Halogen-lithium exchange stereospecificity

Halogen-lithium exchange cf. Section 4.7.3.2) at low temperatures with an alkyllithium followed by quenching with methanol or water results in stereospecific reduction of dihalocyclopropyl compounds.113 Stereospecific reduction has also been achieved by reaction with potassium diphenylphosphide (equation... 

Simple proton abstraction is more difficult in the case of alkenyl substrates (p/iTa 36, but see Scheme 1.110) and a more-common method for the formation of alkenyllithium (vinyllithium) species is halogen-lithium exchange with n- or tert-butyllithium. The exchange reaction is stereospecific, such that the stereochemistry of the starting alkenyl halide determines the stereochemistry of the product alkenyllithium species (1.114). Subsequent reaction with electrophiles also occurs with retention of configuration and this of course is important in order to prepare stereodefined alkenyl products. 

Trigonally hybridized vinyllithium derivatives are characterized by higher configurational stability [34,35]. Panek [36] demonstrated that (E) and (Z)-l-lithium-l-phenyl-l-butene could be prepared stereospecifically, with retention of configuration, by a lithium-halogen exchange reaction between n-butyllithium and (Z) and ( )-l-bromo-l-phenyl-1-butene, respectively (Scheme 7). Both lithium derivatives are configurationally stable at... 

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