Cross-coupling reaction

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:

R−M + R'−X → R−R' + MX (R, R' = organic fragments, usually aryl; M = main group center such as Li or MgX; X = halide)

These reactions are used to form carbon–carbon bonds but also carbon-heteroatom bonds.[1][2][3][4] Cross-coupling reaction are a subset of coupling reactions.

Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki were awarded the 2010 Nobel Prize in Chemistry for developing palladium-catalyzed coupling reactions.[5][6]

Mechanism

Many mechanisms exist reflecting the myriad types of cross-couplings, including those that do not require metal catalysts.[7] Often, however, cross-coupling refers to a metal-catalyzed reaction of a nucleophilic partner with an electrophilic partner.

In such cases, the mechanism generally involves reductive elimination of R-R' from LnMR(R') (L = spectator ligand). This intermediate LnMR(R') is formed in a two step process from a low valence precursor LnM. The oxidative addition of an organic halide (RX) to LnM gives LnMR(X). Subsequently, the second partner undergoes transmetallation with a source of R'−. The final step is reductive elimination of the two coupling fragments to regenerate the catalyst and give the organic product. Unsaturated substrates, such as C(sp)−X and C(sp2)−X bonds, couple more easily, in part because they add readily to the catalyst.

Catalysts

Catalysts are often based on palladium, which is frequently selected due to high functional group tolerance. Organopalladium compounds are generally stable towards water and air. Palladium catalysts can be problematic for the pharmaceutical industry, which faces extensive regulation regarding heavy metals. Many pharmaceutical chemists attempt to use coupling reactions early in production to minimize metal traces in the product.[8] Heterogeneous catalysts based on Pd are also well developed.[9]

Copper-based catalysts are also common, especially for coupling involving heteroatom-C bonds.[10][11]

Iron-,[12] cobalt-,[13] and nickel-based[14] catalysts have been investigated.

Leaving groups

The leaving group X in the organic partner is usually a halide, although triflate, tosylate and other pseudohalide have been used. Chloride is an ideal group due to the low cost of organochlorine compounds. Frequently, however, C–Cl bonds are too inert, and bromide or iodide leaving groups are required for acceptable rates. The main group metal in the organometallic partner usually is an electropositive element such as tin, zinc, silicon, or boron.

Carbon–carbon cross-coupling

Many cross-couplings entail forming carbon–carbon bonds.

ReactionYear Reactant A Reactant BCatalystRemark
Cadiot–Chodkiewicz coupling1957RC≡CHspRC≡CXspCurequires base
Castro–Stephens coupling1963RC≡CHspAr-Xsp2Cu
Corey–House synthesis1967R2CuLi or RMgXsp3 R-Xsp2, sp3 Cu Cu-catalyzed version by Kochi, 1971
Kumada coupling1972RMgBrsp2, sp3R-Xsp2Pd or Ni or Fe
Heck reaction1972alkenesp2Ar-Xsp2Pd or Nirequires base
Sonogashira coupling1975ArC≡CHspR-Xsp3 sp2Pd and Curequires base
Negishi coupling1977R-Zn-Xsp3, sp2, spR-Xsp3 sp2Pd or Ni
Stille cross coupling1978R-SnR3sp3, sp2, spR-Xsp3 sp2Pd or Ni
Suzuki reaction1979R-B(OR)2sp2R-Xsp3 sp2Pd or Nirequires base
Murahashi coupling[15] 1979 R-Li sp2, sp3 R-X sp2 Pd or Ru
Hiyama coupling1988R-SiR3sp2R-Xsp3 sp2Pdrequires base
Fukuyama coupling1998R-Zn-Isp3RCO(SEt)sp2Pd or Nisee Liebeskind–Srogl coupling, gives ketones
Liebeskind–Srogl coupling2000R-B(OR)2sp3, sp2RCO(SEt) Ar-SMesp2Pdrequires CuTC, gives ketones
Cross dehydrogenative coupling2004R-Hsp, sp2, sp3R'-Hsp, sp2, sp3Cu, Fe, Pd etc.requires oxidant or dehydrogenation
Decarboxylative cross-coupling2000sR-CO2Hsp2R'-Xsp, sp2Cu, PdRequires little-to-no base

The restrictions on carbon atom geometry mainly inhibit β-hydride elimination when complexed to the catalyst.[16]

