Heterocyclic Carbene–Metal-catalyzed Csp2–Csp2 and Csp–Csp2 Couplings Using Nonmetallic Substrates

Abstract

The use of carbene ligands for transition-metal complexes has been developed in the last decades, being of special interest those carbenes derived from a nitrogen-containing heterocyclic system. An interesting variety of carbenemetal complexes has been tested in the MizorokiHeck reaction. In comparison, few examples can be found for the MatsudaHeck version of this coupling reaction. Additionally, the Sonogashira coupling has been also catalyzed with different carbenemetal catalysts. Ç IntroductionN-Heterocyclic carbenes (NHCs) have emerged in the last years as a new family of ligands with interesting electronic and structural properties.1 The combination of their strong ·-donor and poor ³-acceptor abilities results in good coordination stability and versatility. Consequently, the use of NHC ligands based on imidazolium ions and related heterocycles has appeared as an alternative to phosphanes in the design of new organometallic catalysts.2 One of the main advantages of employing NHC-based catalysts compared with phosphanebased ones is that they are far less prone to oxidation, allowing their prolonged use, and reuse. Although NHCs were presented as simple tertiary phophane mimics, they have been recognized as much more over the past decade. Therefore, NHC ligands have become an interesting alternative to other types of ligands in different cross-coupling catalytic reactions mediated by transition metals.3 Moreover, the easy introduction of chiral elements in the corresponding precursors have made chiral NHCs promising chiral ligands in metal-based asymmetric catalysis. 4 The comparison between phosphane and NHC ligands, from a mechanistic point of view, has been studied, clear differences being found.5 First, the possibility of carbene ligand dissociation is greatly reduced due to its stronger donor ability; and second, the reactivity of the NHCmetal complex is higher probably caused by the lack of back-donation from the metal to the carbene ligand. Jutand and co-workers studied the reactivity of monocarbene and bis(carbene) Pd(0) complexes toward the oxidative addition of aryl halides, concluding that the electronic and steric properties of the carbene ligand are important factors in this step of the catalytic process. 6 Among the carboncarbon coupling reactions, the activation of a CH from an alkene (MizorokiHeck reaction) or an alkyne (Sonogashira reaction) are of special interest. This highlight review describes the progress on the topic of NHC ligands for transition-metal-catalyzed coupling reactions, employing nonmetallic reagents. Thus, both different NHC precursors and NHCmetal complexes with catalytic activity in the Mizoroki Heck, MatsudaHeck, and Sonogashira reactions are compiled. Ç Mizoroki-Heck ReactionIn 1995, Herrmann and co-workers introduced the use of bis(NHC)Pd complexes as catalysts with long-term stability in the MizorokiHeck reaction.7 Indeed, the carbenes derived from imidazole stabilized the palladium complexes due to their pronounced donor properties, being more stable toward...

Nickel N-heterocyclic carbene complexes and their utility in homogeneous catalysis

“…Heck coupling is the cross-coupling reaction of the activated olefins with aryl halides catalyzed predominantly by palladium [257] though a few isolated reports of catalysis by nickel [258] also exist (Scheme 20). Specifically, the complex (666) [202] displayed activity towards the Heck coupling of bromide and iodide substrates while the complex (756) [234] carried out the Heck coupling of the chloride, bromide and iodide substrates.…”

Comparative Study of Catalytic Systems Formed by Palladium and Acyl‐Substituted Imidazolium Salts

“…Although, the use of ligands in the Matsuda‐Heck (MH) reaction may help stabilize the species involved in the catalytic cycle. Indeed, carbene, macrocyclic olefin and thiourea derivatives have been described as capable ligands in the MH reaction . Thus, MH coupling reaction is interesting for comparing the ability during the catalysis of related catalytic systems.…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…Formation of palladium nanoparticles in the reaction mixture was confirmed by dynamic light scattering and transmission electron microscopy studies and a mercury poisoning experiment. The complexes C5, bearing benzimidazole and pyridine groups have been proved to be a highly efficient catalyst for the coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields (Table 1, entries [16][17][18][19][20][21][22]. 43 Electron-deficient aryl bromides gave a slightly higher yield than electron-rich ones under the optimized conditions.…”

“…Their steric bulk enables stabilization of a low-valent active intermediate and favors reductive elimination, while the strong σ -donor character facilitates the oxidative addition of aryl halides. Complex C96 was proven to be superior to its [Pd(IPr*)(acac)Cl] congener (Table 14, entries [18][19][20][21]. 135 In this context, (IPent Cl )PdCl 2 (o-Picoline) (C97)…”

“…Suzuki coupling reactions carried out using immobilized Pd-NHC catalysts. The palladium catalyst C85 based on modified halloysite nanotubes displayed good activity, allowing the synthesis of several biphenyl compounds in high yield working with only 0.1 mol% palladium loading (Table 12, entries [14][15][16][17][18][19] Applications of a polymer supported air-stable palladium NHC complex with a spacer (catalyst C90,…”