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 on the reaction between azobenzene and palladium(II) dichloride. [1] The potential of palladacycles as catalysts was highlighted by the invention of Herrmann's catalyst in the 1990s, as 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.
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