Alkenyllithiums, formation

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

M. Braun, a-Heteroatom Substituted 1-Alkenyllithium Reagents Carbanions and Carbenoids for C-C Bond Formation , Angew. Chem. Int. Ed. Engl. 1998, 37, 430 451. 

Fig. 14.63. Ketone -> alkane reduction via enol phosphonoamidates (for one way to prepare A, see Figure 10.21) and enol dialkylphosphates (one way to prepare B is to use a combination of the methods depicted in Figures 10.17 and 10.22). The cleavage of the Cspi—O bond of the substrates occurs in analogy to the electron transfers in the formation of methylmagnesium iodide (Figure 14.37). The alkenyllithium intermediates are protonated in the terminating step to afford the target alkenes.

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. 

In contrast, treatment of the arylsulfonyl hydrazone with two equivalents of a strong base, such as BuLi or lithium diisopropylamide (LDA), effects the Shapiro reaction for the formation of alkenes. Double deprotonation of the hydrazone gives the dianion 40, which fragments to the alkenyllithium species 41 (2.36). Addition of an electrophile leads to the alkene product. The second deprotonation takes place syn to the ArSOiN" (aryl sulfonamido anion) group and the less-substituted alkene is typically formed. Thus phenylacetone provides 3-phenylpropene and not the isomeric styrene (2.37). Protonation of the alkenyllithium species 41 to give a 1,2-disubstituted alkene generally leads to predominantly the Z-isomer. 

Xi Z, Song Q, Chen J, Guan H, Li P (2001) Dialkenylation of carbonyl groups by alkenyllithium compounds formation of cyclopentadiene derivatives by the reaction of... 

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