A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.

On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Couplings – Homogeneous or Heterogeneous Catalysis, A Critical Review

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions—A Synthetic Chemist's Perspective

Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N-Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition-metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.

The Measure of All Rings—N‐Heterocyclic Carbenes

Cross-coupling reactions using Pd-NHC (NHC = N-heterocyclic carbene) catalysts are discussed in this critical review and examined in terms of catalytic activity and how these have permitted advances in the area as they developed (95 references).

N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union

N-heterocyclic carbene analogues with low-valent group 13 and group 14 elements: syntheses, structures, and reactivities of a new generation of multitalented ligands.

N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5) react with Ni(CO)4 to give the saturated tricarbonyl complexes Ni(CO)3(IMes) (8), Ni(CO)3(SIMes) (9), Ni(CO)3(IPr) (10), Ni(CO)3(SIPr) (11), and Ni(CO)3(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)3(PR3) by recording their νCO stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1−5 are surprisingly small. Novel, unsaturated Ni(CO)2(IAd) (13) and Ni(CO)2(ItBu) (14) compounds are obtained from the reaction of Ni(CO)4 with IAd (6) and ItBu (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni−C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8−14 are in line wi...

Steric and electronic properties of N-heterocyclic carbenes (NHC) : A detailed study on their interaction with Ni(CO)4

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

Activation and reactivity of (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes

Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety.

The cotalytic application of the newly commercial available complex (IPr)Pd(allyl)Cl (IPr = N,N'-bis(2,6-diisopropyl)imidazol-2-ylidene) is described. This complex is an excellent source in generating the active 12-electron [(IPr)Pd(O)] palladium species under mild catalytic conditions. As a result it performs a number of organic transformations very effectively and allows for the use of the inexpensive aryl chlorides as coupling partners in cross-coupling chemistry. These non-phosphine palladium systems also permit simple workup procedures. Processes incorporating these catalysts seem feasable as several well defined Pd NHC complexes are availiable at industrial scale.

NM Scott

Papers

Steric and electronic properties of N-heterocyclic carbenes (NHC) : A detailed study on their interaction with Ni(CO)4

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Activation and reactivity of (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes

Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety.

[(IPr)Pd(0)] species : a versatile, well-defined catalyst precursor for cross coupling reactions