM AUTHOR]

The formation of substituted 1,2-diamines via the regiospecific nucleophilic ring opening of 2-methylaziridine with methylamine was performed by nucleophilic attack at aziridine carbon atoms. A detailed theoretical study was investigated by density functional theory (DFT) at the B3LYP level and second order Moller Plesset perturbation theory (MP2) by using the 6-311G(d,p) basis set. The third Grimme correction term (D3) was used to take into account weak interactions. Solvent effects were computed in methanol and dimethylsulfoxide using the polarizable continuum model (PCM). Emphasis was placed on the ring opening mechanisms of neutral aziridines and aziridinium ions obtained through N-complexation with the BF3 Lewis acid. Moreover, the effect of substituent groups on the regioselectivity of the ring opening was investigated. The nucleophilic attack was carried out via two pathways (frontside attack M1 and backside attack M2) where activation barriers proved the preference for ring opening through the backside attack at the C3 aziridine carbon atom. The obtained results showed that the frontside attack with methylamine takes place along a concerted mechanism that leads to formation of products through one transition state. However, the backside attack is carried via a stepwise process in which the methylamine attack takes place in an SN2 fashion where the leaving group is the ring nitrogen. It first conduces a ring opening considered as the rate-determining step followed by formation of a zwitterionic intermediate. This latter undergoes a rotation to allow the proton transfer step and finally leads to formation of the thermodynamic products. [ABSTRACT FROM AUTHOR]