Reactivity of Polar Organometallic Intermediates
Conversions via polar organometallic intermediates are among the most frequently applied procedures in organic synthesis. Since chemists usually strive to attain satisfactory yields of products with the highest possible purity, considerable effort must be expended in finding the most favorable reaction conditions. This may be done by varying concentrations, temperature or other conditions systematically. A much easier and often applied procedure is to mix the reactants in high dilution (on the order of 0.1 mol per litre or less) at a low temperature, and then either allow the temperature to rise slowly, or maintaining the low temperature for a long period. Indeed, some reactions give good results only when carried out in a relatively large amount of solvent at very low temperatures (e.g. the generation of o-Li—C6H4—Br and of o-Li—C6H4—NO2 from BuLi and 1,2-dibromobenzene  or l-bromo-2-nitrobenzene , respectively. In most cases, however, synthetic operations can be performed successfully at more practical and economical concentrations of 0.5 to 1 mol/1 or higher. Then heating effects are often significant and routine observations with a thermometer can serve as a probe of the course of the reaction (more reliable than the appearance of a thin precipitate or a colour change); the selection of the most favorable temperature range is thus facilitated. Some conversions do not give problems, since they generally are fast, even at low temperatures (e.g. carboxylations of RLi with CO2, coupling with carbonyl compounds, reactions with disulfides and trialkylchlorosilane). Selections of reaction conditions, such as solvent and counter ion, can, in principle, be derived from analogous reactions described in the literature. However, this often requires an extensive, and complicated literature search.
KeywordsLithium Iodine Acetylene Epoxide Bromine
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