Nucleophilic Attack vs. General Base Assisted Hydrolysis in the Reactions of Acetic Anhydride with Primary and Secondary Amines. pH-Yield Studies in the Recognition and Assessment of the Nucleophilic and General Base Reactions

Heterobimetallic dinuclear lanthanide alkoxide complexes as acid–base bifunctional catalysts for synthesis of carbamates under solvent-free conditions

Heterobimetallic dinuclear lanthanide alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)] were used as efficient acid–base bifunctional catalysts for the synthesis of carbamates and the N-Boc protection of amines.

Functioning of nanovalves on polymer coated mesoporous silica Nanoparticles

Nanomachines activated by a pH change can be combined with polymer coatings on mesoporous silica nanoparticles to produce a new generation of nanoparticles for drug delivery that exhibits properties of both components. The nanovalves can trap cargos inside the mesoporous silica nanoparticles without premature release and only respond to specific stimuli, resulting in a high local concentration of drugs at the site of release. The polymer surface coatings can increase the cellular uptake, avoid the reticuloendothelial uptake, provide protected space for storing siRNA, and enhance the biodistribution of nanoparticles. Two nanovalve-polymer systems are designed and their successful assembly is confirmed by solid state NMR and thermogravimetric analysis. The fluorescence spectroscopy results demonstrate that the controlled release functions of the nanomachines in both of the systems are not hindered by the polymer surface coatings. These new multifunctional nanoparticles combining stimulated molecule release together with the functionality provided by the polymers produce enhanced biological properties and multi-task drug delivery applications.

Effect of modification of the electrophilic center on the α effect

We report on a nucleophilic study of esters R-C(=X)-Y-Ar in which the electrophilic center has been modified by replacing O by S in the leaving group or carbonyl center: 4-nitrophenyl acetate (1), S-4-nitrophenyl thioacetate (2), 4-nitrophenyl benzoate (3), and O-4-nitrophenyl thionobenzoate (4). The studies include O– and S– nucleophiles as well as α nucleophiles in H2O at 25.0 ± 0.1 °C. The sulfur nucleophile (4-chlorothiophenoxide, 4-ClPhS–) exhibits significant enhanced reactivity for the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. On the contrary, the common nucleophile OH– is much less reactive towards 2 and 4 compared with 1 and 3. The effect of changing both the electrophilic center and the nucleofugic center on the reactivity of the other oxygen nucleophiles is not so significant: 4-chlorophenoxide (4-ClPhO–) is four to six times more reactive in the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. The α effects exhibited by butan-2,3-dione monoximate (Ox–) and HOO– are strongly dependent on the nature of the electrophilic center of the substrates, indicating that the difference in the ground-state solvation energy cannot be fully responsible for the α effect. Our results clearly emphasize the strong dependence of the α effect on the substrate structure, notably, the nature of the electrophilic center. The impact of change in the nucleofuge (1→2) and the electrophilic center (3→4) on reactivity indicates that α nucleophiles will need to be “purpose built” for decontamination and nucleophilic degradation of specific biocides.Key words: α effect, nucleophilicity, nucleofuge effect, electrophilicity, polarizability.

Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Chloroformates in Aqueous Solution

The reactions of secondary alicyclic amines with phenyl and 4-nitrophenyl chloroformates (PClF and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are followed spectrophotometrically at 210-270 nm (PClF) and at 310-400 nm (NPClF). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. From linear plots of k(obsd) vs free amine concentration, the second-order rate coefficients (k(N)) for aminolysis are obtained. For the aminolysis of both substrates, linear Brönsted-type plots (log k(N) vs amine pK(a)) of slopes 0.23 (PClF) and 0.26 (NPClF) are found. The values of the slopes are consistent with stepwise mechanisms where the formation of a zwitterionic tetrahedral intermediate (T(+/-)) is the rate-determining step (k(1) step). In contrast, the aminolysis (anilines) of the same substrates in acetonitrile are concerted, which is attributed to destabilization of T(+/-) in the latter solvent due to a faster expulsion of the amine from T(+/-) in acetonitrile compared to water. The values of k(1) are larger for the title reactions compared to the same aminolysis of the corresponding thionochloroformates, and this is attributed to the relatively hard character of these amines which prefer to bind to the harder carbonyl group (relative to thiocarbonyl). There is no change in mechanism by the change of S(-) by O(-) in T(+/-), which should destabilize this intermediate. By comparison with the stepwise pyridinolysis of methyl chloroformate, it is concluded that the changes of methoxy by phenoxy (or 4-nitrophenoxy) and a pyridine by an alicyclic amine in T(+/-) do not greatly affect the stability of these intermediates.