Organozinc compounds secondary alkyl

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

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. 

Other examples of ZnR2-initiated radical additions to C=N bond-containing compounds can be found in a recent review by Miyabe et al 7 Overall, radical additions to imines and related derivatives may be considered as a valuable alternative to nucleophilic additions, particularly for the introduction of secondary and tertiary alkyl groups. In general, organozinc-mediated radical reactions are a new and significant direction with great potential.382,385... 

Organozincs free of complexing solvents may be produced using Zn-Cu couples in a mixed hydrocarbon-carbonyl medium, e.g., MeCOjEt, or MeCOjMe. For primary alkyl iodides, benzene or toluene is recommended, but for secondary compounds, a nonaromatic hydrocarbon is preferred ... 

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