Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes
A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex.
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