Allylic Amination of Pd(II)-Allyl Complexes via High-Valent Pd Intermediates

Organometallic high-valent Pd­(allyl) complexes have been
recently
proposed to act as intermediates in catalytic allylic functionalization
reactions. While a few PdIV(η1-allyl)
complexes have been isolated and characterized, PdIII(η3-allyl) or PdIII(η1-allyl) complexes
have not been detected or isolated to date. Reported herein is the
synthesis, characterization, and reactivity of a series of PdII(η3-allyl) complexes supported by the tetradentate
pyridinophane ligands N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane (tBuN4) and N,N′-di-tert-methyl-2,11-diaza­[3.3]­(2,6)­pyridinophane (MeN4). These PdII(η3-allyl) complexes exhibit
accessible oxidation potentials and upon oxidation generate PdIII(allyl) complexes that were characterized by electron paramagnetic
resonance (EPR) spectroscopy. Interestingly, the [(MeN4)­PdIII(η3-allyl)]2+ complex undergoes
a rearrangement to the [(MeN4)­PdIII(η1-allyl)]2+ species at low temperatures. Moreover,
fast allylic amination occurred within 15 min at room temperature
upon the reaction of [(MeN4)­PdII(η3-allyl)]+ complexes with N-fluorobenzenesulfonimide
(NFSI), and the C–N bond formation step is proposed to occur
at the Pd­(IV) oxidation state, likely via a PdIV(η1-allyl) intermediate.