Models of superbase-catalyzed acetylene transformations: A quantum-chemical insight

Nadezhda M. Vitkovskaya,* Vladimir B. Kobychev,* Vladimir B. Orel,* Alexander S. Bobkov* and Boris A. Trofimov**

* Irkutsk State University, 1 Karl Marx Str., 664003 Irkutsk, Russia
** A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, 1 Favorsky Str., 664033 Irkutsk, Russia


Progress in modern acetylene chemistry is largely due to the systematic use of supercbasic catalytic systems MOH/DMSO and t-BuOM/DMSO, M = Na, K allowing complex one-pot cascade transformations without the use of transition metals, which can be controlled by varying the properties of the medium and the nature of the catalyst.
A correct description of the superbasic medium is the key to successful quantum-chemical modeling of these reactions.
Our studies have made it possible to propose a general scheme for catalysis by the MOH/DMSO and t-BuOM/DMSO (M = Na, K) superbases. According to this scheme, all reaction stages, in particular for the vinylation reactions, can be realized in the coordination sphere of the alkali metal cation.
We have introduced models for an adequate description of the superbasic reaction centers KOH•5DMSO, tBuOK•5DMSO and NaOH•4DMSO, tBuONa•4DMSO. The structures of the MOH/DMSO superbasic environment and the effect of water on vinylation and ethynylation reactions in KOH/DMSO and NaOH/DMSO systems have been analyzed. The decrease in the activity of O-, S-, and C-nucleophiles with the introduction of water into the superbasic system has been explained. On the other hand, the mobility of water molecules in the coordination sphere of the alkali metal provides free transport of the proton to the carbanionic center in the final stages of the vinylation reactions with the formation of the product and the regeneration of the superbasic catalyst. At contrast, the molecule of tert-butanol can easily migrate to the outer coordination sphere; for the reaction of nucleophilic addition to phenylacetylene this migration allows the stabilization of the transition state by the metal cation and reduces the activation barrier.
The subsequent analysis demonstrated the feasibility of using simplified, monosolvate and even anionic, models in a number of cases. Using these models, we have studied a number of reactions of acetylenes, including cascade transformations involving ketones, which are carried out in the superbasic environment. The stereochemical aspects of the corresponding cascade assemblies have been considered, and an explanation has been offered for the high diastereo specificity of these reactions. Thus, the use of the anion model instead of the resource-intensive pentacoordination one allowed us to investigate the mechanism of the nucleophilic addition of acetophenone to methyl propargyl and methyl allenyl ethers in the superbase medium KOH/DMSO within the framework of the precision CBS-Q//B3 approach. The results obtained explain the unusual composition of the reaction products and indicate the predominance of the allene form in the reaction, which essentially supplements the experimental data.
This work was supported by Grant No. 4.1671.2017/4.6 from the Ministry of education and science of the Russian Federation and RFBR Grant No. 18-03-00573 à