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

Abstract

A zwitterionic palladium complex [[Ph2BP2]Pd(THF)2][OTf] (1) (where [Ph2BP2] ) [Ph2B(CH2PPh2)2] -) reacts with trialkylamines to activate a C-H bond adjacent to the amine N atom, thereby producing iminium adduct complexes [Ph2BP2]Pd(N,C:η 2 -NR2CHR′). 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 {[Ph2BP2]Pd}2 (4) as the dominant palladium product. Low-temperature toluene-d8 experiments proceed more cleanly, and intermediates assigned as [Ph2BP2]Pd(NEt3)(OTf) and the iminium hydride species [[Ph2BP2]Pd(H)(Et2NdCHCH3)][OTf] are directly observed. The complex (Ph2-SiP2)Pd(OTf)2 ( 14) was also studied for amine activation and generates dimeric [(Ph2SiP2)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.

Synthesis, Interionic Structure, and Reactivity toward CO and p‐Methylstyrene of Palladacyclic Compounds Bearing α‐Diimine Ligands

“…with the proposed structure for complex 18; in particular, the chemical shift of C c (d 76.6) is similar to that recently reported by Lu and Peters [31] for analogous Pd complexes that, in some cases, were structurally characterized in the solid state by X-ray diffraction studies.…”

Direct Synthesis of Naltrexone by Palladium‐Catalyzed N‐Demethylation/Acylation of Oxymorphone: The Benefit of CH Activation and the Intramolecular Acyl Transfer from C‐14 Hydroxy

“…The coordination of the nitrogen with the low valent palladium and the overlap of the metal orbital with the α‐CH bond induces CH activation and the formation of the η 2 ‐iminium hydride metal complex, as was proven by racemization experiments at the chiral α carbon of tertiary amines and deuterium‐labeling experiments at the α and β positions 17a,23a. The isolation of such a Pd‐iminium ion complex and its structure determination by X‐ray diffraction were reported 24. A similar CH activation in a ruthenium cluster‐catalyzed alkyl‐exchange reaction was also demonstrated 25.…”

Palladium(0)/NHC‐Catalyzed Reductive Heck Reaction of Enones: A Detailed Mechanistic Study

“…[2g, 32] Moreover,t he Peter group suggested that the b-hydride elimination from aP d II speciesi st he CÀH bond-cleaving step. [33] For complexes based on other metals, similar reactions have been reported, such as the formation of Os II /iminium hydride speciest hrough b-hydride elimination of an Os III /amine complex [34] and formation of iridium III /dihydride complexes by the dehydrogenation of tertiary amines to produce enamines. [35] More recently,t he Peter group reported dehydrogenation of av ariety of trialkylamines by ac ationic (or zwitterionic) Pd complex through a b-hydride elimination/iminium salt reinsertion process to give the corresponding threemembered palladacycle.…”

“…[35] More recently,t he Peter group reported dehydrogenation of av ariety of trialkylamines by ac ationic (or zwitterionic) Pd complex through a b-hydride elimination/iminium salt reinsertion process to give the corresponding threemembered palladacycle. [33] It was also shownt hat the selectivity in the activation of tertiary and secondary CÀHb onds in trialkylamines was solely towards secondary CÀHb onds owing to sterichindrance. [34b] The H/D scrambling that occurred during the reaction may be caused by the Pd II ÀHs peciest hat is present in the reaction mixture as am inor byproduct, which was formed from either ac lassical Heck reactiono rr eactionw ith traces of water in the reaction mixture, as hydride anionse xchange quickly in palladium complexes.…”