Imino-N-Heterocyclic Carbene Palladium(II) Complex-Catalyzed Direct Arylation of Electron-Deficient Fluoroarenes with “On and Off” Chelating Effect Assistance

An
imino-N-heterocyclic carbene palladium­(II) complex with a bulky
substituted group on the imino nitrogen was used to catalyze the direct
arylation of electron-deficient fluoroarenes with aryl halides. A
series of electron-poor substrates and aryl bromides could be coupled
in good to excellent yields with satisfactory position selectivity
(20 examples, up to 93%). These arylations could proceed at a relatively
low temperature (80 °C, 20 examples, up to 95%) with mono-N-protected
amino acid assistance. Some of them even gained higher yields than
those at high temperature (110 °C). Otherwise, some aryl iodides
can forge cross-coupling products in yields of nearly 30% under the
optimized conditions. The rate profiles for arylation of electron-poor
arenes were measured in the presence of the imino-N-heterocyclic carbene
palladium­(II) complex or Pd­(OAc)2 as the catalyst, which
showed that the former could keep catalytic activity for a longer
time. Computational studies indicated that the imino nitrogen in the
imino-N-heterocyclic carbene ligand can detach from and attach to
the central metal in the catalytic cycle. Thus, the coordination site
could be protected, and this effect may be responsible for decreasing
the rate of palladium black formation.