Direct insertion of zinc metal

The most significant obstacle to be overcome in the preparation of organozinc halides by direct insertion of zinc metal into an organic halide is the intrinsic lack of reactivity of commercially available zinc metal. This is primarily due to a surface coating, for example of zinc oxide, which prevents direct reaction with an organic substrate. The success of attempts to insert zinc metal into an organic halide is entirely dependent upon the care with which the zinc metal is... 

Primary and secondary alkylzinc iodides (RZnI) are best prepared by direct insertion of zinc metal (zinc dust activated by 1,2-dibromoethane or chlorotrimethylsilane) into alkyl iodides or by treating alkyl iodides with Rieke zinc. The zinc insertion shows a remarkable functional group tolerance, permitting the preparation of polyfunctional... 

Alkylzinc bromides bearing various functional groups can be readily prepared by the direct insertion of zinc metal (dust, powder, or shot) to alkyl bromides by performing the reaction in the presence of iodine (1-5 mol%) and in a polar solvent like DMAC (Scheme 2-95, eq. (a)). It is also possible to use alkyl chlorides as starting material. In this case, the reaction is best performed in the presence of Bu4NBr (1 equiv.) (Scheme 2-95, eq. (b)). 

Alkylzinc bromides bearing various functional groups [31] can be readily prepared by the direct insertion of zinc metal (dust, powder or shot) to alkyl bromides by performing the reaction in the presence of iodine (1-5 mol%) in a polar solvent... 

A novel approach to perfluoroarylzinc reagents was recently reported by Miller, Platonov and coworkers. This route involves direct insertion of zinc into carbon-fluorine bonds in the presence of metal salts, such as SnCl2, CuCl2 and ZnBr2. These reactions are accelerated by ultrasound (equation 75)71. The reaction rate decreased in THF or if ZnBr2 or CuCl2 were used. 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

Direct Reaction of Zn with Alkyl Halides. The direct insertion see Insertion) reaction of Zn metal into alkyl halides - alkyl iodides being the ideal snbstrates - is a nseful reaction to prepare simple or polyfunctional organozinc halide compounds (equation 1). With primary alkyl iodides, the reaction requires an excess of Zn dnst (ca. 3 eqniv), previonsly treated with few mol % of 1,2-dibromoethane and TMSCl, and a temperature of 40 °C in THF. In these conditions, secondary alkyl iodides react at room temperatnre and benzylic and allylic bromides at 0 °C. The insertion see Insertion) into less activated C-X bonds may reqnire more reactive forms of zinc (Riecke zinc), higher temperatures, or the use of polar see Polar Compounds) solvent or cosolvent. 

Numerous modifications of the direct zinc insertion procedure can be found in the hterature. For example, simple diaUcylzincs can be used as reagents instead of metallic Zn, bnt in this case the reaction is accelerated by catalytic qnantities of zinc salts or transition metal see Transition Metals) salts. Whereas the Cu -catalyzed iodine-zinc exchange reaction provides diorganozincs, the Pd, Mn and nF catalyzed iodine- or bromine-zinc exchange leads to organozinc halides. 

Insertion compounds of hydrogen such as HxMo03 can also be prepared, either by direct reaction of the host with H2 in the presence of a platinum catalyst, or by reduction with metallic zinc in aqueous acid. The structural features are different from those containing alkali metals however. One would not expect the very small FTf ion to occupy an intersitial site in the same way as a metal cation, but rather to form a covalent bond with oxygen. Techniques such as IR spectroscopy (see Topic B7) do indeed show the presence of OH groups, so that the compound above should be formulated as Mo03 x(OH)x. 

The direct insertion ofZn metal in the presence of LiCl can be quite conveniently performed in THF as solvent. A broad range of aromatic and heteroaromatic substrates undergoes this process, offering aromatic zinc halides, mostly in excellent yields and functional group compatibility. 

Figure 2 Partitioning SODl between the mitochondria and cytosol via the action of CCS. The double line represents the outer membrane of the mitochondria. Light arrows indicate maturation steps in the metallation of SODl, whereas heavy arrows indicate movement of SODl across the outer mitochondrial membrane. The apo or unmetallated form of SODl can readily traverse the membrane in either direction. Once copper is inserted via CCS, SODl can no longer cross the membrane and becomes trapped within the corresponding compartment, either the cytosol or intermembrane space (IMS) of the mitochondria as indicated. This model shows zinc loading of SODl prior to copper metallation, but the exact order by which this occurs in vivo is not clear...

Surprisingly, zinc has been inserted directly into C—F bonds using ultrasound techniques, and in the presence of metal salts, e.g. SnCl2. The reactivity of the system appears to depend at least partly on the electron affinity of the aromatic system, because hexa-fluorobenzene is relatively unreactive in the process [57] (Figure 10.16). 

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