A rapidly developing approach adding new dimensions to acetylene chemistry relying on employment of high basicity media such as alkali metal hydroxide suspensions in dimethyl sulfoxide (DMSO) has been, for the first time, investigated theoretically using ab initio models. Extending our recently introduced model of superbase catalysis with a nondissociated KOH (or NaOH) participation, we present here a model for a superbasic reaction center with the first solvation shell explicitly included. The alkali metal hydroxides in a DMSO solution were found to form KOH·5DMSO and NaOH·4DMSO complexes that are stabilized due to the interligand interaction. Our present MP2/6‐311++G**//B3LYP/6‐31+G* computations show that 1 and 2 water molecules can build themselves into the MOH close surrounding without substantially perturbing the DMSO ligands and easily travel between different insertion positions. Our results predict that the activation energies in the series of reactions of nucleophilic addition to a triple bond with water, methanol, methanethiol, sodium hydrosulfide, and acetone in the presence of dihydrated complexes should be larger than those obtained with the participation of monohydrated ones, which is in fair agreement with the experimental findings. The present model also explains an increase in the ethynylation reaction yield in the presence of water by suppression of the competitive enolization reaction.
ABSTRACT:The reaction mechanism of methanol vinylation with acetylene involving a nondissociated KOH molecule was studied using the MP2/6-311þþG**// HF/6-31þG* approach with the DMSO molecule explicitly included into reaction system. The whole conversion cycle of methanol vinylation including active nucleophile generation, bonding of the latter to the acetylene moiety yielding an intermediate carbanion, final methyl vinyl ether formation, and potassium hydroxide regeneration could be realized in the coordination shell of nondissociated KOH.
ABSTRACT:The mechanism of base-catalyzed nucleophilic addition of methanol to acetylene triple bond (vinylation) in dimethyl sulfoxide (DMSO) and methanol solution was studied using the MP2/6-311ϩϩG**//B3LYP/6-31G* calculations with solvent effects included via continuum model. The proton abstraction from methanol by nondissociated alkali in DMSO surrounding media to form alkali metal methoxides CH 3 OM (M ϭ Li, Na, K) was found to occur with a negligible activation barrier. The reasons for facilitation of base-catalyzed alcohol vinylation in the DMSO medium are discussed in the light of both poor solvation of methoxide ion and a specific coordination of reactants by nondissociated alkali in the MOH/DMSO mixtures.
Three positional isomers of hydroxybenzoic acid, as well as phenol and benzoic acid, were studied using core-level photoemission and X-ray absorption spectroscopies, supported by quantum chemical calculations. While 2-hydroxybenzoic (salicylic) acid exists as a single conformer with an internal hydrogen bond, 3and 4-hydroxybenzoic acids are mixtures of multiple conformers. The effects due to isomerism are clearly seen in the C 1s and O 1s photoelectron spectra, whereas the conformational effects on the binding energies are less pronounced. The O 1s photoelectron spectrum of salicylic acid is significantly different from that of the other two isomers, providing a signature of the hydrogen bond. In contrast, the oxygen K edge X-ray absorption spectra of the three hydroxybenzoic acids show only minor differences. The salicylic acid absorption spectrum at the carbon K edge shows a more resolved vibrational structure than the spectra of the other molecules, which can be explained in part by the existence of a single conformer. Our theoretical study of vibrational excitations in the lowest C 1s absorption bands of salicylic and 4-hydroxybenzoic acids indicates that the observed structure can be assigned to 0−0 lines of various electronic transitions since most of the totally symmetric vibrational modes with sufficiently large frequencies to be resolved are predicted to be inactive. Significant sensitivity of the C 1s excitations in 3-hydroxybenzoic acid to rotational conformerism was predicted but not observed due to spectral crowding.
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