Coupling reaction

A coupling reaction in organic chemistry is a catch-all term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst. In one important reaction type a main group organometallic compound of the type RM (R = organic fragment, M = main group centre) reacts with an organic halide of the type R'X with formation of a new carbon-carbon bond in the product R-R' ^[1] Contributions to coupling reactions by Ei-ichi Negishi and Akira Suzuki were recognized with the 2010 Nobel Prize in Chemistry, which was shared with Richard F. Heck.^[2]

Broadly speaking, two types of coupling reactions are recognized:

• cross couplings involve reactions between two different partners, for example bromobenzene (PhBr) and vinyl chloride to give styrene (PhCH=CH₂).
• homocouplings couple two identical partners, for example, the conversion of iodobenzene (PhI) to biphenyl (Ph-Ph).

Mechanism

The reaction mechanism usually begins with oxidative addition of one organic halide to the catalyst. Subsequently, the second partner undergoes transmetallation, which places both coupling partners on the same metal centre. The final step is reductive elimination of the two coupling fragments to regenerate the catalyst and give the organic product. Unsaturated organic groups couple more easily in part because the add readily. The intermediates are also less prone to beta-hydride elimination.^[3]

In one computational study, unsaturated organic groups were shown to undergo much easier coupling reaction on the metal center.^[4] The rates for reductive elimination followed the following order: vinyl-vinyl > phenyl-phenyl > alkynyl-alkynyl > alkyl-alkyl. The activation barriers and the reaction energies for unsymmetrical R-R′ couplings were found to be close to the averages of the corresponding values of the symmetrical R-R and R′-R′ coupling reactions; for example: vinyl-vinyl > vinyl-alkyl > alkyl-alkyl. Another mechanistic approach proposes that specifically in aqueous solutions, coupling actually occurs via a radical mechanism rather than a metal-assisted one.^[5]

Catalysts

The most popular metal catalyst is palladium, but some processes often use nickel and copper. A common catalyst is tetrakis(triphenylphosphine)palladium(0). Palladium catalysed reactions have several advantages including functional group tolerance, low sensitivity of organopalladium compounds towards water and air.

Reviews have been written for example on cobalt,^[6] palladium ^{[7][8][9][10][11]} and nickel ^[12] mediated reactions and on applications ^[13][14]

Leaving groups

The leaving group X in the organic partner is usually bromide, iodide or triflate. Ideal leaving groups are chloride, since organic chlorides are cheaper than related compounds. The main group metal in the organometallic partner usually is tin, zinc, or boron.

Operating conditions

While many coupling reactions involve reagents that are extremely susceptible to presence of water or oxygen, it is unreasonable to assume that all coupling reactions need to be performed with strict exclusion of water. It is possible to perform palladium-based coupling reactions in aqueous solutions using the water-soluble sulfonated phosphines made by the reaction of triphenyl phosphine with sulfuric acid. Another example of coupling in aqueous media, with the main reacting agent being trimolybdenum-alkylidyne clusters, is that of Bogoslavsky et al..^[15] In general, the oxygen in the air is more able to disrupt coupling reactions, because many of these reactions occur via unsaturated metal complexes that do not have 18 valence electrons. For example, in nickel and palladium cross couplings, a zerovalent complex with two vacant sites (or labile ligands) reacts with the carbon halogen bond to form a metal halogen and a metal carbon bond. Such a zerovalent complex with labile ligands or empty coordination sites is normally very reactive toward oxygen.

Coupling types

Coupling reactions include (not exhaustive):

Reaction	year	Reactant A		Reactant B		homo/cross	catalyst	remark
Wurtz reaction	1855	R-X	sp³	R-X	sp³	homo	Na
Glaser coupling	1869	RC≡CH	sp	RC≡CH	sp	homo	Cu	O2 as H-acceptor
Ullmann reaction	1901	Ar-X	sp²	Ar-X	sp²	homo	Cu	high temperatures
Gomberg-Bachmann reaction	1924	Ar-H	sp²	Ar-N2X	sp²	homo		requires base
Cadiot-Chodkiewicz coupling	1957	RC≡CH	sp	RC≡CX	sp	cross	Cu	requires base
Castro-Stephens coupling	1963	RC≡CH	sp	Ar-X	sp²	cross	Cu
Gilman reagent coupling	1967	R2CuLi		R-X		cross
Cassar reaction	1970	Alkene	sp²	R-X	sp³	cross	Pd	requires base
Kumada coupling	1972	Ar-MgBr	sp², sp³	Ar-X	sp²	cross	Pd or Ni
Heck reaction	1972	alkene	sp²	R-X	sp²	cross	Pd	requires base
Sonogashira coupling	1975	RC≡CH	sp	R-X	sp³ sp²	cross	Pd and Cu	requires base
Negishi coupling	1977	R-Zn-X	sp³, sp², sp	R-X	sp³ sp²	cross	Pd or Ni
Stille cross coupling	1978	R-SnR3	sp³, sp², sp	R-X	sp³ sp²	cross	Pd
Suzuki reaction	1979	R-B(OR)2	sp²	R-X	sp³ sp²	cross	Pd	requires base
Hiyama coupling	1988	R-SiR3	sp²	R-X	sp³ sp²	cross	Pd	requires base
Buchwald-Hartwig reaction	1994	R2N-R SnR3	sp	R-X	sp²	cross	Pd	N-C coupling, second generation free amine
Fukuyama coupling	1998	RCO(SEt)	sp2	R-Zn-I	sp3	cross	Pd
Coupling reaction overview. For references consult satellite pages

