Arene C–H Functionalization by p‑Block Metal Tl(III) Occurs at the Borderline of C–H Activation and Electron Transfer

M06 density functional theory calculations
reveal that arene C–H
functionalization by the p-block main-group-metal complex TlIII(TFA)3 (TFA = trifluoroacetate) occurs by a C–H
activation mechanism akin to transition-metal-mediated C–H
activation. For benzene, toluene, and xylenes a one-step C–H
activation is preferred over electron transfer or proton-coupled electron
transfer. The proposed C–H activation mechanism is consistent
with calculation and comparison to experiment, of arene thallation
rates, regioselectivity, and H/D kinetic isotope effects. For tetramethyl-
and pentamethyl-substituted arenes, electron transfer becomes a competitive
pathway and thermodynamic and kinetic calculations correctly predict
the experimentally reported electron transfer crossover region. These
calculations show that p-block metals activate strong hydrocarbon
C–H bonds through organometallic intermediates and changes
in arene functional groups can result in a shift from C–H activation
to electron transfer.