Metallation of Various Olefins with Strongly Basic Reagents

7 Metallation of Various Olefins with Strongly Basic Reagents 

In Table 4 reaction conditions for the metallation of a number of olefinic compounds, as derived from our investigations, are summarized. The deprotonation of non-conjugated olefins proceeds rather sluggishly and satisfactory rates of conversion can be attained only when the substrate is used in a large excess. Under these conditions attack of the solvent (THF ) by the base is suppressed. 

In most cases the excess of starting compound can be readily separated from the derivatization product by distillation. Problems may arise, however, when a substrate with a relatively high molecular weight is to be metallated and subsequently functionalized, since the difference in the boiling points of the starting compound and end product is then much smaller. In such a case one may consider to perform the metallation with an excess of base (if there is no chance on dimetallation or other complications) and to destroy the excess by reaction with THF at somewhat higher temperatures (resulting in the formation of H2C=CH2 and H2C=CH—OM) prior to the further synthetic operations. Such a consideration also makes sense when the substrate is expensive or not readily available. 

In this experiment a number of metallations in Table 4 are described in somewhat more detail. 

Freshly distilled limonene (300% excess) is metallated as described in procedure a (see also Table 4). The red solution obtained is cooled to — 50 °C, after which freshly distilled trimethylchlorosilane (10% excess) is added. After the usual work-up 

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