Arylzinc compounds

Arylzinc compounds electrochemically generated in the presence of Ni-bpy and zinc salts, already mentioned in Sect. 3.1, can also react with (CF3C0)20 to give the corresponding aryltrifluoromethylketones [55]. 

FIGURE 27. Structures of the arylzinc compounds 54, 55 and 56 in the solid state... 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

Mechanism of the electrochemical conversion of aryl halides to arylzinc compounds by nickel catalysis5 45... 

The electrochemical analysis allowed the determination of kinetic constants for this reaction46. Thus, in the presence of bromobenzene, the rate constant for the oxidative addition was found to be equal to about 70 M 1 s 1. The a-arylnickel complexes are unstable, except those obtained from o-tolyl or mesityl bromide as starting substrates. In these particular cases, the arylnickel complexes can be prepared by electrolysis from an ArBr/NiBr2(bpy) equimolar ratio. However, the exhaustive electrolysis of an aromatic iodide in the presence of ZnBr2, in DMF and at —1.4 V/SCE, leads to the corresponding arylzinc compound but the yield remains low (<20%). Indeed, the aryl iodide is mainly converted to ArH according to, very likely, a radical process (Scheme 11). 

Reactivity of eiectrochemicaiiy prepared arylzinc compounds via a nickel catalysis... 

The slow addition of the arylzinc compound to a solution containing both an aryl hahde, Ar X, and the palladium complex furnished the biaryl in good to excellent yields. The very reactive Pd(0) complex was likely formed in situ by reduction of the starting palladium(II) complex by ArZnX. The reactions were very rapid (ca 1 or 2 h) compared to most usual Pd-catalyzed reactions involving ArZnX (ca 24 h). The reduction of Pd(II) could account for the formation of a small amount of Ar-Ar (2-5%) in the last non-electrochemical step while no homocoupling of Ar X occurred. 

TABLE 6. Preparation of arylzinc compounds substituted by electron-withdrawing groups... 

After the electrochemical preparation of ZnBr2 and the introduction of both 0 2 and aryl halide, it was seen that arylzinc compounds were detected in small amounts without engaging electricity. This phenomena was interpreted as follows the zinc stemming from electroreduction of ZnBr2 can reduce cobalt(II) hahde to form low-valent cobalt Co(I), which can activate aryl bromides to form arylzinc compounds via ArConX. The mechanism would be similar to that proposed by the electrochemical approach. In this case, the reduced zinc becomes reactive and can replace electricity. From this electrochemical... 

Based on Knochel s work59, which has employed cobalt halides as catalyst for the acylation of dialkylzinc reagents into ketones, this catalyst has been investigated for the coupling reaction of electrochemical arylzinc compounds formed in acetonitrile (equation 71)60. 

The amount of cobalt complex in this step influences the reaction rate, but not the yields. Indeed, with only 0.3 equivalent of cobalt catalyst, the arylzinc compound is consumed after 24 h instead of 10 h when 1 equivalent was used. An excess of the activated olefin is required to optimize the yield of the conjugate addition. Under these conditions, this process has been studied with various aryl halides (X = Br, Cl) and activated olefins. Yields range from 40 to 80%. 

Figure 5.20. Biaryl formation from resin-bound aryl bromides and arylzinc compounds [32,204], and by oxidative coupling of phenols [205],...

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 5 can be prepared in excellent yields (Equation (5)). An electrochemical conversion of aryl halides to arylzinc compounds can also... 

Hossain KM, Takagi K (1999) Novel Rh(I)-Catalyzed reaction of arylzinc compounds with methyl halides. Chem Lett 1241-1242... 

Arylamines. Arylzinc compounds are converted into arylamines on Cu-catalyzed reaction with the reagent. 

Methylation. On promotion by the Rh complex, arylzinc compounds undergo methylation with MeX to afford ArMe. 

As with nickel catalysts, Pd(0) complexes are also effective in the coupling of aryl iodides with arylzinc compounds as demonstrated in the synthesis of biphenomycin by Schmidt et al. [47]. 

Aryl triflates are also useful as electrophiles in the reaction of arylzinc compounds [48]. Pd(0) and Ni(0)catalysed cross-coupling and homocoupling reactions of aryl triflates were reviewed recently [49]. Aryl fluorosulfonates gave biaryls in this reaction too. 

Using this exchange reaction, some functionalizations of aryl halides were examined. As an example of 1,2-addition to a carbonyl group, the arylzinc prepared from 4-iodobenzoate and diethylzinc in the presence of Bu-P4 base in THF was reacted with benzaldehyde to give the benzhydrol derivative in 78% yield. As for the 1,4-addition reaction, the arylzinc prepared similarly in THF was reacted with chalcone and the 1,4-adduct was obtained in 71% yield under copper-free reaction conditions. Allylation was also carried out in the absence of copper additive, and allylbenzoate was obtained in 98% yield. It has been reported that arylzinc compounds are inert to 1,4-addition and allylation reaction in the absence of additives and conventionally the employment of copper species has been widely used. However, in this case the Bu-P4 base is considered to promote the reactivity of arylzinc compounds toward electrophiles [59] (Scheme 5.38). 

The arylzinc compounds prepared in DMF can also be used in the palladium catalysed Negishi coupling reaction and the reaction of the arylzinc with iodobenzene in the presence of palladium catalyst gave the corresponding biarylcarboxylated in 53% yield [59] (Scheme 5.39). 

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