Aryl Grignard reagents homocoupling

The homocoupling of aryl Grignard reagents triggered by organic halides in the presence of catalytic quantities of cobalt chloride or other transition metal halogenides was studied initially by Kharasch and Fields in 1941 [36]. They... 

Heterocyclic aryl chlorides can be coupled with aryl magnesium chlorides using a Pd(dba)2-dppf catalyst system. Even unactivated aryl tosylates have been successfully coupled with aryl Grignard reagents in the presence of as little as 0.1% of a catalyst prepared from Pd(dba)2 and chelating phosphines of the Josiphos-type. Syrmnetrical biaryls can be prepared from the direct homocoupling of aryl iodides and bromides using a combination of phosphine-free Pd(dba)2 and TBAF in DMF. ... 

Table 5. Synthesis of symmetrical biaryls by thallium(I) bromide-promoted homocoupling reaction of aryl Grignard reagents [14]...

In analogy to the reaction described in the previous section, iron-catalyzed homocoupling of aryl Grignard reagents can be achieved (Scheme 4-253). Two variations of this transformation have been described both using iron(III) chloride as catalyst. In the first procedure, the reaction is performed in tetrahydrofuran at low temperatures between -40 °C and 0 °C employing 1,2-diiodoethane as an oxidant in stoichiometric amounts. This method tolerates nitrile, ester, and nitro groups. An intramolecular version has been employed for the total synthesis of the natural product N-... 

However, arylmagnesium organometaUics can only be used in a few cases " like an iron-catalyzed homocoupling (equation 92) . Iron catalysis is further used for the dechlorination of electron-rich aryl chlorides by Grignard reagents . ... 

TABLE 9. Homocoupling of aryl and heteroaryl Grignard reagents... 

Microwaves were utilized both in the preparation of the Grignard reagent and in the Kumada couplings [161] with aryl chlorides. It was noted in this case that a higher amount of homocoupling side-products was typically formed when microwaves were used as a heating source than when the reaction was carried out by employing ultrasound at ambient temperature (Scheme 74) [156]. 

Completely aromatic, hyperbranched polyphenylenes were synthesized as monodendrons from AB2 type monomers by Kim and Webster [111, 112]. These dendrimers were prepared either by the homocoupling of 3,5-dibromophenyl boronic acid under modified Suzuki conditions, or by aryl-aryl coupling reactions involving 3,5-dihalo-phenyl Grignard reagents in the presence of Ni(II) catalysts as shown in Scheme 7. 

Total basicity is measured by standard acid-base titration techniques. The activity divided by the total basicity should be greater than 90%. If it is not, then the Grignard reagent should be checked for unreacted alkyl or aryl halide, homocoupled product, hydrolysis products, and oxidation products. 

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