Lithium halide-organolithium

Crystal Structures of Lithium Halide and Mixed Lithium Halide -Organolithium Complexes... 

Fig. 25. Structures of selected lithium halide and lithium halide-organolithium complexes.

The preparation of an organolithium compound (shown in Figure 14-12) is similar to the preparation of a Grignard reagent except that lithium replaces the magnesium. Two moles of lithium per mole of halide are necessary One lithium yields an organolithium, and the other yields a lithium halide. 

Organolithium compounds may be made by a similar oxidative insertion reaction from lithium metal and alkyl halides. Each inserting reaction requires two atoms of lithium and generates one equivalent of lithium halide salt. As with Grignard formation, there is really very little limit on the types of organolithium that can be made this way. 

The reaction of strong bases with aryl halides results in the generation of an aryne through loss of HX, Arynes can also be formed by reaction of aryl halides with lithium and organolithium compounds. 

Organolithium compounds RLi (R = Me, Et, //-Bu, Ph) are excellent dehydrohalo-genating agents of vinyl halides in ether or THE solution under very mild conditions at temperatures below 0 °C. In a first step the acidic geminal hydrogen is replaced by lithium in a slow step to give an alkenyllithium which can be isolated at low temperature. In a second fast step lithium halide is eliminated to furnish an acetylene (equation 61). Two equivalents of alkyllithium are needed according to this equation. 

Although the direct iodine-to-Zn-exchange allows cyclizations of functionalized molecules, the iodine-to-organolithium and iodine-to-organozinc iodide exchanges were studied on the simple l-iodo-5-hexene, and the tremendous importance of the nature of the lithium halides present in solution was noticed. Indeed, as shown in Scheme 7-44, the cyclization that is somewhat similar to that of Scheme 7-41 does not operate any more, although performed in ether The only difference lies in the presence of Lil (foimed in the first step) and LiBr (formed in the transmetallation step). 

Wittig reactions employing alkylphosphonium salts can be made stereoselective by utilizing a modified procedure. The betaine is formed at low temperature and in the presence of lithium halide. Under these conditions, betaine does not rapidly eliminate triphenylphosphine oxide. The intermediate is then treated with a second mole of an organolithium compound to form a new ylide ... 

The exact formulation of the reactive intermediate in a-elimination reactions using organolithium compounds as bases has been difficult. Apart from the free carbene, various carbenoids are possible, including the a-haloorganolithium formed on metalation, and carbene-lithium halide complexes of various degrees of association. In the case of the dichlorocarbene-trichloromethyllithium equilibrium, the... 

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