Palladium-Catalyzed C−O Coupling Involving Unactivated Aryl Halides. Sterically Induced Reductive Elimination To Form the C−O Bond in Diaryl Ethers

The reductive elimination of ethers from transition metal complexes is a rare elementary reaction that is limited to special cases. The importance of aryl ether substructures has created a synthetic challenge to prepare the aryl-oxygen linkage in a general fashion under mild conditions. Pd-catalyzed C-O coupling could be a solution to this synthetic problem, but the reductive elimination of acyclic ethers that is the crucial step for this catalytic process has been limited to palladium complexes with aromatic systems that are highly activated and undergo direct, uncatalyzed nucleophilic aromatic substitution chemistry. Thus, it is unactivated aryl halides that are crucial substrates to include in transition metal-catalyzed C-O coupling chemistry, and they report that Pd complexes with sterically hindered alkylphosphines (1) undergo thermal reductive elimination to form the C-O bond in diaryl ethers from complexes with unactivated metal-bound aryl groups and (2) catalyze the formation of diaryl ethers and protected phenols from unactivated aryl halides. These findings demonstrate the concept that sterically hindered alkylphosphines accelerate reductive elimination.