Palladacyclic N-heterocyclic carbene precatalysts for transition metal catalysis

A General C–N Cross-Coupling to Synthesize Heteroaryl Amines Using a Palladacyclic N-Heterocyclic Carbene Precatalyst

“…C2 with a DiMeOxa Ph skeleton possessed a shorter Pd–C Ar bond (1.999 Å vs 2.097 Å) and a smaller 13 C chemical shift of the aryl carbon (173.44 ppm vs 174.57 ppm) compared to those of C1 . In one word, C1 can deliver the active AnIPr-Pd(0) species more readily due to the stronger Pd–C AnIPr and weaker Pd–C Naphthyl bonds according to the previously reported activation mechanism, favoring the oxidative addition step. Furthermore, the percent buried volumes (% V bur ) , and steric maps of C1–C3 were also calculated with SambVca 2.1 (Table S4).…”

“…Since the seminal studies by Cope and Herrmann, − palladacycles have received a great deal of attention from chemists due to the following characteristics: (a) high stability against air and moisture and (b) easy modulation of the electronic and steric properties via variation of the size of metalacyclic rings, the coordinated donor atoms to the Pd center, and the nature of the metaled carbon atom and ancillary ligands. − Nolan and co-workers presented seminal work on NHC palladacycles and were the first to show that these catalysts at a parts per million level revealed excellent activity in transition metal catalysis, such as Suzuki–Miyaura cross-coupling, Buchwald–Hartwig amination, and other couplings. − The introduction of a NHC, , a strong σ-donating and weak π-accepting ligand, not only contributes to the structural stability of the active LPd(0) species but also provides the Pd center with increased electronic density facilitating the difficult oxidative addition step. However, to the best of our knowledge, the full evaluation of NHC palladacycles in the amination of heteroaryl chlorides with heteroaryl amines has so far been neglected.…”

“…Importantly, the combination of the AnIPr and DiMeOxa Naphthyl palladacyclic fragments guarantees the high catalytic capability of C1 . Then, on the basis of previous literature reports, ,,, we proposed a rational reaction mechanism consisting of four sequential steps (Scheme S6). First, C1 was activated under the reaction conditions, releasing the actually active species AnIPr-Pd(0), which was subsequently available for the oxidative addition of the heteroaryl chloride to form a Pd­(II) intermediate.…”

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

“…Nolan and other groups have already presented interesting research works by incorporating NHCs into palladacycles, demonstrating their excellent catalytic activity in Suzuki–Miyaura cross-coupling reactions, Buchwald–Hartwig amination and so on. 11 b , e ,15 Very recently, we designed a novel type of NHC-ligated palladacycle, termed NHC–Pd cycles –Oxa, by first introducing oxazoline palladacyclic motifs. Its catalytic ability was elegantly tuned by the modification of oxazoline and NHC skeletons.…”

Pd-Loaded Cellulose NanoSponge as a Heterogeneous Catalyst for Suzuki–Miyaura Coupling Reactions

“…Many efforts have been devoted over the years to the design and synthesis of Pd(II)-precatalysts by selecting proper ligands such as N -heterocyclic carbenes [ 5 , 6 , 7 ], phosphine [ 8 , 9 ], palladacycles [ 10 ], the PEPPSI (pyridine-enhanced precatalyst preparation) system [ 11 ], and allyl-based ligands [ 12 ], in order to improve the efficiency of this catalysis under homogeneous conditions.…”