Palladacycle, as a class of metallacycles, refers to complexes containing at least one carbon-palladium bond. Palladacycles are invoked as intermediates in catalytic or palladium mediated reactions. They have been investigated as pre-catalysts for homogeneous catalysis and synthesis.
In the 1960s, Arthur C. Cope and Robert W. Siekman reported the cyclopalladation reaction between aromatic azobenzenes and palladium(II) dichloride. [1] The potential of palladacycles as catalysts was highlighted by Herrmann's catalyst in 1990s. Derivatives of tris(o-tolyl)phosphine proved effective in Heck reactions. [2]
There are two distinct types of palladacycle: four-electron donor (CY) and six-electron donor (YCY) complexes.
The palladacycles can be neutral, cationic, or anionic. Depending on the nature of the coordinating ligands, the neutral palladacycles can be monomers, dimers, or bis-cyclopalladated.
Palladacycles with ring-sizes range from 3 to 10 have been synthesized and characterized, whereas only 5-/6-membered ones are commonly used. Palladacycles of 3-/4-/>6-membered ring-sizes are usually unstable due to their ring strains.
The palladacycles could also be classified by the donor atoms. For example, the Herrmann’s catalyst discussed before is a phosphine-derived palladacycle. Other types of palladacycles such as phosphite palladacycle, imine palladacycle, oxime palladacycle, CS-/CO-palladacycles are also effective in catalytic reactions. Palladacycles derived from 2-aminobiphenyl have been used in a variety of cross-coupling reactions.
Several methods are available for the preparation of palladacycles. A simple and direct method is C–H activation. [3] The cyclopalladation of aromatic derivatives is usually considered to go through an electrophilic aromatic substitution pathway. [4] The oxidative addition of aryl halides is another useful method. [5] However, the accessibility of the aryl halides starting material is a major drawback.
Other types of reactions such as transmetalation [6] and nucleopalladation [7] also turned out to be effective methods in the synthesis of palladacycles.
Palladacycles are used as pre-catalysts, usually by the reductive elimination from palladium(II) to the catalytically active palladium(0). In the example of 2-aminobiphenyl palladacycles, a kinetically active 12-electrons Pd(0) species is formed, allowing for further oxidative addition with reactants. [8] A series of 2-aminobiphenyl bearing various X and L groups were synthesized to better understand the electron/steric effect.
By employing palladacycles as pre-catalysts, high reactivity and selectivity have been achieved in Heck reaction[2] and a variety of cross-coupling reactions, such as Suzuki, [9] Sonogashira, [10] Stille, [11] Buchwald–Hartwig reactions. [12]
Total synthesis containing palladacycles have been demonstrated. [13] [14]
Except their abilities in catalyzing organic reactions, palladacycles have also shown their potential in medicinal and biological chemistry after the success of cis-Pt(NH3)2Cl2 as an anticancer agent. Additionally, they can also be used in CO/SCN- sensing. [15]
Bruneau, Alexandre; Roche, Maxime; Alami, Mouad; Messaoudi, Samir (2015-02-06). "2-Aminobiphenyl Palladacycles: The "Most Powerful" Precatalysts in C–C and C–Heteroatom Cross-Couplings". ACS Catalysis. 5 (2): 1386–1396. doi:10.1021/cs502011x. ISSN 2155-5435.
The Heck reaction is the chemical reaction of an unsaturated halide with an alkene in the presence of a base and a palladium catalyst to form a substituted alkene. It is named after Tsutomu Mizoroki and Richard F. Heck. Heck was awarded the 2010 Nobel Prize in Chemistry, which he shared with Ei-ichi Negishi and Akira Suzuki, for the discovery and development of this reaction. This reaction was the first example of a carbon-carbon bond-forming reaction that followed a Pd(0)/Pd(II) catalytic cycle, the same catalytic cycle that is seen in other Pd(0)-catalyzed cross-coupling reactions. The Heck reaction is a way to substitute alkenes.
The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling a halide (R1-X) with an organoboron species (R2-BY2) using a palladium catalyst and a base. The organoboron species is usually synthesized by hydroboration or carboboration, allowing for rapid generation of molecular complexity.
The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vinyl halide.
In organic chemistry, a coupling reaction is a type of reaction in which two reactant molecules are bonded together. Such reactions often require the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M reacts with an organic halide of the type R'-X with formation of a new carbon-carbon bond in the product R-R'. The most common type of coupling reaction is the cross coupling reaction.
