Activation and Reactivity of (NHC)Pd(allyl)Cl (NHC = N-Heterocyclic Carbene) Complexes in Cross-Coupling Reactions

Abstract

Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…Treatment of the corresponding p-allylpalladium chloride dimers with free carbenes at room temperature resulted in the formation of monomeric Pd-NHC species in high yields (Scheme 44). [122][123][124][125][126] As the handling of the sensitive free carbene limits the practicality and the scale of the precatalyst preparation, Nolan and coworkers have recently addressed this concern by developing a one-pot procedure: the carbene was generated on a large scale from the imidazolium salt and KOtBu in technical grade 2-propanol followed by addition of [{Pd(p-allyl)Cl} 2 ]. [122] However, in this case an excess of the ligand precursors (1.4 equiv NHC·HCl versus 1.1 equiv NHC, Scheme 44) had to be used.…”

“…Simple substitution of the chloride ligand with tert-butoxide at the Pd center followed by reductive elimination or, alternatively, direct S N 2' attack on C1/C3 of the allyl moiety both lead to formation of allyl-tert-butyl ether (isolated from the reaction) and the singly ligated [(IPr)Pd] species (trapped as an tricyclohexylphosphane adduct). [126] Substitution of the pallyl group at C3 leads to a lowering of the symmetry and an elongation of the C3 À Pd bond. Overall, the allyl group becomes much more susceptible towards either nucleophilic attack or reductive elimination.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…Ein einfacher Austausch von Chlorid gegen tert-Butoxid am Palladiumzentrum mit anschließender reduktiver Eliminierung, oder alternativ ein direkter S N 2'-Angriff auf C1/C3 der Allylgruppe, führt zu einem Allyl-tert-butylether (der isoliert werden konnte) und einer einfach ligierten Ipr-Pd-Spezies (die als Tricyclohexylphosphan-Addukt abgefangen wurde). [126] Eine Substitution an C3 der p-Allylgruppe vermindert die Symmetrie und verlängert die C3-Pd-Bindung. Insgesamt werden nucleophile Angriffe oder die reduktive Eliminierung der Allylgruppe erleichtert.…”

Improved Turnover Numbers in Palladium‐Catalyzed Bisdiene Cyclization‐Trapping using N‐Heterocyclic Carbene Ligands

“…The chloride-exchanged catalysts, prepared in situ via exchange of APC with NaBF 4 or NaPF 6 , give results similar to those obtained with 14; the NaSbF 6 -exchanged catalyst is inferior, consistent with data in Table 3. Using the procedure of Nolan, [23] we prepared and isolated the mononuclear (IPr)Pd(allyl)Cl complex (16). Unfortunately, this preformed complex did not prove better than the corresponding in situ preparation (Table 4); neither did the corresponding isolated (IPr)Pd(allyl)X complexes (X ¼ BF 4 , PF 6 , SbF 6 ; data not shown).…”