Cyclometalated 2-Phenylimidazole Palladium Carbene Complexes in the Catalytic Suzuki–Miyaura Cross-Coupling Reaction

Abstract

We present the syntheses of 12 cyclometalated palladium C ∧ N 2-phenylimidazole carbene complexes with different N-1 groups as well as different substituents at the C-2 phenyl group of the cyclometalating imidazole. We investigated the influence of these substituents by comparing the catalytic performance of the complexes in the Suzuki−Miyaura cross-coupling reaction of aryl chlorides. We can show a strong dependence between the steric demand of the N-1 substituent of the cyclometalating imidazole and the catalytic activity in the cross-coupling reaction. The most active complex shows a wide substrate scope, where several aryl as well as benzyl chlorides could be coupled with different boronic acids in excellent yields using very low catalyst concentrations of 0.05 mol %.

Highly Efficient Bulky α‐Diimine Palladium Complexes for Suzuki‐Miyaura Cross‐Coupling Reaction

“…However, it is unlikely that a quantitative in-situ complex formation occurs in a short reaction time, which leads to a waste of palladium source and ligand. [36][37][38] A series of benzimidazolium-pyrazole palladium complexes have been successfully developed by Andy Hor and his collaborators, [39] which showed high catalytic activity for cross-coupling of aryl halides with arylboronic acids under homogeneous conditions. [27][28][29] Recently, several air-stable phosphine-free palladium complexes have been reported for use in the Suzuki-Miyaura cross-coupling reaction.…”

Piperazine‐ and DABCO‐bridged dinuclear N‐heterocyclic carbene palladium complexes: synthesis, structure and application to Hiyama coupling reaction

“…For example, Organ and co‐workers reported that (NHC)PdCl 2 (pyridine) were more active than their 3‐chloropyridine analogues, due to either a higher dissociation rate of pyridine or a higher tendency to re‐coordinate to [(NHC)Pd(0)] species . Based on this, other types of N‐donors have received attention and been introduced into the coordination with NHC–Pd complexes, including triethylamine, diethylamine, 1‐methylimidazole, 2‐phenyl‐4,5‐dihydrooxazole, morpholine, 2‐phenylimidazole, acetonitrile, benzoxazole (benzothiazole), http://pubs.rsc.org/en/content/articlelanding/2016/ob/c6ob00013d, 1‐methylpyrazole (1‐methylindazole), etc. All of them have shown promising catalytic activity in organic transformations.…”

Synthesis of a cyclometalated 1,3,5-triphenylpyrazole palladium dimer and its activity towards cross coupling reactions

“…The first report on an application of a cyclopalladated trio-tolyl-phosphine complex for cross-coupling reactions by Herrmann and co-workers 32 raised high expectations for the palladacycles. Since then several groups have shown interest in synthesizing CN, [33][34][35][36][37][38][39][40][41][42][43][44][45][46] CS, 47,48 and CP 32,[49][50][51][52][53] palladacycle catalysts or precatalysts and pincer [54][55][56][57] palladium complexes. The CN palladacycles [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] gained attention since the pioneering work of Milstein and co-workers, 33 who introduced a wide variety of imine-based palladacycles for the cross-coupling reactions.…”

“…Since then several groups have shown interest in synthesizing CN, [33][34][35][36][37][38][39][40][41][42][43][44][45][46] CS, 47,48 and CP 32,[49][50][51][52][53] palladacycle catalysts or precatalysts and pincer [54][55][56][57] palladium complexes. The CN palladacycles [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] gained attention since the pioneering work of Milstein and co-workers, 33 who introduced a wide variety of imine-based palladacycles for the cross-coupling reactions. The importance of CN palladacycles was expanded significantly by Buchwald, 40 Gladysz, 36 Najera,…”