Arene activation with mercury(II) and thallium(III) electrophiles. Mechanistic relevance of charge-transfer transitions in .pi.-complexes as intermediates

Abstract

The activation of various aromatic hydrocarbons by mercuration and thallation proceeds via ..pi..-complexes, as observed by the transient charge-transfer (CT) absorption spectra. Quantitative spectrophotometric analysis of (a) the association constants K and 9b) the second-order rate constants k/sub 2/ establishes the reactive forms of the electrophiles to be Hg(O/sub 2/CCF/sub 3/)/sub 2/ and Tl(O/sub 2/CCF/sub 3/)/sub 2//sup +/, both in ..pi..-complex formation as well as in aromatic metalation. The direct participation by these isoelectronic and isostructural species presents a unique opportunity to examine arene activation with electrophiles which primarily differ only in the charge they bear. A detailed comparison of the CT excitation energies and the reactivities of various arenes relative to steric, kinetic isotope, and solvent effects in mercuration and thallation reveals unusual similarities. At the same time the observation of arene cations as key intermediates in thallation, but not in mercuration, leads to puzzling incongruities. The paradox is analyzed in the context of merging stepwise (electron-transfer) and concerted (electrophilic) mechanisms.