Metallation of Olefinic Compounds

Some representative structures of olefinic hydrocarbons are given below. They are placed in order of increasing kinetic acidity. This order is based on literature data, on our experiments and on general knowledge on inductive stabilization or destabilization of negative charge. The carbon atoms from which preferential abstraction of a proton occurs are underlined, pertinent literature is indicated in brackets under the formula. 

Using superbasic reagents such as BuLi t-BuOK and Me3SiCH2K, olefins with a poor kinetic acidity can be metallated relatively easily [1-4]. BuLi TMEDA is a less efficient deprotonating reagent, though good results have been reported with the metallation of limonene [5], propene, and isobutene [7]. 

Conjugated or skipped dienes and trienes undergo ionization with alkali amides in liquid ammonia. In the cases of allylbenzene, heptatriene, indene, and cyclopentadiene, the equilibrium RH + MNH2 RM +NH3 is probably completely on the right side (with M = Na or K), but the thermodynamic acidities of cyclohexadiene and pentadiene are probably very close to that of ammonia, so that metallation with alkali amides (even potassium amide) may be incomplete. 

When lithium bromide is added prior to the reactions with alkyl halides and oxirane, the deprotonation equilibrium shifts completely to the left side and functionalization of isoprene does not take place. 

A similar strong influence of the counterion (Li+ or K+) on the metallation equilibrium is observed when a-methylstyrene, PhC(CH3)=CH2, is allowed to interact with LDA or with a 1 1 molar mixture of LDA and t-BuOK whereas LDA does not cause any ionization , addition of the olefin to the mixture of 

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