Arylzinc iodides

FIGURE 59. Structure of arylzinc iodide 119 in the solid state... 

A similar insertion of zinc with aryl iodides requires N,N-dimethylformamide or -acetamide as solvent. These arylzinc iodides react with CuCN-2LiCl to form mixed zinc and copper organometallics, ArCu(CN)ZnI, which react with a variety of electrophiles to give functionalized aromatics.4... 

Early preparations of active zinc utilized the potassium or sodium metal reduction of anhydrous zinc salts in refluxing THF or DME (Protocol l).3,8 These highly divided zinc powders displayed high reactivity towards organic halides in oxidative addition. Alkyl iodides and bromides reacted with the zinc powders at room temperature. Even aryl bromides and iodides would react to form the corresponding arylzinc iodides or bromides at refluxing... 

This form of active zinc prepared in the presence of an electron carrier can readily form arylzinc iodides at room temperature from aryl iodides. Aryl bromides reacted far more efficiently with reduced reaction times at refluxing temperatures. The newer two-pot lithium reduction procedures for the preparation of active zinc requires 3-4 h for a total preparation time. Whilst the old one-pot preparation of active zinc from the reduction of zinc salts with lithium... 

Scheme 3.71 Asymmetric a-arylation of racemic a-bromoketones with arylzinc iodides in the presence of NiCl2 glyme and a pybox ligand [242b].

Arylzinc bromides are also capable of reacting with functionalized primary alkyl iodides in the presence of Ni(acac)2-410 The couplings readily occurred in the presence of an equimolar amount of 4-trifluoromethylstyrene at -15 °C. 

Table 1 Polyfunctional products obtained by the Ni(ll)-cat zedcross<50UFling of Arylzinc bromides and alkyl iodides in the presence of 4-(trifluoromethyl)styrene. 

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). 

Unsymmetrical biaryls.14 The Pd(0)- or Ni(0)-catalyzed coupling of arylzinc derivatives with aryl halides appears to be the method of choice for synthesis of unsymmetrical biaryls (equation I). Both Pd[P(C6H5)3]4 and Ni[P(C6H5)3]4 are usually equally effective with aryl iodides, but only activated aryl bromides can be coupled with Pd(0). 

A variety of additional organozinc-mediated couplings to iT-allylpalladium complexes have been reported, including those of perfluoroalkylzinc iodides,150 vinylzinc bromides,151 allylzinc acetates152 and arylzinc bromides (equations 35-37).153,154... 

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