Influences on the Relative Rates for C−N Bond-Forming Reductive Elimination and β-Hydrogen Elimination of Amides. A Case Study on the Origins of Competing Reduction in the Palladium-Catalyzed Amination of Aryl Halides

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Abstract

Typical decomposition by β-hydrogen elimination has limited the productive catalytic organometallic chemistry of late transition metal amido complexes. However, one reaction that has been shown to involve a late metal amido complex with β-hydrogens and elude extensive β-hydrogen elimination is the palladium-catalyzed amination of aryl bromides to give arylamines. The primary side products formed in these catalytic aminations are arenes, the products of aryl halide reduction. It would seem reasonable that both arylamine and arene products result from competitive reductive elimination of amine and β-hydrogen elimination from a common amido aryl intermediate. Our results do substantiate competitive β-hydrogen elimination and reductive elimination involving an amido group, but also reveal a second pathway to reduction that occurs when employing Pd(II) precursors. This second pathway for aryl halide reduction was shown principally by the observations that (1) stoichiometric reactions of aryl halide complexes or catalytic reactions employing [P(o-tolyl)3]2Pd(0) showed less arene side product than did catalytic reactions employing Pd(II) precursors, (2) increasing amounts of Pd(II) catalyst gave increasing amounts of arene product, and (3) reactions catalyzed by Pd(II) precursors showed amine:arene ratios at early reaction times that were lower than ratios after complete reaction. In addition to data concerning arene formation during Pd(II) reduction, we report data that demonstrate how electronic and steric factors control the relative rates for amine vs arene formation. The relative amounts of reduction product and amination product depend on the size of the phosphine and substitution pattern of the amide ligands. Systematic variation of phosphine size demonstrated that increasing the size of this ligand gave increasing amounts of arylamine product, increasing size of the amido group gave increasing amounts of arylamine product, while decreased nucleophilicity of the amide gave decreased amounts of arylamine product. Further, the presence of electron withdrawing groups on the palladium-bound aryl ring accelerated the reductive elimination reaction, relative to β-hydrogen elimination, and this result is consistent with previously observed acceleration of carbon−heteroatom bond-forming reductive eliminations with isolable palladium complexes.

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