Dimetallic Palladium‐NHC Complexes: Synthesis, Characterization, and Catalytic Application for Direct C−H Arylation Reaction of Heteroaromatics with Aryl Chlorides

Abstract

A series of dimetallic palladium(II)-NHC complexes comprised of 1,4-naphthalenyl or 9,10anthracenyl spacer sandwiched between two imidazole rings was successfully synthesized. These complexes were characterized by 1 H and 13 C{ 1 H} NMR spectroscopy and elemental analysis. The structures of two dimetallic palladium complexes and a related mononuclear palladium complex to be used for comparative studies were further characterized by X-ray diffraction. The dimetallic palladium complex with the 9,10anthracenyl linker was very efficient in catalyzing direct CÀ H arylation reactions of heteroaromatic compounds (imidazoles, imidazo[1,2-a]pyridine, and thioazole) with a broad range of aryl chlorides, employing a mild monopalladium loading of 1.5 mol%. It allows for the effective use of aryl chlorides to prepare arylated heterocycles, previously only accessible with the more reactive bromide counterparts. Importantly, the catalytic activity of the dimetallic precatalyst was found to be higher than that of an analogous mononuclear complex. Scheme 1. Synthesis of ligand precursors. Scheme 2. Synthesis of Dinuclear Palladium-NHC Complexes. Scheme 3. Synthesis of Mononuclear Palladium-NHC Complexes.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

“…Research areas where metal mediated organic molecule transformations were conducted using bidentate NHC ligands to a lesser degree include reduction of carbonyl compounds using hydrosilylation process [ 209 , 214 , 215 , 216 , 217 , 218 , 219 ], isomerization of allylic alcohols [ 220 ], cross metathesis [ 221 ], arene borylation [ 222 , 223 ], amination [ 224 ], reduction of sulfoxides [ 225 ], alcohol oxidation [ 132 , 226 , 227 ], CO 2 reduction [ 228 , 229 , 230 , 231 ], norbornene polymerization [ 232 , 233 , 234 ], alkylation of amines [ 235 ], C-H borylation [ 236 ], CO 2 activation [ 237 ], hydroamination [ 238 ], functionalization of propane [ 239 , 240 ], ylide cycloaddition [ 74 ], three component coupling [ 241 ], hydroxylation of benzene [ 242 ], hydroaminoethlyation [ 243 ], water oxidation [ 244 ], amine alkylation [ 245 ], homopolymerization of ethylene [ 246 ], destruction of chemical warfare [ 247 ], Friedel-Crafts alkylation [ 248 ], C–H arylation [ 249 ], C–H oxidation [ 250 ], SABRE catalyst [ 251 ], and alcohol amidation reactions [ 211 ]. S...…”

Direct arylation reaction catalyzed by a PEPPSI-type palladium complex with an amido-functionalized triazole-based mesoionic carbene ligand

“…To make sure that the catalytic properties of these complexes are worthy of investigation, we first screened the new complexes against reported Pd complexes including free Pd(OAc) 2 and a PEPPSI-type complex (Table 1). The reaction between 1,2-dimethylimidazole and 4-chloroacetophenone was taken as the benchmark reaction, following similar conditions 48 established previously: 2 mol% Pd precatalyst; N , N -dimethylacetamide (DMA) as a solvent; K 2 CO 3 as a base; pivalic acid (PivOH) as an additive; a heating temperature of 130 °C for a duration of 18 h. Among all the complexes tested, MIC complex 4a afforded the best yield of the quantitative formation of the coupled product (entry 1). This activity is superior to that delivered by free Pd(OAc) 2 and the PEPPSI-type complex bearing an IMes ligand, 57 which only afforded yields of 45 and 64%, respectively (entries 5 and 6).…”

“…39 Several palladium carbene complexes have successfully catalyzed the direct arylation reaction of imidazoles with aryl halides but only a few of them could use less reactive aryl chlorides as substrates (vide infra). 34,[40][41][42][43][44][45][46][47][48] Herein, we were interested in the preparation of new palladium complexes with carbene moieties changed to a triazolebased carbene donor instead. The target complexes are also shown in Fig.…”

“…39 Several palladium carbene complexes have successfully catalyzed the direct arylation reaction of imidazoles with aryl halides but only a few of them could use less reactive aryl chlorides as substrates ( vide infra ). 34,40–48…”

Ligand‐assisted Palladium Catalyzed C5 Arylation Reaction of Imidazoles with Aryl Chlorides

“…Thus, recent advances in palladium‐ based complex catalyzed C5 arylation of imidazole were achieved by Doucet, [13] Lee, [14] Liu, [15] etc [16] . In the reported literature, direct arylation with aryl chlorides mostly using pivalic acid as additives [12k,14d,17c,f] . Despite major progress, stoichiometric amounts of pivalic acid reagents were required, which promoting the tolerance of synthetically useful functional groups of aryl bromides and chlorides [17]…”