Aryl coupling reaction

This reaction is known as the Ullmann coupling. It is believed to involve the intermediacy of aryl copper complexes rather than radical species. The reaction is best suited to the preparation of symmetrical biaryls ( homo-coupled products). Attempts to couple two different halides (Ar X and Ar X) in this way can lead to mixtures of the desired cross-coupled product (Ar -Ar ) and the two homo-coupled species (Ar -Ar and Ar -Ar ). 

The stannane coupling partnef in these reactions can be prepared by a number of methods, Particularly useful is the transmetallation from a Grignard or a/y/lrthrum compound using a t rial ky Ichlorostan n ane, 

Ti e borcn compounds used in iuzuk coupling reactions may b prepared by reaction of the appropriate aryllithium compound with trimethyl borate, followed by reaction with hydrochloric acid. 

Coupling reactions such as this can all be explained in a general way by the sequence (i) oxidative addition (ii) transmetallation (iii) reductive elimination. 

Reactions of this type are not limited to two aryl components. Phenylethenes can be prepared in this way, with either the aromatic or the alkenic component being used as the organometallic compound 

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In addition to sodium, other metals have found application for the Wurtz coupling reaction, e.g. zinc, iron, copper, lithium, magnesium. The use of ultrasound can have positive effect on reactivity as well as rate and yield of this two-phase reaction aryl halides can then even undergo an aryl-aryl coupling reaction to yield biaryls. ... 

Although the copper mediated Ullmann reaction is a well known method for biaryl synthesis, drastic conditions in the range of 150-280 °C are required. Zerovalent nickel complexes such as bis(l,5-cyclooctadiene)nickel or tetrakis(triphenylphosphine)nickel have been shown to be acceptable coupling reagents under mild conditions however, the complexes are unstable and not easy to prepare. The method using activated metallic nickel eliminates most of these problems and provides an attractive alternative for carrying out aryl coupling reactions(36,38). 

The arylation of heteroaromatic compounds is also achieved by aryl-aryl coupling reaction. The arylation of A-methylimidazole with bromobenzene occurs under palladium catalysis (Equation (62)).72 The arylation of thiazole with aryl iodide occurs at the 2-position under PdCl2(PPh3)2/CuI catalysis.73 In this case, tetrabutylammonium fluoride improves the activity of the catalyst. Alternatively, thiazoles and benzothiazole are efficiently arylated... 

For an excellent review of aryl-aryl coupling reactions see reference Ic, section 4-3 and references therein... 

A. Aryl-Aryl, Aryl-Alkenyl and Alkenyl-Aryl Coupling Reactions. 483... 

Some of the notable recent developments in the Pd- or Ni-catalyzed aryl-alkenyl and alkenyl-aryl coupling reactions include the use of aryl and alkenyl chlorides along the lines of earlier discussions. lust a couple of representative examples of the aryl-alkenyl coupling reaction with alkenyl chlorides are shown in Scheme 33. 

The Pd-catalyzed aryl-alkenyl and alkenyl-aryl coupling reactions involving Zn, A1 and Zr have been applied to the syntheses of a fair number of natural products26. Recent examples published in the last several years include the syntheses of UB-165107 and (—)-diazonamide A108 (Scheme 35). 

Table 5.6 Iron-catalyzed aryl-aryl coupling reaction developed by Knochel s group.

Scheme 14.4 Synthesis of a main chain poly[2] catenane based on an aryl coupling reaction.27...

Scope, Limitations and Fundamental Examples 214 1.4 Aryl-Aryl Coupling Reactions 223... 

Anodic treatment of 1,2- or 1,4-dihydroxy-substituted benzenes to form the corresponding quinones or masked congeners is well known, since they represent valuable synthetic intermediates [64]. Benzoquinone ketals of electron rich arenes like 18 can be challenging since the oxidative aryl-aryl coupling reaction usually competes. When using BDD anodes the benzoquinone ketal 19 is obtained in an almost quantitative manner, demonstrating the superior properties of this electrode material. Despite the basic conditions, no deblocking of the silyl-protected phenol moiety is observed [65] (Scheme 9). 

Many reports on tributyltin hydride-mediated intramolecular aryl-aryl coupling reactions have appeared in the literature in the 1990s [27, 121], One of the latest articles, in which many references on previous work have been included, focuses on the preparation of aporphines (Scheme 23) [122], Due to steric repulsion, an undesired 5-endo cyclization of the aryl radical on the imine nitrogen atom occurs only when more bulky substituents R are present in the benzyldihydroisoquinoline 64. With R = H, the cyclization to 65 proceeds in high yields. 

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