Investigations of a New and Improved Precataylst for Palladium Catalyzed Cross Coupling Reactions
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Little attention has been given to the formation of the putative PdL2 species required for Pd-catalyzed cross-coupling reactions. Active species are generally difficult to store due to air-sensitivity and are therefore formed in situ at unknown rates and in unknown yields via a variety of palladium precatalysts. Commonly employed Pd(0) and Pd(II) precatalysts are often relatively ineffective because they generate only low concentrations of the bis(phosphine) species PdL2 under most reaction conditions. This thesis describes the use of the easily synthesized and easily handled Pd(η3-1-Ph-C3H4)(η5-C5H5) (I) as a superior precursor than any other documented system for the in situ formation of PdL2. Rapid and quantitative formation of active catalyst solutions allow us to demonstrate that I is also the best precatalyst known for PdL2-catalyzed crosscoupling reactions. We discuss the Suzuki-Miyaura reaction of 4-bromoanisole with phenylboronic acid and demonstrate that, under mild reaction conditions, higher initial rates and higher conversions with I can be obtained compared with other common precatalysts (Pd(OAc)2, Pd(PPh3)4, Pd2dba3, etc.) containing a variety of phosphine ligands. This methodology has also been extended to other cross-coupling reactions, as we demonstrate that higher initial rates and higher conversions with I can be obtained for a variety of Mizoroki-Heck arylations and Buchwald-Hartwig aminations.