Solvent-Induced Reduction of Palladium-Aryls, a Potential Interference in Pd Catalysis

The decomposition of the Pd-aryl complex (NBu4)2[Pd2(μ-Br)2Br2(C6F5)2] (1) to the reduction product C6F5H was checked in different solvents and conditions. 1 is not stable in N-alkyl amides (DMF, NMP, DMA), cyclohexanone, and diethers (1,4-dioxane, DME) at high temperatures (above 80 °C). Other solvents such as nitriles, THF, water, or toluene are safe, and no significant decomposition occurs. The solvent is the source of hydrogen, and the decomposition mechanisms have been identified by analyzing the reaction products coming from the solvent. β-H elimination involving the methyl group in a N-coordinated amide is the predominant pathway for amides. An O-coordinated diether undergoes β-H elimination and subsequent deprotonation of the resulting oxonium salt to give an enol ether. A palladium enolate from cyclohexanone leads to cyclohexenone, a reaction favored by the presence of a base. Oxygen strongly increases the extent of decomposition, and we propose this occurs by reoxidation of the Pd(0) species formed in the process and regeneration of active Pd­(II) complexes.