C O M M U N I C A T I O N S
t
Replacing Pd(OAc)2 with Pd(OPiv)2 (Piv ) BuCO) further
(Scheme 1).7 Reductive elimination from the Pd(IV) complex then
occurs, affording the desired product. An intramolecular isotope effect
(kH/kD ) 5.7) was observed (eq 5), indicating that the cleavage of the
C-H bond is involved in the rate-determining step.
improved the yield (Table 1, footnote b). Under the Pd(OPiv)2/
HOTf/Piv2O conditions, ortho arylation occurred smoothly with
phenol esters carrying various substituents (Table 2). Importantly,
the iodo, bromo, and chloro substitutions (4e-h,j) were tolerated,
making possible additional modification reactions at the halogenated
positions. For substrates containing strong electron-withdrawing
groups (4r, 4s), the corresponding dimethylcarbamates were used
because their esters are relatively unstable. Even so, the ortho-
arylated products were already hydrolyzed into phenols during the
reaction. Notably, the selectivity of mono- versus diarylation could
be controlled by tuning the reaction temperature and reactant ratio
(4o vs 4o′). Moreover, the scope of the reaction with respect to the
arylation reagent is presented in Table 3. Both electron-rich and
electron-deficient phenyl groups could be incorporated into the
phenol esters. However, ortho substituents on the arylation reagents
reduced the coupling yields.
In summary, we have characterized by X-ray crystallography the
first cyclopalladation complex formed from a simple phenol ester. A
promising protocol for the ortho C-H activation/aryl-aryl coupling
of phenol esters that is not sensitive to moisture or air has been
established. Because substituted phenols are important organic inter-
mediates, this reaction is likely to find broad synthetic utility.
Acknowledgment. We thank the 973 Program (2007CB210205)
and NSFC (20972148, 20832004, 20932006) for support.
Table 3. Reaction Scope with Respect to the Arylation Reagent
Supporting Information Available: Experimental details and
compound characterizations. This material is available free of charge
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Scheme 1. Proposed Mechanism
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The mechanism of the arylation reaction most likely involves an
acyloxy-directed Pd insertion into the C-H bond and subsequent
oxidation of the Pd(II) complex to a Pd(IV) intermediate by Ar2I+OTf-
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