Crucial role of elusive isomeric eta-complexes in gas-phase electrophilic aromatic alkylations.
[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.
Marcantoni
Enrico
E
Dipartimento di Scienze Chimiche, Università degli Studi di Camerino, V. S. Agostino 1, I-62032 Camerino (Mc), Italy.
Roselli
Graziella
G
Lucarelli
Laura
L
Renzi
Gabriele
G
Filippi
Antonello
A
Trionfetti
Cristiano
C
Speranza
Maurizio
M