Intramolecular C−H Activation by Alkylpalladium(II) Complexes: Insights into the Mechanism of the Palladium-Catalyzed Arylation Reaction

Abstract

The cyclization of [ArOCH2PdL2Cl] complexes proceeds at room temperature in CH3CN in the presence of base, such as KOPh or carbonate, to form palladacycles. The effect of substituents on the aryl moiety (p-MeO > H >p-NO2) is as expected for an electrophilic aromatic substitution by electrophilic Pd(II). The absence of isotopic effect is also consistent with this proposal. Cyclopalladation proceeds with bidentate ligands (dppf, COD and phen); although the C-H activation reactions are slower in these cases. The starting [ArOCH2Pd(PPh3)2Cl] and [ArOCH2Pd(PPh3)Cl]2 complexes were prepared by transmetalation of organostannanes [(ArOCH2)4Sn] with [Pd(PPh3)2Cl2] or [Pd(PPh3)Cl2]2, respectively. Cleavage of palladacycles with HCl also gave [ArOCH2PdL2Cl] complexes.

Palladium-Catalyzed IntramolecularArylation Reaction: Mechanism and Application for theSynthesis of Polyarenes

“…In addition, the fact that a cationic complex analogous to 43b, but with an acetonitrile ligand, failed to activate the aryl C-H bond suggests that chloride dissociation does not take place before C-H activation and that formation of alkylpalladium(II) complexes with a carbonato or OPh ligand is not a productive pathway. 46 A mechanism for the C-H activation event consistent with all these results is shown in Scheme 20. The electrophilic substitution reaction of complexes VI could give either VII or VIII by substitution of the anionic or neutral ligand, respectively.…”

“…We examined the intramolecular C-H activation on a family of aryloxymethylpalladium(II) complexes 43-44 46 Although poor results were obtained with a variety of bases (NaOH, py, 2,6-di-tert-butylpyridine, DBU, NaOAc, K 2 CO 3 , NaHCO 3 , n-Bu 4 NOH), satisfactory results were obtained by using KOPh, Cs 2 CO 3 , or Ag 2 CO 3 in MeCN at 23°C to afford palladacycles 45 and 46, 46 in good to excellent yields (Scheme 18). 47…”

Insights into Directing Group Ability in Palladium-Catalyzed C−H Bond Functionalization

“…The cyclopalladation reaction (step ii ) is believed to proceed by an electrophilic12b,c,15 mechanism and/or by formation of an agostic intermediate followed by deprotonation 16,17. Both mechanisms involve the Pd acting as an electrophile; as such, the rate of this step is expected to be accelerated by electron withdrawing ancillary ligands (L) 12b,d.…”

Palladium Catalyzed Allylic C-H Alkylation: A Mechanistic Perspective

“…C-H bonds in aromatic systems are relatively easy to activate and the research field has received considerable attention, which has lead to interesting applications of the generally used palladium-diacetate catalyst. As the bis- ortho -arylation of biphenyl diamines published by Stahl and coworkers [ 46 ] or the cyclization of aryl complexes by intramolecular C-H activation [ 47 ]. The use of nitrogen atoms for coordination to Pd in the arene reagents has been well documented by Sanford and coworkers [ 48 , 49 , 50 , 51 ], especially the regioselective C-H activation in benzo[ h ]quinolone systems [ 52 , 53 , 54 ].…”