Can aromaticity trigger thermal [1,5]‐halogen shift towards the forbidden antarafacial mode?: A density functional case study

Abstract

[1,5]‐H shift of (Z,Z) 1,3‐pentadienes are known to take suprafacial mode. The stereochemical fate of the reaction when halogens are involved in the [1,5]‐shift in similar (Z,Z) 1,3‐pentadienes is presented in this work. Interesting tendency of fluorine to take up the forbidden and relatively expensive antarafacial path as an economical route for [1,5]‐fluorine shift is owed to the aromaticity of the transition state. Remarkable correlations between various aromaticity indices (bond alternation coefficient [BAC], harmonic oscillator model of aromaticity [HOMA], nucleus independent chemical shift (NICS), and ellipticity [ξ]) with activation barriers pave solid foundation to the fact that aromaticity of transition states (TS) in pericyclic reaction can manoeuvre the stereochemical course of a reaction. The differential effect of fluorine to other halogens is exemplified by the antara migration while the other halogens (Cl and Br) prefer supra mode. NBO studies and the topological parameter, ellipticity used as a measure of aromatic criterion support the above fact.

[1,5]-Sigmatropic Shifts Regulated by Built-in Frustration

“…A similar joint effect of acceptor (captor) and donor, also known as captodative effect, was shown to influence radical stabilities . Thus, like our previously reported superactivated Cope substrate, this polarizing substitution generates a highly activated 1,3-pentadiene that was predisposed to undergo [1,5]-hydrogen shift.…”

“…F, Cl, and Br. Kalpana and Akilandeswari 54 showed that among halogens, fluorine prefers an antarafacial path whereas chlorine and bromine prefer a suprafacial shift. Although the TS for the antarafacial shift was still shown to be aromatic, 112 it is likely to be electronically different from the rest and will not be further discussed in this work.…”

“…53 More recently, Kalpana and Akilandeswari reported a [1,5]-fluorine shift that proceeded via an aromatic TS but through an antarafacial pathway. 54 Our curiosity was triggered by our previous investigation of transition metal-catalyzed pericyclic rearrangements 55−60 and, in particular, the boronyl-activated Cope rearrangement, where we demonstrated how cooperation between donor and acceptor groups (frustrated Lewis pairs, FLP) placed at C2 and C5 carbons of a Cope substrate significantly lowered the activation barrier. Remarkably, it also revealed a new approach to interrupting and aborting pericyclic reactions.…”

“…Using Marcus theory, the Alabugin group predicted the activation energies for the migration of several groups in aromaticity-driven [1,5]-sigmatropic shifts . More recently, Kalpana and Akilandeswari reported a [1,5]-fluorine shift that proceeded via an aromatic TS but through an antarafacial pathway …”

“…Inspired by our previous donor–acceptor design for Cope substrates, an electron-donating group at C1 carbon was expected to weaken the σ-bond between C1 and the migrating group and to assist in a smooth transfer of this group to the migration terminus. Previously, we reported a special case of such boronyl based donor–acceptor systems, also known as “frustrated Lewis pairs” or FLPs, where the intramolecular communication in a Lewis acid–base pair is made unfavorable by stereoelectronic effects. , The resulting chemical reactivity in these cases provides the way to satisfy innate “frustration” due to the unfulfilled potential for the donor/acceptor self-quenching. When such “chemical frustration” is applied to 1,3-pentadienyl framework, the remotely located boronyl group (C4) and bases (C1) passed on their electronic properties to the adjacent π- and σ-systems respectively and interacted through a cyclic transition states consisting of 3-center bridge.…”