The Hiyama coupling is a palladium-catalyzed cross-coupling reaction of organosilanes with organic halides used in organic chemistry to form carbon–carbon bonds. This reaction was discovered in 1988 by Tamejiro Hiyama and Yasuo Hatanaka as a method to form carbon-carbon bonds synthetically with chemo- and regioselectivity. The Hiyama coupling has been applied to the synthesis of various natural products.
The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (C-C) in the process. A palladium (0) species is generally utilized as the metal catalyst, though nickel is sometimes used. A variety of nickel catalysts in either Ni0 or NiII oxidation state can be employed in Negishi cross couplings such as Ni(PPh3)4, Ni(acac)2, Ni(COD)2 etc.
In organic chemistry, the Kumada coupling is a type of cross coupling reaction, useful for generating carbon–carbon bonds by the reaction of a Grignard reagent and an organic halide. The procedure uses transition metal catalysts, typically nickel or palladium, to couple a combination of two alkyl, aryl or vinyl groups. The groups of Robert Corriu and Makoto Kumada reported the reaction independently in 1972.
Organonickel chemistry is a branch of organometallic chemistry that deals with organic compounds featuring nickel-carbon bonds. They are used as a catalyst, as a building block in organic chemistry and in chemical vapor deposition. Organonickel compounds are also short-lived intermediates in organic reactions. The first organonickel compound was nickel tetracarbonyl Ni(CO)4, reported in 1890 and quickly applied in the Mond process for nickel purification. Organonickel complexes are prominent in numerous industrial processes including carbonylations, hydrocyanation, and the Shell higher olefin process.
XPhos is a phosphine ligand derived from biphenyl. Its palladium complexes exhibit high activity for Buchwald-Hartwig amination reactions involving aryl chlorides and aryl tosylates. Both palladium and copper complexes of the compound exhibit high activity for the coupling of aryl halides and aryl tosylates with various amides. It is also an efficient ligand for several commonly used C–C bond-forming cross-coupling reactions, including the Negishi, Suzuki, and the copper-free Sonogashira coupling reactions. It is especially efficient and general when employed as a (2-aminobiphenyl)-cyclometalated palladium mesylate precatalyst complex, XPhos-G3-Pd, which is commercially available and stable to bench storage. The ligand itself also has convenient handling characteristics as a crystalline, air-stable solid.
In organic chemistry, a cross-coupling reaction is a reaction where two different fragments are joined. Cross-couplings are a subset of the more general coupling reactions. Often cross-coupling reactions require metal catalysts. One important reaction type is this:
Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.
The Catellani reaction was discovered by Marta Catellani and co-workers in 1997. The reaction uses aryl iodides to perform bi- or tri-functionalization, including C-H functionalization of the unsubstituted ortho position(s), followed a terminating cross-coupling reaction at the ipso position. This cross-coupling cascade reaction depends on the ortho-directing transient mediator, norbornene.
In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds in which palladium forms a coordination complex with N-heterocyclic carbenes (NHCs). They have been investigated for applications in homogeneous catalysis, particularly cross-coupling reactions.
In organic chemistry, the Fujiwara–Moritani reaction is a type of cross coupling reaction where an aromatic C-H bond is directly coupled to an olefinic C-H bond, generating a new C-C bond. This reaction is performed in the presence of a transition metal, typically palladium. The reaction was discovered by Yuzo Fujiwara and Ichiro Moritani in 1967. An external oxidant is required to this reaction to be run catalytically. Thus, this reaction can be classified as a C-H activation reaction, an oxidative Heck reaction, and a C-H olefination. Surprisingly, the Fujiwara–Moritani reaction was discovered before the Heck reaction.
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Cobalt(II)–porphyrin catalysis is a process in which a Co(II) porphyrin complex acts as a catalyst, inducing and accelerating a chemical reaction.
Herrmann's catalyst is an organopalladium compound that is a popular catalyst for the Heck reaction. It is a yellow air-stable solid that is soluble in organic solvents. Under conditions for catalysis, the acetate group is lost and the Pd-C bond undergoes protonolysis, giving rise to a source of "PdP(o-tol)3".
F. Dean Toste is the Gerald E. K. Branch Distinguished Professor of Chemistry at the University of California, Berkeley and faculty scientist at the chemical sciences division of Lawrence Berkeley National Lab. He is a prominent figure in the field of organic chemistry and is best known for his contributions to gold chemistry and asymmetric ion-pairing catalysis. Toste was elected a member of the National Academy of Sciences in 2020, and a member of the American Academy of Arts and Sciences in 2018.
Organogermanium compounds in cross-coupling reactions refers to a type of cross-coupling reaction where one of the coupling partners is an organogermanium compound. Usually these reactions are catalyzed by transition metal complexes.
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