The direct reaction of an imidazole-2-ylidene in a predominantly aqueous environment [about 0.1 M solution in a H(2)O (>60%)/THF solvent system] was investigated for the first time. The reaction yielded a stable solution of the corresponding imidazolium-hydroxide of pH 13, which is in agreement with results from an ab initio molecular dynamics simulation. In contrast, hydrolysis of the carbene in a mainly aprotic environment (>80% THF) gives a hydrogen-bridged carbene-water complex which could be detected by NMR and IR spectroscopies for the first time. This complex converts slowly to two isomeric ring opened products and is at higher water concentration in dynamic equilibrium with the imidazolium hydroxide. A computational mechanistic study of the carbene hydrolysis with a gradually increasing number of water molecules revealed that the imidazolium-hydroxide structure can only be optimized with three or more water molecules as reactants, and with the increasing number of water molecules its stability is increasing with respect to the carbene-water complex. In agreement with the experimental results, these findings point out that solvent stabilization and basicity of the hydroxide ion plays a crucial role in the reaction. With increasing number of water molecules the barriers connecting the reaction intermediates are getting smaller, and the ring opened hydrolysis products can be derived from imidazolium-hydroxide type intermediates. Computational studies on the hydrolysis of a nonaromatic imidazolidine-2-ylidene analogue clearly indicated the analogous ring-opened product to be by 10-12 kcal/mol more stable than the appropriate ion pair and the carbene-water complex, in agreement with the known aromatic stabilization of imidazol-2-ylidenes. Accordingly, these molecules hydrolyze with exclusive formation of the ring-opened product.

The properties of the water−dichloromethane (DCM) liquid/liquid interface are investigated by molecular dynamics computer simulation. The results are analyzed in terms of the novel identification of truly interfacial molecules (ITIM) method. In this way, the molecules constituting the first molecular layer beneath the interface as well as those belonging to the consecutive molecular layers in both phases are identified, and the properties of interest are calculated separately for these separate molecular layers. The obtained results reveal that the influence of the interface on almost all properties of both phases disappears beyond the first molecular layer. Thus, the roughness of the first layer as well as the dynamics of the molecules belonging to this layer turn out to be considerably different from what is found in the consecutive layers in both phases. The orientational preferences of the water molecules also vanish beyond the first molecular layer. Further, water molecules form a strongly percolating two-dimensional, lateral hydrogen-bonding network in the first layer, but this lateral, intralayer percolation network does not exist in the subsequent molecular layers. The two surfaces covering the two liquid phases are found to behave largely independently from each other. Thus, at some parts of the interface, typically at positions where the water surface is locally convex and the DCM surface is locally concave, i.e., where the water phase forms tips penetrating somewhat into the DCM phase, the two surface layers can be in close contact with each other. On the other hand, at some other points of the interface, typically where the water surface is locally concave or the DCM phase is locally convex, relatively large voids can be located between the two phases.

The target of the present computational study was the acid catalyzed bond cleavage of the Si-O and C-O bonds in siloxane, alkoxysilane and ether in aqueous media. In the present study the effect of water as a solvent has been modeled using a full primary solvate shell built up from water molecules connected via hydrogen bonds around the reacting molecules. The interaction energy between the embedding water cluster and the "solvated" molecule gives an estimate for solution effects. The cleavage of the Si-O bonds in these molecular clusters proceeds with low barriers; furthermore the reaction energies corrected with the solvent interaction energies gives a reaction thermodynamics, which is in accordance with the experimental results. Molecules with a Si-O bond form stable pentavalent silicon with the solvent water molecules if protonated, while in the case of the neutral molecules tetracoordinate silicon is obtainable. The summary of the calculated reaction paths gives a possible route of siloxane formation from methoxysilane in aqueous media. The same computational methodology predicts that the hydrolysis of dimethyl ether is hindered by a substantial barrier.

No abstract

Aromatic or heteroaromatic ring precursors with 2-3 identical functionalities are often used in sequential derivatization depending on the reactivity difference or the selective execution of the reaction such as nucleophilic aromatic substitution. Continuous flow chemistry offers an enhanced parameter space (pressure and temperature) with rapid parameter optimization that ensures selectivity in many cases. We developed a flow chemistry procedure to carry out a stepwise aromatic nucleophilic substitution of difluoro-benzenes having an activating group in meta position to the fluorines. The mono-aminated products were obtained in high yield and selectivity in an extremely short reaction time, while applying higher temperature, longer reaction zone (or time), and employing higher excess of another amine reactant, the subsequent introduction of the second amino group was also successfully achieved leading to an unsymmetrically substituted 3,5-diamino-benzonitrile library.