Exploring acetylene chemistry in superbasic media: A theoretical study of the effect of water on vinylation and ethynylation reactions with acetylene in KOH/DMSO and NaOH/DMSO systems

Abstract

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.

Quantum‐chemical models of KOH(KOBut)/DMSO superbasic systems and mechanisms of base‐promoted acetylene reactions

“…Previously, we proposed a general concept of superbase (KOH/DMSO and KOBu t /DMSO) catalysis . According to this concept, the reaction center is represented by a nondissociated molecule of KOH or KOBu t , whose environment can be described by models of various levels of complexity.…”

“…According to this concept, the reaction center is represented by a nondissociated molecule of KOH or KOBu t , whose environment can be described by models of various levels of complexity. The most complete description of the reaction center includes the nearest solvation shell: the pentasolvate model ( PENTA ) for potassium complexes—KOH·5DMSO, KOBu t ·5DMSO (Figure ) …”

“…The use of the sophisticated pentasolvate model is very resource‐intensive and can be affordable only for molecular systems of moderate size. Modeling the superbase‐promoted vinylation of O ‐, S ‐, and C ‐nucleophiles and ethynylation of ketones shows that the main reaction step, which is the addition of the formed nucleophilic particle to the triple bond, occurs on the periphery of the reaction system . This fact suggests that the simple anionic model ( ANION ), which neglects the presence of the cation and considers the solvent only as a polarizable continuum, can be applicable.…”

Two classes of heterocycles—6,8‐dioxabicyclo[3.2.1]octanes and cyclopentenols from the same reagents: A quantum‐chemical comparison of mechanism

“…Previously, we have shown that in the superbase‐catalyzed reactions, the catalyst (KOH/DMSO) can be approximated by the stable complexes of 5DMSO·KOH, in which K + is located in closest pseudooctahedral surrounding formed by DMSO oxygen atoms and hydroxide ions (pentasolvate model) …”

“…According to the pentasolvate model, the activation barrier of addition to the triple bond СС of a methoxide‐ion ( O ‐vinylation) is estimated to be by 3.0 kcal/mol higher than that of acetone carboanion ( C ‐vinylation) . The anionic model shows that this difference does not almost change and this value is 2.6 kcal/mol.…”