Mechanisms in the Reaction of Palladium(II)–π‐Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

Abstract

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(eta(3)-C(3)H(5))(tmiy)(PR(3))]BF(4) (tmiy = tetramethylimidazolin-2-ylidene, PR(3) = phosphane), precursors to monoligated Pd(0) species, with aryl electrophiles yielding 2-arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd(IV) and sigma-bond metathesis pathways, and that transmetalation is the rate-determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd(II) complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin-2-ylidene).

N-heterocyclic carbenes bearing two, one and no nitrogen atoms at the ylidene carbon: insight from theoretical calculations

“…2-4, 8 It is now recognized that the synthetic availability, ease of functionalization, strong s-donor and tunable steric properties of NHCs account for their success as supporting ligands for transition metal catalysts. Moreover, NHCs exhibit interesting reactivity patterns either in the free form 27, 28 or as coordinated species, [29][30][31] and they are also active organocatalysts. 28,[32][33][34][35][36] The isolation of free NHCs of type 1 and their successful utilization in catalysis stimulated the search for stable carbenes of different types.…”

Mechanistic Insights into the Oxidative Coupling of N‐Heterocyclic Carbenes within the Coordination Sphere of Copper Complexes

“…Reaction of 4 (Scheme 2) with CuBr 2 leads to the occurrence of a CÀC bondforming reaction under exceptionally mild conditions. This reactivity is particularly unusual, especially when considering that the majority of the C carbene ÀC bond-forming reactions described so far in the literature have involved either C carbene ÀC reductive elimination reactions from Group 10-NHC complexes, [5,[17][18][19][20][21][22][23][24][25] or migratory insertion reactions. [9,10,26] Previous work has shown that the NHC, 1,3-dimesityl-imidazol-2-ylidene (IMes), reacts with tetrahydrofuran in the presence of an oxidizing uranyl triflate complex to give a bis-imidazolium linked by 1,3-butadiene.…”

Dioxygen adducts of rhodium N-heterocyclic carbene complexes

“…23 In particular, NHC complexes of Rh and other transition metals can undergo cleavage of the metal-carbon bond of the NHC under remarkably mild conditions. 16,[24][25][26][27][28][29] In addition, we have also demonstrated that Rh-NHC complexes can react rapidly with oxygen, in some cases providing Rh-O 2 complexes with highly unusual bonding modes (Fig. 1).…”