Nickel- vs palladium-catalyzed synthesis of protected phenols from aryl halides
We report the nickel-catalyzed formation of alkyl and silyl ethers from aryl halides in one step. These ethers can act as precursors to phenols by reaction with Brønsted or Lewis acids or with fluoride. A combination of Ni(COD)2 and DPPF mediates the formation of tert-butyl aryl, methyl aryl, and tert-butyldimethyl silyl aryl ethers efficiently from aryl halides and sodium alkoxides or sodium siloxides under mild reaction conditions. Reactions to form the tert-butyl aryl ethers mediated by nickel complexes have lower turnover numbers than the analogous palladium-catalyzed reactions. However, reactions to form methyl aryl ethers show higher yields when catalyzed by a combination of Ni(COD)2 and BINAP than when catalyzed by BINAP or DPPF and palladium catalyst precursors. Similarly, the formation of silyl aryl ethers occurs in higher yields and under milder conditions when catalyzed by a combination Ni(COD)2 and DPPF than when catalyzed by palladium complexes. We also report improved yields from our previous results for the palladium-catalyzed conversion of aryl halides to tert-butyl aryl ethers.
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