jo050019q_si_001.pdf (210.65 kB)
Crucial Role of Elusive Isomeric η-Complexes in Gas-Phase Electrophilic Aromatic Alkylations
journal contribution
posted on 2005-05-13, 00:00 authored by Enrico Marcantoni, Graziella Roselli, Laura Lucarelli, Gabriele Renzi, Antonello Filippi, Cristiano Trionfetti, Maurizio SperanzaThe 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 °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 η-type intermediates of defined structure and stability,
lying well below the classical π-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.
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Crucial Rolebutyl moietyElectrophilicstabilityactivation parameterskineticcomplexunimolecularAromaticsite selectivityarene alkylationnoncovalentIsomericoccurrenceComplete racemizationmetasecElusiveAlkylationComplexesubstitutionisomerization reactionshiftgas phaseformationPESortho isomerselectrophilicintermediatetypeab initio calculationsevidencecationTorralkyl grouprolestereochemistryfluorophenyl
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