Can Aromaticity of Fused Aromatic Ring in 1,3-Pentadiene Modulate its Reactivity towards [1,5]-Halo Shift? - A DFT Study

Abstract

In (Z)-1,3-pentadienes, [1,5]-H migration is suprafacially allowed while fluorine shift in this system takes place by a Contra Hoffmann antarafacial pathway for which aromaticity is the driving force. If aromaticity of the transition structure (TS) can drive a reaction towards a disallowed pathway as found in the case of fluorine, the role of aromatic ring annealed to (Z)-1,3-pentadienes in determining the reaction pathway and barrier is worth noting. Hence, the combined role of aromaticity of transition state and the loss in aromaticity of the annealed ring has been explored during the [1,5]-X (X = H, F, Cl, Br) shifts in aromatic (benzene/naphthalene) annealed 1,3-pentadiene system. Notable correlations between various aromaticity index NICS(0,1) with activation barriers show that aromaticity of transition structure in pericyclic reaction can drive the stereochemical course of a reaction. The distinct effect of fluorine to other halogens is the antara migration while the other halogens (Cl & Br) prefer supramode.