Crucial Role of Elusive Isomeric η-Complexes in Gas-Phase Electrophilic Aromatic Alkylations

Abstract

[isomers: see text] The kinetics and stereochemistry of the protonation-induced unimolecular isomerization of (R)-1-D1-3-(p-fluorophenyl)butane have been investigated in the gas phase at 40-100 degrees C and 70-760 Torr. This process leads to the formation of the relevant meta and ortho isomers with partial racemization of the migrating sec-butyl moiety. Complete racemization is observed, instead, when the isomerization reaction involves a 1,2-H shift in the moving alkyl group. These results, together with the relevant activation parameters, fully confirm the previous evidence of the occurrence in the alkyl cation/arene PES of noncovalent eta-type intermediates of defined structure and stability, lying well below the classical pi-complexes, as confirmed by ab initio calculations. Their crucial role in determining the positional selectivity of gas-phase electrophilic aromatic substitutions clearly emerges from the comparison of the present results with the site selectivity measured in the corresponding bimolecular arene alkylation carried out at the same temperatures and pressures.

Regioselectivity in Electrophilic Aromatic Substitution: Insights from Interaction Energy Decomposition Potentials

“…This suggests that different reaction mechanisms or prereactive complexes compete depending on the nature of the system electrophile/arene. [7,[10][11][12][14][15][16] For soft complexes our analysis indicates that highly ordered π-complex like structures are promoted, as observed with X-ray crystallography experiments by Kochi et al [13] In these structures the electrophile is oriented over the atom with remarkable high affinity for positions rendering, in a later stage of the reaction, the stablest σ-complexes. This stability is in turn strongly related to the regioselectivity observed for these rings.…”

“…Two years later Speranza and co-workers put forward a similar idea, for electrophilic aromatic alkylations. [14] They suggested the existence of well-defined arene/cation π-complex like structures lying mechanistically in between the classical π-complexes and σ-complexes. Based on computational evidence these complexes, called η-complexes, are believed to play a major role in the selectivity of arenes as movement on their almost flat PES from η meta , η para and η ortho is suggested to be relatively easy compared to movement on the PES of the oriented σ-complexes (Scheme 2), from there the differently oriented σ-complexes are formed.…”

Lipase catalysed kinetic resolutions of 3-aryl alkanoic acids

“…Treatment of the pure acid chloride with ethanol in the presence of triethylamine led to analytically pure ester (Scheme 1). An alternative route via a Wadsworth-Emmons reaction was employed in synthesising ethyl 3-(4-fluorophenyl)butanoate (±)-3i (Scheme 2) 32 . Acids 1a-e and 1h have been previously Table 7 for chiral HPLC conditions.…”