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

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.