Syntheses and Thermolysis of Arylpalladium Hydroxide Complexes: Implications for C(sp2)-OH Bond-Forming Reductive Elimination To Generate Phenol Derivatives

The decomposition of arylpalladium hydroxide complexes gave the corresponding phenolic products, which may form through a C(sp(2))-OH bond-forming reductive elimination either by treatment of arylpalladium halide complexes with cesium hydroxide or by heating arylpalladium hydroxide complexes. Treatment of a p-nitrophenylpalladium iodide complex possessing a (t)BuXPhos ligand with cesium hydroxide formed a mixture of p-nitrophenol and 4,4'-di-p-nitrobiphenyl. The reaction of a (DBPP)-B-t-ligated p-nitrophenylpalladium iodide complex with cesium hydroxide gave a mixture of p-nitrophenylpalladium hydroxide complex, cesium p-nitrophenoxide, and (DBPP)-B-t-bridged dinuclear Pd(0) complex. Gradual decomposition of p-nitrophenylpalladium hydroxide complex to cesium p-nitorophenoxide and a (DBPP)-B-t-bridged dinuclear Pd(0) complex suggested that the C(sp(2))-OH bond-forming reductive elimination took place. While an isolated p-tolylpalladium hydroxide complex gave no phenolic product upon heating, thermolysis of an isolated trifluoromethyl-substituted arylpalladium hydroxide complex enabled us to observe p-trifluoromethylphenol directly. Although an ester-substituted hydroxide complex did not form free p-methoxycarbonylphenol, its invisibility enabled us to handle the kinetic equation to estimate the rate constant k(1) for reductive elimination. Polar solvents such as THF and DMF accelerated the reductive elimination with a large negative entropy of activation. Comparison of these results with those in the literature suggested direct C(sp(2))-OH bond-forming reductive elimination with a concerted three-centered pathway. DFT calculations also predicted the hydrogen bond between the hydroxo ligand and the solvent molecule to stabilize the transition state.