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Synthetic, Structural, and Mechanistic Aspects of an Amine Activation Process Mediated at a Zwitterionic Pd(II) Center

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A zwitterionic palladium complex [[Ph_(2)BP_(2)]Pd(THF)_(2)][OTf] (1) (where [Ph_(2)BP_(2)] = [Ph_(2_B(CH_(2)PPh_(2))_(2)]-) reacts with trialkylamines to activate a C−H bond adjacent to the amine N atom, thereby producing iminium adduct complexes [Ph_(2)BP_(2)]Pd(N,C:η^(2)-NR_(2)CHR‘). In all cases examined the amine activation process is selective for the secondary C−H bond position adjacent to the N atom. These palladacycles undergo facile β-hydride elimination/olefin reinsertion processes as evident from deuterium scrambling studies and chemical trap studies. The kinetics of the amine activation process was explored, and β-hydride elimination appears to be the rate-limiting step. A large kinetic deuterium isotope effect for the amine activation process is evident. The reaction profile in less polar solvents such as benzene and toluene is different at room temperature and leads to dimeric {[Ph_(2)BP_(2)]Pd}_(2) (4) as the dominant palladium product. Low-temperature toluene-d8 experiments proceed more cleanly, and intermediates assigned as [Ph_(2)BP_(2)]Pd(NEt_(3))(OTf) and the iminium hydride species [[Ph_(2)BP_(2)]Pd(H)(Et_(2)NCHCH_(3))][OTf] are directly observed. The complex (Ph2SiP2)Pd(OTf)2 (14) was also studied for amine activation and generates dimeric [(Ph_(2)SiP_(2))Pd]_(2)[OTf]_(2) (16) as the dominant palladium product. These collective data are discussed with respect to the mechanism of the amine activation and, in particular, the influence that solvent polarity and charge have on the overall reaction profile.

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