Lithiation a to silicon

Allyltrimethylsilane 126 and benzyltrimethylsilane 127 are lithiated with BuLi in the presence of TMEDA or HMPA. Methyllithium, however, attacks the silicon atom.93 95 

Without additional stabilisation of the organolithium, deprotonation a to silicon is difficult. It can occur however when there is a nearby coordinating heteroatom, as in the formation of 129 from 128.96 The organolithium 129 is unstable and cyclises to give 130 which hydrolyses to 131. Treatment with BF3 gives the stable flurorsilyl compound 132. 

Attempted double silylation of 1,8-dilithionaphthalene 133 was frustrated by a deprotonation a to silicon in the intermediate 134 which rearranged to 135 faster than it reacted with Me3SiCl.97 

Although allylic lithiation by deprotonation of non-heterosubstituted compounds is possible using superbases (see section 2.6), in most cases allylic lithiation requires a directing heteroatom. (Non-heterosubstituted allyllithiums are best produced by reductive lithiation of allyl ethers or allyl sulfides - see section 4.4.) One of the few cases where this heteroatom is not a to the new organolithium is shown below the p-lithiation of a homoallylic amide 137. The reaction is particularly remarkable because of the possibility of competing deprotonation 

The lithiation of allylic positions directed by remote carbonyl groups is observable even when enolates are formed - labelling shows that only the cis methyl group of 140 is deprotonated during the formation of the extended enolate 141.46 

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