Arylzinc halide

Low-valent cobalt pyridine complexes, electrogenerated from CoCl2 in DMF containing pyridine and associated with a sacrificial zinc anode, are also able to activate aryl halides to form arylzinc halides.223 This electrocatalytic system has also been applied to the addition of aryl bromides containing an electron-withdrawing group onto activated alkenes224 and to the synthesis of 4-phenylquinoline derivatives from phenyl halides and 4-chloroquinoline.225 Since the use of iron as anode appeared necessary, the role of iron ions in the catalytic system remains to be elucidated. 

The aryl bromide 40, prepared from cross-coupling between 1,2-dibromobenzene and (trimethylsilyl)acetylene, was converted to the corresponding arylzinc halide 41a and arylboronic acid 41b for subsequent coupling with the haloallenes 42 to produce the benzannulated enyne-allene 43 in -40% yield (Scheme 20.10) [38]. Desilylation with tetrabutylammonium fluoride (TBAF) then afforded 44 in 67% yield. 

Sehemc4.t t. Catalytic cycle for the reduction of 317 halides by palladium complexes. Outer circle represents the electrochemical process, I. = Hly,P and. some anionic ligands are omitted for clarity. Tlie inner shunt is the non-elcciroehemieal reaction with arylzinc halides. 

Interestingly, low-valent cobalt species obtained by the in situ reduction of CoBrz with zinc catalyze the reaction of aryl bromides with zinc dust. The reaction allows the preparation of a range of functionalized arylzinc halides such as 44 (Scheme 19)". ... 

SCHEME 28. Preparation and uses of arylzinc halides via an I/Mg exchange... 

Under these standard reaction conditions, the acetonitrile/pyridine mixture can replace the DMF/pyridine one. This solvent mixture is also quite convenient for running the preparation of arylzinc halides. Yields are good to excellent (60-90%) and even higher than those obtained in DMF. However, with bromophenol, no organozinc species was formed in acetonitrile as observed in DMF. The formation of arylzinc species is also effective in a mixture of solvents of acetonitrile-DMF-pyridine (8/1/1) in the presence of C0CI2 (13%) as the catalyst precursor and zinc bromide (30%). This method has also been applied to the formation of organozinc halides from alkyl and alkenyl halides. So far, only low yields have been obtained using the standard reaction conditions in DMF-pyridine. Results are reported in Table 8. 

TABLE 8. Preparation of arylzinc halides from alkyl and alkenyl halides... 

Arylzinc halides, and zinc dialkyls and di-aryls may be prepay by the action of the Grignard reagent on anhydrous zinc chloride in ether solution.6... 

The TT-allyl-cr-arylpalladi urn-phosphine complexes have been of particular interest in the study of the mechanism of their reductive elimination to yield allylated aromatics.148 149 Their preparation can be achieved by the addition of a triarylthallium or arylzinc halide to the corresponding allylpalladium phos-phino halides (equation 34). 

If the reaction is run in the presence of a zinc salt (ZnBr2 or ZnCl2), an Mg or Zn anode, and excess bipyridine, compared to the nickel catalyst, the arylzinc halide is the major product (Equation 8.18).2... 

In the case of ortho-substituted aryl halides, which are less reactive towards Ni°(bpy)n the formation of the arylzinc intermediate likely involves the occurrence of a Ni°-bpy-Zn(II) complex, which by reduction leads directly to the oxidative addition-transmetallation process. According to this, the nickel catalyst is NiBr2bpy, without extra bipyridine. It is thus possible to prepare arylzinc halides from not easily reduced 2-chlorotoluene or 2-chloroanisole, but also, more importantly, from aryl bromides or chlorides bearing reactive functional groups (COR, C02R, CN). These compounds can then be added... 

Thirty-eight years after Winkler developed the organozinc method for the synthesis of tetraethylgermane, Dennis and Hance160 reproduced it, but this method for synthesis of aliphatic germanium derivative was not used later. However, in the years 1927-1935 arylzinc halides were used for the synthesis of tetraarylgermanes23,161-165. 

Fu s group showed more recently that ot-bromo ketones 6b can be coupled with arylzinc halides catalyzed by 5 mol% NiCl2 DME and 6.5 mol% of PyBOX ligand 5e with good asymmetric induction providing benzylic ketones in 71-93% yield and 72-96% ee (entry 12) [54]. The lower ee values were obtained with orf/io-substituted aryl bromo ketones, while orf/to-substituted arylzinc reagents as well as a-branched bromo ketones did not react. 

Nickel (0) reagents have been employed in the symmetrical coupling of aryl halides in sterically unencumbered cases. An efficient cross coupling reaction between an arylzinc halide and an ortho-iodoaryllmlne under mild conditions mediated by N1(0) has been reported. Thallium(III)... 

The cross-coupling of organozinc compounds has been accomplished using a number of metal enolate complexes, namely Rh(acac)(H2C=CH2)2 (6) , Co(acac)3 (30) °, Ni(acac)2 (47) , Li(acac) (107) and Cr(acac)3 (108) . For instance, complex 47 proved to be an extremely effective catalyst system for the Negishi cross-coupling of arylzinc halides (109) with aryl (110), heteroaryl and alkenyl halides, triflates and nonaflates to give the corresponding biaryl compounds (111) (equation 30). The solvent played an important role in these reactions and optimal conditions were found with 8 1 mixtures of THF and A-ethylpyrrolidinone (nep). Only 0.05 mol% of the nickel complex... 

Later, in 2014 McCann and Organ investigated the role of halide additives in the Negishi reaction catalyzed by 29 involving sp -hybridized zinc reagents. Diarylzinc compounds were found to couple in low dielectric solvents (i.e., THF) with zero salt present whereas arylzinc halides failed to couple in THF alone, requiring the addition of salt. Further, unlike alkyl-alkyl coupling there was no evidence to support the... 

The direct synthesis of arylzinc halides from the halide and activated zinc <90TL4413,9lJOCl445> can also be applied to heteroaromatic systems, (e.g. bromo- and iodopyridines). In the electrophilic pyrimidine 2-position zincation and coupling of the iodo derivative took place to the extent of 26%... 

The coupling of arylzinc halides w ith i 3-Arylindoles are obtained from 3-indols 1.4-disubstituted butadienes - starts from i After Pd(0)-catalyzed coupling with alke Grignard reaction in the presence of 1 Ph. P... 

In 2004, Walla and Kappe described a more straightforward means of access to arylzinc halides - microwave-accelerated Rieke Zn insertion into aryl halides [67]. 

D.ii.d. Termination by Organozinc Compounds. Arylzinc halides, which are readily... 

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