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C O M M U N I C A T I O N S
bromoarene would then form Pd[P(t-Bu)3]2 and free aryl bromide.
As shown in eq 2, this pathway predicts first-order behavior in 1b,
zero-order behavior in bromoarene, first-order behavior in P(t-Bu)3
at low [P(t-Bu)3], and zero-order behavior at high [P(t-Bu)3]. Path
C involves reversible reductive elimination of aryl bromide from
1b with or without an intermediate haloarene complex,18 followed
by trapping of the palladium(0) intermediate by P(t-Bu)3. As shown
in eq 3, this pathway predicts the same behavior in the concentration
of 1b and P(t-Bu)3 as path B. However, path C predicts that the
observed rate constant will depend inversely on [ArBr] when the
k2 step competes with the k-1 step. The inverse dependence of the
rate constant on the concentration of [ArBr] shows that the reaction
occurs by path C.
Figure 1. Plot of 1/kobs vs 1/[P(t-Bu)3] and 1/kobs vs [ArBr] for the reductive
elimination of aryl bromide from complex 1b.
Scheme 1
The y-intercept of the plot of 1/kobs vs [ArBr] corresponds to
1/k1. The value k1 is the rate constant for reductive elimination of
2-bromotoluene from 1b. The ratio of k-1/k2, describes the rela-
tive rate constants for the oxidative addition of this bromoarene to
Pd[P(t-Bu)3] and the coordination of P(t-Bu)3 to Pd[P(t-Bu)3]. The
data in Figure 1 show that the rate constant for oxidative addition
of bromoarene exceeds the rate constant for ligand association by
a factor of 65 ( 25. Thus, oxidative addition to Pd[P(t-Bu)3] is
even faster than simple coordination of ligand to this highly
unsaturated intermediate.
In summary, we have shown that slow activation of chloroarenes
results from a transition state that is high in energy for reasons
beyond the strength of the ArCl bond and that oxidative addition
of aryl halides to Pd[P(t-Bu)3] is faster than simple ligand
coordination. Further studies with other hindered alkylphosphines
and the effects of haloarene electronics on the rates and equilibria
for addition and elimination will comprise future studies.
create a low-energy transition state. Instead, the softness or
nucleophilicity of the halogen in the transition state for addition
and elimination is likely to be more important. The rate of reaction
of the iodide 1c appears to result from a combination of ground-
and transition-state effects.
Reductive elimination is typically faster from three-coordinate
than related four-coordinate complexes,1,16 and the steric hindrance
of P(t-Bu)3 makes elimination from a four-coordinate species after
ligand association unlikely. Thus, reductive elimination of bro-
moarene from the starting arylpalladium bromide complex seemed
the most likely mechanism.17 This mechanism is shown as Path A
in Scheme 1. Under conditions in which the reaction is driven far
to the side of reductive elimination, formation of bromoarene from
the three-coordinate complex would correspond to a rate equation
that is zero order in the concentration of ligand and aryl bromide.
To evaluate this mechanistic assumption, rate constants for the
reductive elimination of bromoarene from {Pd[P(t-Bu)3](o-tolyl)-
(Br)} (1b) at 17 mM initial concentrations were measured over 5
Acknowledgment. We thank the NIH (GM-55382) for support,
Johnson-Matthey for a gift of PdCl2, and Merck Research Labor-
atories for unrestricted support.
Supporting Information Available: Experimental methods and
spectral data for 1a-d (PDF). This material is available free of charge
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1
half-lives by H NMR spectroscopy at 65 °C. Excellent fits to a
first-order appearance of product were obtained. The concentration
of P(t-Bu)3 was varied from 0.13 to 0.61 M while maintaining a
constant concentration of added bromoarene by addition of 0.069
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