Ullmann reaction

Ullmann reaction

The Ullmann reaction or Ullmann coupling [cite journal
title = The Ullmann Synthesis of Biaryls
author = P.E. Fanta
journal = Synthesis
volume = 1974
issue =
pages = 9–21
year = 1974
url =
doi = 10.1055/s-1974-23219 ] is a coupling reaction between aryl halides with copper. The reaction is named after Fritz Ullmann. [cite journal
title = Ueber Synthesen in der Biphenylreihe
author = F. Ullmann, Jean Bielecki
journal = Chemische Berichte
volume = 34
issue = 2
pages = 2174–2185
year = 1901
url =
doi = 10.1002/cber.190103402141 ]

A typical example is the coupling of o-chloronitrobenzene to 2,2'-dinitrobiphenyl with a copper - bronze alloy. [ Reynold C. Fuson and E. A. Cleveland, "2,2'-dinitrobiphenyl", "Organic Syntheses", Coll. Vol. 3, 339. [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0339 Online article] ]

The traditional version of the Ullmann reaction requires harsh reaction conditions, and the reaction has a reputation for erratic yields. Since its discovery some improvements and alternative procedures have been introduced. [cite journal
title = Aryl-Aryl Bond Formation One Century after the Discovery of the Ullmann Reaction
author = J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire
journal = Chemical Reviews
volume = 102
issue =
pages = 1359–1470
year = 2002
url =
doi = 10.1002/cber.190103402141 ]

The reaction mechanism of the Ullmann reaction is extensively studied. Electron spin resonance rules out a radical intermediate. The oxidative addition / reductive elimination sequence observed with palladium catalysts is unlikely for copper because copper(III) is rarely observed. The reaction probably involves the formation of an organocopper compound (RCuX) which reacts with the other aryl reactant in a nucleophilic aromatic substitution. Alternative mechanisms do exist such as σ-bond metathesis [Derek van Allen, PhD Thesis, University of Massachusetts at Amherst 2004. [http://www.people.umass.edu/dv/group/pdf/dvathesis.pdf Electronic thesis] ] .

The Ullmann reaction is limited to electron deficient aryl halides and requires harsh reaction conditions. In organic synthesis this reaction is often replaced by palladium coupling reactions such as the Heck reaction, the Hiyama coupling and the Sonogashira coupling

In a variation of the Ullmann reaction, 2-bromovinylstyrene is reacted with imidazole in an ionic liquid to an "n"-vinylimidazole. [Zhiming Wang, Weiliang Bao and Yong Jiang, "L-Proline promoted Ullmann-type reaction of vinyl bromides with imidazoles in ionic liquids", "Chemical Communications", 2005, 2849-51 [http://www.rsc.org/publishing/journals/CC/article.asp?doi=b501628b Abstract] ] [BMIMBF4 stands for the ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate] The reaction requires "(L)"-proline catalysis.

References


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