Arylzinc compounds reactivity

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 . ... 

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. 

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... 

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). 

Both the lifetime (around some seconds to several minutes) and the reactivity in solution of these arylcobalt complexes ArjCo depend on the nature of the substituent on the aromatic nucleus. In the presence of zinc salts, this intermediate leads to the stable arylzinc compound by simple metal exchange according to ... 

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... 

---