Palladacycle

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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.

History of the palladacycle discovery[edit]

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]

The first palladacycle from aromatic azo compounds and the Herrmann’s catalyst.

Classes of palladacycles[edit]

There are two distinct types of palladacycle: four-electron donor (CY) and six-electron donor (YCY) complexes.

CY-/YCY-type palladacycles

Neutral, cationic and anionic palladacycles[edit]

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.

Examples of neutral, cationic and anionic palladacycles.

Palladacycles with various ring-sizes[edit]

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.

Examples of palladacycles with various ring-sizes

Palladacycles with various donor groups[edit]

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.

Synthesis of palladacycles[edit]

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.

Preparation of palladacycles via C-H activation and oxidative addition.

Other types of reactions such as transmetalation[6] and nucleopalladation[7] also turned out to be effective methods in the synthesis of palladacycles.

Applications as precatalysts[edit]

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.

Activation of Buchwald palladacycle pre-catalysts.

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]

Palladacycles as intermediate and pre-catalyst in total synthesis

Other applications[edit]

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]

Further reading[edit]

  • Beletskaya, Irina P.; Cheprakov, Andrei V. (November 2004). "Palladacycles in catalysis – a critical survey". Journal of Organometallic Chemistry. 689 (24): 4055–4082. doi:10.1016/j.jorganchem.2004.07.054.
  • Dupont, Jairton; Consorti, Crestina S.; Spencer, John (2005-06-01). "The Potential of Palladacycles: More Than Just Precatalysts". Chemical Reviews. 105 (6): 2527–2572. doi:10.1021/cr030681r. ISSN 0009-2665. PMID 15941221.

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.

References[edit]

  1. ^ Cope, Arthur C.; Siekman, Robert W. (July 1965). "Formation of Covalent Bonds from Platinum or Palladium to Carbon by Direct Substitution". Journal of the American Chemical Society. 87 (14): 3272–3273. doi:10.1021/ja01092a063. ISSN 0002-7863.
  2. ^ Herrmann, Wolfgang A.; Brossmer, Christoph; Reisinger, Claus-Peter; Riermeier, Thomas H.; Öfele, Karl; Beller, Matthias (August 1997). "Palladacycles: Efficient New Catalysts for the Heck Vinylation of Aryl Halides". Chemistry - A European Journal (in German). 3 (8): 1357–1364. doi:10.1002/chem.19970030823.
  3. ^ Trofimenko, S. (1973-06-01). "Cyclopalladation reaction". Inorganic Chemistry. 12 (6): 1215–1221. doi:10.1021/ic50124a001. ISSN 0020-1669.
  4. ^ Parshall, George W. (1970-04-01). "Intramolecular aromatic substitution in transition metal complexes". Accounts of Chemical Research. 3 (4): 139–144. doi:10.1021/ar50028a004. ISSN 0001-4842.
  5. ^ Rodríguez, Gema; Albrecht, Martin; Schoenmaker, Jeroen; Ford, Alan; Lutz, Martin; Spek, Anthony L.; van Koten, Gerard (May 2002). "Bifunctional Pincer-type Organometallics as Substrates for Organic Transformations and as Novel Building Blocks for Polymetallic Materials". Journal of the American Chemical Society. 124 (18): 5127–5138. doi:10.1021/ja0177657. hdl:1874/14925. ISSN 0002-7863. PMID 11982378. S2CID 31662816.
  6. ^ Grove, David M.; Van Koten, Gerard; Louwen, Jaap N.; Noltes, Jan G.; Spek, Anthony L.; Ubbels, Henk J. C. (December 1982). "Trans-2,6-bis[(dimethylamino)methyl]phenyl-N,N',C complexes of palladium(II) and platinum(II). Crystal structure of [PtI[MeC6H3(CH2NMe2)2-o,o']]BF4: a cyclohexadienyl carbonium ion with a .sigma.-bonded metal substituent". Journal of the American Chemical Society. 104 (24): 6609–6616. doi:10.1021/ja00388a022. ISSN 0002-7863.
  7. ^ Holton, Robert A.; Kjonaas, Richard A. (June 1977). "Carbopalladation-depalladation of allylic amines and sulfides". Journal of the American Chemical Society. 99 (12): 4177–4179. doi:10.1021/ja00454a057. ISSN 0002-7863.
  8. ^ 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.
  9. ^ Lu, Ting-Yi; Xue, Cuihua; Luo, Fen-Tair (February 2003). "Palladium-catalyzed cross-coupling reaction of aryldioxaborolane with 2-bromo-N,N-dimethylacetamide". Tetrahedron Letters. 44 (8): 1587–1590. doi:10.1016/S0040-4039(03)00066-2.
  10. ^ Brun, Virginie; Legraverend, Michel; Grierson, David S (September 2002). "Traceless solid-phase synthesis of 2,6,9-trisubstituted purines from resin bound 6-thiopurines". Tetrahedron. 58 (39): 7911–7923. doi:10.1016/S0040-4020(02)00905-5.
  11. ^ Prinz, Peter; Lansky, Annegret; Knieriem, Burkhard; de Meijere, Armin; Haumann, Thomas; Boese, Roland; Noltemeyer, Matthias (1997-07-04). "Palladium-Catalyzed Sixfold Alkenylation of Hexabromobenzene: An Interesting Case of Self-Organization". Angewandte Chemie International Edition in English. 36 (12): 1289–1292. doi:10.1002/anie.199712891. ISSN 0570-0833.
  12. ^ Zim, Danilo; Buchwald, Stephen L. (July 2003). "An Air and Thermally Stable One- Component Catalyst for the Amination of Aryl Chlorides". Organic Letters. 5 (14): 2413–2415. doi:10.1021/ol034561h. ISSN 1523-7060. PMID 12841743.
  13. ^ Holton, Robert A. (November 1977). "Prostaglandin synthesis via carbopalladation". Journal of the American Chemical Society. 99 (24): 8083–8085. doi:10.1021/ja00466a069. ISSN 0002-7863.
  14. ^ De Meijere, Armin; Schelper, Michael; Knoke, Mario; Yucel, Baris; Sünnemann, Hans Wolf; Scheurich, René Peter; Arve, Lars (2003-12-07). "Palladium-catalyzed cross-coupling reactions and electrocyclizations—efficient combinations for new cascade reactions". Journal of Organometallic Chemistry. 687 (2): 249–255. doi:10.1016/j.jorganchem.2003.07.007. ISSN 0022-328X.
  15. ^ Kapdi, Anant (2019). Palladacycles : catalysis and beyond. Debabrata Maiti (First ed.). Amsterdam. ISBN 978-0-12-816516-4. OCLC 1104998787.{{cite book}}: CS1 maint: location missing publisher (link)
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