Lithiation a to sulfur

The lithiation of sulfoxides and sulfones gives stabilised species which are arguably not organolithiums but anions with O-Li bonds.74 76 Lithiation a to phosphorus likewise gives anions which are outside the scope of this book. 

Organolithiums a to sulfide S can be produced with difficulty - thioanisole 108 requires BuLi and DABCO in THF at 0 °C to produce the organolithium 10977 (without DABCO only a low yield is formed78). 

The effect of the sulfur atom is principally one of acidification - a phenylsulfanyl (PhS) substituent increases the acidity of a-protons by about 5-10 pATa units.79 This effect is now generally considered not to be a result of interaction with sulfur s d-orbitals, but rather of sulfur s polarisability80 and of hyperconjugation with the antiperiplanar S-C a orbital (110).81 

Benzylsulfides may be lithiated with PhLi,83 and the acetal group of 111 both assists lithiation and imparts stereoselectivity on the reaction (the configurational stability of organolithiums such as 112 is discussed in section 5.1).84 

Allyl sulfides are readily lithiated, and give species 113 which are configurationally stable about the allyl system at 0 °C.1 

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