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

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.