Carbon–heteroatom coupling

Many cross-couplings entail forming carbon–heteroatom bonds (heteroatom = S, N, O). A popular method is the Buchwald–Hartwig reaction:

(Eq.1)
ReactionYear Reactant A Reactant BCatalystRemark
Ullmann-type reaction1905 ArO-MM, ArNH2,RS-M,NC-Msp3Ar-X (X = OAr, N(H)Ar, SR, CN)sp2Cu
Buchwald–Hartwig reaction[17]1994R2N-Hsp3R-Xsp2PdN-C coupling,
second generation free amine
Chan–Lam coupling[18]1998Ar-B(OR)2sp2Ar-NH2sp2Cu

Miscellaneous reactions

Palladium-catalyzes the cross-coupling of aryl halides with fluorinated arene. The process is unusual in that it involves C–H functionalisation at an electron deficient arene.[19]

Applications

Cross-coupling reactions are important for the production of pharmaceuticals,[4] examples being montelukast, eletriptan, naproxen, varenicline, and resveratrol.[20] with Suzuki coupling being most widely used.[21] Some polymers and monomers are also prepared in this way.[22]

Reviews
  • Fortman, George C.; Nolan, Steven P. (2011). "N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union". Chemical Society Reviews. 40 (10): 5151–69. doi:10.1039/c1cs15088j. PMID 21731956.
  • Yin; Liebscher, Jürgen (2007). "Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts". Chemical Reviews. 107 (1): 133–173. doi:10.1021/cr0505674. PMID 17212474. S2CID 36974481.
  • Jana, Ranjan; Pathak, Tejas P.; Sigman, Matthew S. (2011). "Advances in Transition Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-organometallics as Reaction Partners". Chemical Reviews. 111 (3): 1417–1492. doi:10.1021/cr100327p. PMC 3075866. PMID 21319862.
  • Molnár, Árpád (2011). "Efficient, Selective, and Recyclable Palladium Catalysts in Carbon−Carbon Coupling Reactions". Chemical Reviews. 111 (3): 2251–2320. doi:10.1021/cr100355b. PMID 21391571.
  • Miyaura, Norio; Suzuki, Akira (1995). "Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds". Chemical Reviews. 95 (7): 2457–2483. CiteSeerX 10.1.1.735.7660. doi:10.1021/cr00039a007.
  • Roglans, Anna; Pla-Quintana, Anna; Moreno-Mañas, Marcial (2006). "Diazonium Salts as Substrates in Palladium-Catalyzed Cross-Coupling Reactions". Chemical Reviews. 106 (11): 4622–4643. doi:10.1021/cr0509861. PMID 17091930. S2CID 8128630.
  • Korch, Katerina M.; Watson, Donald A. (2019). "Cross-Coupling of Heteroatomic Electrophiles". Chemical Reviews. 119 (13): 8192–8228. doi:10.1021/acs.chemrev.8b00628. PMC 6620169. PMID 31184483.
  • Cahiez, Gérard; Moyeux, Alban (2010). "Cobalt-Catalyzed Cross-Coupling Reactions". Chemical Reviews. 110 (3): 1435–1462. doi:10.1021/cr9000786. PMID 20148539.
  • Yi, Hong; Zhang, Guoting; Wang, Huamin; Huang, Zhiyuan; Wang, Jue; Singh, Atul K.; Lei, Aiwen (2017). "Recent Advances in Radical C–H Activation/Radical Cross-Coupling". Chemical Reviews. 117 (13): 9016–9085. doi:10.1021/acs.chemrev.6b00620. PMID 28639787.

References
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  11. Evano, Gwilherm; Blanchard, Nicolas; Toumi, Mathieu (2008). "Copper-Mediated Coupling Reactions and Their Applications in Natural Products and Designed Biomolecules Synthesis". Chemical Reviews. 108 (8): 3054–3131. doi:10.1021/cr8002505. PMID 18698737.
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  14. Rosen, Brad M.; Quasdorf, Kyle W.; Wilson, Daniella A.; Zhang, Na; Resmerita, Ana-Maria; Garg, Neil K.; Percec, Virgil (2011). "Nickel-Catalyzed Cross-Couplings Involving Carbon−Oxygen Bonds". Chemical Reviews. 111 (3): 1346–1416. doi:10.1021/cr100259t. PMC 3055945. PMID 21133429.
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