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.