# Coupling reaction

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 (where R = organic group, M = main group centre metal atom) 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.
Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki were awarded the 2010 Nobel Prize in Chemistry for developing palladium-catalyzed cross coupling reactions.
Broadly speaking, two types of coupling reactions are recognized:
* Homocouplings joining two identical partners. The product is symmetrical R−R
* Heterocouplings joining two different partners. These reactions are also called cross-coupling reactions. The product is unsymmetrical, R−R'.

## Homo-coupling types

Coupling reactions are illustrated by the Ullmann reaction:
| Reaction | Year | Organic compound | Organic compound | Coupler | Remark |
| Wurtz reaction | 1855 | R-X | sp$^3$ | Na as reductant | dry ether as medium |
| Pinacol coupling reaction | 1859 | R-HC=O or R$_2$(C=O) | | various metals | requires proton donor |
| Glaser coupling | 1869 | RC≡CH | sp | Cu | O$_2$ as H-acceptor |
| Ullmann reaction | 1901 | Ar-X | sp$^2$ | Cu | high temperatures |
| Fittig reaction | | Ar-X | sp$^2$ | Na | dry ether as medium |
| Scholl reaction | 1910 | ArH | sp$^2$ | NaAlCl$_4$(l) | O$_2$ as H-acceptor; presumably trace Fe$^{3+}$ catalyst; requires high heat |

## Cross-coupling types

| Reaction | Year | Reactant A | Reactant A | Reactant B | Reactant B | Catalyst | Remark |
| Grignard reaction | 1900 | R-MgBr | sp, sp$^2$, sp$^3$ | R-HC=O or R(C=O)R$_2$ | sp$^2$ | not catalytic |
| Gomberg–Bachmann reaction | 1924 | Ar-H | sp$^2$ | Ar'-N$_2$$^+$X$^−$ | sp$^2$ | not catalytic |
| Cadiot–Chodkiewicz coupling | 1957 | RC≡CH | sp | RC≡CX | sp | Cu | requires base |
| Castro–Stephens coupling | 1963 | RC≡CH | sp | Ar-X | sp$^2$ | Cu | |
| Corey–House synthesis | 1967 | R$_2$CuLi or RMgX | sp$^3$ | R-X | sp$^2$, sp$^3$ | Cu | Cu-catalyzed version by Kochi, 1971 |
| Cassar reaction | 1970 | Alkene | sp$^2$ | R-X | sp$^3$ | Pd | requires base |
| Kumada coupling | 1972 | Ar-MgBr | sp$^2$, sp$^3$ | Ar-X | sp$^2$ | Pd or Ni or Fe | |
| Heck reaction | 1972 | alkene | sp$^2$ | Ar-X | sp$^2$ | Pd or Ni | requires base |
| Sonogashira coupling | 1975 | RC≡CH | sp | R-X | sp$^3$ sp$^2$ | Pd and Cu | requires base |
| Murahashi coupling | 1975 | RLi | sp$^2$, sp$^3$ | Ar-X | sp$^2$ | Pd or Ni | Pd-catalyzed version by Murahashi, 1979 |
| Negishi coupling | 1977 | R-Zn-X | sp$^3$, sp$^2$, sp | R-X | sp$^3$ sp$^2$ | Pd or Ni | |
| Stille reaction | 1978 | R-SnR$_3$ | sp$^3$, sp$^2$, sp | R-X | sp$^3$ sp$^2$ | Pd | |
| Suzuki reaction | 1979 | R-B(OR)$_2$ | sp$^2$ | R-X | sp$^3$ sp$^2$ | Pd or Ni | requires base |
| Hiyama coupling | 1988 | R-SiR$_3$ | sp$^2$ | R-X | sp$^3$ sp$^2$ | Pd | requires base |
| Buchwald–Hartwig amination | 1994 | R$_2$N-H | sp$^3$ | R-X | sp$^2$ | Pd | N-C coupling, second generation free amine |
| Fukuyama coupling | 1998 | R-Zn-I | sp$^3$ | RCO(SEt) | sp$^2$ | Pd or Ni | |
| Liebeskind–Srogl coupling | 2000 | R-B(OR)$_2$ | sp$^3$, sp$^2$ | RCO(SEt) Ar-SMe | sp$^2$ | Pd | requires CuTC |
| (Li) Cross dehydrogenative coupling(CDC) | 2004 | R-H | sp, sp$^2$, sp$^3$ | R'-H | sp, sp$^2$, sp$^3$ | Cu, Fe, Pd etc | requires oxidant or dehydrogenation |
| Wurtz–Fittig reaction | 1864 | R-X | sp$^3$ | Ar-X | sp$^2$ | Na | dry ether |

## Applications

Coupling reactions are routinely employed in the preparation of pharmaceuticals. Conjugated polymers are prepared using this technology as well.