Palladium-Catalyzed C–H Activation: Mass Spectrometric Approach to Reaction Kinetics in Solution

We
report a new method for determination of rate constants of processes
in solution using electrospray ionization mass spectrometry (ESI-MS).
The investigated reaction is C–H activation of acetanilides
by palladium­(II)­trifluoroacetate leading to stable organopalladium
complexes. The rate constants can be determined from an experiment
with a couple of differently substituted acetanilides being in competition
for being activated by the palladium salt. The formed organopalladium
complexes can be detected by ESI-MS. The time dependence is achieved
by adding one of the acetanilides to the reaction mixture with a time
delay. The kinetics can be then evaluated from the evolution of the
ratio of the ESI-MS signals of differently substituted complexes as
a function of the time delay. The Hammett analysis of the rate constants
obtained for a series of meta- and para-substituted acetanilides provides a ρ value of −1.5,
which is in agreement with values reported for similar C–H
activations. We have also investigated the very same reaction with
UV–vis spectroscopy that gave us about three times smaller
rate constants but the same trend with the ρ value of −1.6.
The rate constants determined by ESI-MS are directly linked to the
occurrence of organopalladium complexes, whereas the UV–vis
data are associated with an absorption spectra change that could involve
more reaction steps. DFT calculations support the interpretation of
the reaction mechanism as cyclopalladation and provide the ρ
value in the same range. The rate-determining step corresponds to
the agostic C–H transition structure.