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Can aromaticity trigger thermal [1,5]‐halogen shift towards the forbidden antarafacial mode?: A density functional case study
Author(s) -
Kalpana P.,
Akilandeswari L.
Publication year - 2019
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3991
Subject(s) - aromaticity , chemistry , pericyclic reaction , halogen , fluorine , natural bond orbital , computational chemistry , transition state , density functional theory , molecule , organic chemistry , alkyl , catalysis
[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.