Miscellaneous reactions

In one study, an unusual coupling reaction was described in which an organomolybdenum compound, [Mo₃(CCH₃)₂(OAc)₆(H₂O)₃](CF₃SO₃)₂ not only sat on a shelf for 30 years without any sign of degradation but also decomposed in water to generate 2-butyne, which is the coupling adduct of its two ethylidyne ligands. This, according to the researchers, opens another way for aqueous organometallic chemistry.^[16]

One method for palladium-catalyzed cross-coupling reactions of aryl halides with fluorinated arenes was reported by Keith Fagnou and co-workers. It is unusual in that it involves C-H functionalisation at an electron deficient arene.^[17]

Applications

Many coupling reactions have found their way into pharmaceutical industry ^[18] and into conjugated organic materials ^[19]

References

1. ^ Organic Synthesis using Transition Metals Rod Bates ISBN 978-1-84127-107-1
2. ^ "The Nobel Prize in Chemistry 2010 - Richard F. Heck, Ei-ichi Negishi, Akira Suzuki". NobelPrize.org. 2010-10-06. http://nobelprize.org/nobel_prizes/chemistry/laureates/2010/. Retrieved 2010-10-06. 
3. ^ Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 189138953X
4. ^ V. P. Ananikov, D. G. Musaev, K. Morokuma, “Theoretical Insight into the C-C Coupling Reactions of the Vinyl, Phenyl, Ethynyl, and Methyl Complexes of Palladium and Platinum” Organometallics 2005, 24, 715. doi:10.1021/om0490841
5. ^ Benny Bogoslavsky, Ophir Levy, Anna Kotlyar, Miri Salem, Faina Gelman and Avi Bino (published online: October 26, 2011). "Do Carbyne Radicals Really Exist in Aqueous Solution?". Angewandte Chemie International Edition. doi:10.1002/anie.201103652. PMID 22031005. 
6. ^ Cobalt-Catalyzed Cross-Coupling Reactions Grard Cahiez and Alban Moyeux Chem. Rev., 2010, 110 (3), pp 1435–1462 Publication Date (Web): February 11, 2010 (Review) doi:10.1021/cr9000786
7. ^ Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts Lunxiang Yin and Jürgen Liebscher Chem. Rev., 2007, 107 (1), pp 133–173 Publication Date (Web): December 21, 2006 (Article) doi:10.1021/cr0505674
8. ^ Advances in Transition Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-organometallics as Reaction Partners Ranjan Jana, Tejas P. Pathak, and Matthew S. Sigman Chem. Rev., 2011, 111 (3), pp 1417–1492 doi: 10.1021/cr100327p
9. ^ Efficient, Selective, and Recyclable Palladium Catalysts in Carbon−Carbon Coupling Reactions rpd Molnr Chem. Rev., 2011, 111 (3), pp 2251–2320 doi:10.1021/cr100355b
10. ^ Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds Norio. Miyaura, Akira. Suzuki Chem. Rev., 1995, 95 (7), pp 2457–2483 doi:10.1021/cr00039a007
11. ^ Diazonium Salts as Substrates in Palladium-Catalyzed Cross-Coupling Reactions Anna Roglans, Anna Pla-Quintana, and Marcial Moreno-Mañas Chem. Rev., 2006, 106 (11), pp 4622–4643 doi:10.1021/cr0509861
12. ^ Nickel-Catalyzed Cross-Couplings Involving Carbon−Oxygen Bonds Brad M. Rosen, Kyle W. Quasdorf, Daniella A. Wilson, Na Zhang, Ana-Maria Resmerita, Neil K. Garg, and Virgil Percec Chem. Rev., 2011, 111 (3), pp 1346–1416 doi:10.1021/cr100259t
13. ^ Selected Patented Cross-Coupling Reaction Technologies Jean-Pierre Corbet and Gérard Mignani Chem. Rev., 2006, 106 (7), pp 2651–2710 2006 (Article) doi:10.1021/cr0505268
14. ^ Copper-Mediated Coupling Reactions and Their Applications in Natural Products and Designed Biomolecules Synthesis Gwilherm Evano, Nicolas Blanchard and Mathieu Toumi Chem. Rev., 2008, 108 (8), pp 3054–3131 doi:10.1021/cr8002505
15. ^ Benny Bogoslavsky, Ophir Levy, Anna Kotlyar, Miri Salem, Faina Gelman and Avi Bino (published online: October 26, 2011). "Do Carbyne Radicals Really Exist in Aqueous Solution?". Angewandte Chemie International Edition. doi:10.1002/anie.201103652. PMID 22031005. 
16. ^ A. Bino, M. Ardon and E. Shirman (2005). "Formation of a Carbon-Carbon Triple Bond by Coupling Reactions In Aqueous Solution". Science 308 (5719): 234–235. doi:10.1126/science.1109965. PMID 15821086. 
17. ^ M. Lafrance, C. N. Rowley, T. K. Woo and K. Fagnou (2006). "Catalytic Intermolecular Direct Arylation of Perfluorobenzenes". J. Am. Chem. Soc. 128 (27): 8754–8756. doi:10.1021/ja062509l. PMID 16819868. 
18. ^ R.H. Crabtree, The Organometallic Chemistry of the Transition Metals 4th Ed.
19. ^ Organotransition Metal Chemistry: From Bonding to Catalysis John Hartwig