Theoretical Examination of the Plausible Reaction Process for Stereoselective Synthesis of Hexapole Helicene via a Palladium-Catalyzed [2 + 2 + 2] Cyclotrimerization of [5]Helicenyl Aryne.

Abstract

The reaction mechanisms of the plausible reaction process for the synthesis of hexapole helicene via a palladium-catalyzed [2 + 2 + 2] cyclotrimerization of [5]helicenyl aryne were examined using a theoretical approach. In a previous experimental study, this reaction selectively produced C2-symmetrical hexapole helicene, even though the D3-symmetrical structure is thermodynamically more stable. To clarify the mechanism underlying this reaction, density functional theory (DFT) and transition-state-theory calculations were used to evaluate the reaction profile and kinetic rate constants of the primary reactions. The thus obtained results suggest that the first step of the [2 + 2 + 2] cyclotrimerization is not a Diels-Alder reaction but an insertion of the helicenyl aryne into a metallacyclopentadiene. Subsequently, we clarified that the formation of the D3-symmetrical product is precluded by the high free-energy barrier of this reaction, while the C2-symmetrical product can be obtained at 300 K. Simulations of the time evolution of the molar fractions of the isomers were carried out based on the evaluated kinetic rate constants. The experimental result that the C2-symmetrical product is formed predominantly at 300 K was successfully reproduced in the simulations, while the isomerization into the more stable D3 hexapole helicene structure is predicted to occur at 400 K.