Allyl halides with organozinc compounds

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]. 

When diorganozinc and organozinc halide compounds react with allylic halides in the presence of Cu salts, the Sn2 see Nucleophilic Addition Rules for Predicting Direction) products are selectively obtained. In contrast, the reactions catalyzed by Ni or Pd lead to the Sn2 products (Scheme 20). 

An indirect addition converts alkynes to an organozinc compound using a palladium catalyst, which then reacts with allylic halides. " Similarly, the reaction of an alkyne with Ti(0/Pr)4/2 /PrMgCl followed by addition of an alkyne leads to a conjugated diene. 

Instead of quenching with deuterium chloride, the intermediary organozinc compound 16 can be used as a nucleophile. Not only allylic halide but also alkenyl or aryl halide can be used as an electrophile in the reaction with gem-dizinc. In Scheme 8.27, the sequential coupling reactions of fois(iodozincio)methane are summarized. In the case of the coupling with a bromoalkene, a nickel catalyst was more effective than a palladium catalyst. 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

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. 

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