Vinyllithium, from vinyl bromides

Lithium-halogen exchange with t-BuLi may also be used to prepare vinyllithiums from vinyl bromides or iodides with complete retention of configuration. Pioneering studies by the Seebach... 

The synthesis of vinyllithiums 116 and 118 from vinyl bromides 115 and 117 with retention of double bond geometry is generally best done using the method of Seebach 102 the vinyl bromide is treated with two equivalents of r-BuLi in Et20 or THF at low temperature (-78 °C, for non-terminal bromides <-110 °C for terminal bromides, which otherwise lithiate a to Br and generate alkynes). Choice of solvent is essential only with alkyl iodides as electrophiles, for which the use of THF leads to attack at iodine instead of at carbon. Clean alkylation in Et20 is obtained on warming slowly to room temperature.24... 

Neuman, H. Seebach, D. Chem. Ber., 1978, III, 2785, report a detailed study of generation of vinyllithium reagents from vinyl bromides, and reactions thereof. 

This reaction can be extended to di- and trisubstituted alkenes by use of substituted vinyllithium reagents. These can be prepared quantitatively from vinyl chlorides or bromides by reaction in ether with lithium powder (containing 2% sodium) or from vinyl bromides by halogen—metal exchange with an alkyl-lithium. 

Vinyllithiums cf. 13, 142).10 An improved route to vinyllithiums involves coupling of vinyl and aryl triflates with tributyl- or trimethylstannylcuprates of the formula (R3Sn)2Cu(CN)Li2, prepared from LDA, R,SnH, and CuCN in THF at -50° to -60°. The higher-order cuprates couple with vinyl and aryl triflates at -20° to provide a vinyltin intermediate. Work-up of the intermediate with AgOAc provides a vinyltin, with bromine a vinyl bromide, or with CF,COOH an aryltin. 

A moderately stereoselective vinyllithium cyclisation has been used to make laurene 349 from a vinyl bromide 347.165 The transition state 348 leading to the major isomer has a pseudo-axial tolyl group and a pseudo-equatorial methyl - in accordance with precedent for similar cyclohexyl systems. 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

Or you might be more concerned about which uses the cheaper, or more readily available, starting materials—this was probably also a factor in the choice of methylmagnesium chloride and the unsaturated aldehyde in the second example. Both can be bought commercially, while the alternative route to this secondary alcohol would require a vinyllithium or vinylmagnesium bromide reagent that would have to be made from a vinyl halide, which is itself not commercially available, along with difficult-to-dry acetaldehyde. 

Vinyllithiums of type 663 (R2 = R3 = H) reacted with primary alkyl bromides, carbonyl compounds, carbon dioxide, DMF, silyl chlorides, stannyl chlorides, disulfides and phenylselenyl bromide142,970-979. Scheme 173 shows the synthesis of dihydrojasmone 669 from the corresponding 1,4-diketone. a-(Phenylsulfanyl)vinyllithium 665, prepared from phenyl vinyl thioether, reacted with hexanal and the corresponding adduct 666 was transformed into its acetoacetate. This ester 667 underwent a Carrol reaction to produce the ketone 668, which was transformed into the cyclopentenone 669 by deprotection either... 

Application of this crotyltitanation reaction was essential in the preparation of the required terminal Z-diene. As previously described by our group, the direct vinylation of a Z-O-enecarbamate moiety was performed in the presence of Ni(acac)2 with commercially available or laboratory-prepared vinylmagnesium bromide. To our dismay, from carbamate 44, this reaction gave highly variable but mostly disappointingly low yields. Substantial optimization studies demonstrated that the reaction with vinyllithium species (without magnesium salts), produced from tetravinyltin and MeLi, led to... 

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