Organolithium reagents, addition alkenes

The reaction of alkyllithium reagents with acyclic and cyclic tosylhydrazones can lead to mixtures of elimination (route A) and addition (route B) products (Scheme 22). The predominant formation of the less-substituted alkene product in the former reaction (Shapiro Reaction) is a result of the strong preference for deprotonation syn to the N-tosyl group. Nucleophilic addition to the carbon-nitrogen tosyl-hydrazone double bond competes effectively wiA a-deprotonation (and alkene formation) if abstraction of the a-hydrogens is slow and excess organolithium reagent is employed. Nucleophilic substitution is consistent with an Su2 addition of alkyllithium followed by electrophilic capture of the resultant carbanion. 

Evidence for a radical coupling mechanism (as opposed to a carbanionic carbonyl addition mechanism) in the intramolecular Smh-promoted Barbier reactions has come from studies on appropriately functionalized substrates in the 3-keto ester series. It is well known that heterosubstituents are rapidly eliminated when they are adjacent to a carbanionic center. Indeed, treatment of a 3-methoxy organic halide (suitably functionalized for cyclization ) with an organolithium reagent leads only to alkene (equation 48). No cyclized material can be detected. On the other hand, treatment of the same substrate with Sml2 provides cyclized product and a small amount of reduced alcohol, with none of the alkene detected by gas chromatographic analysis (equation 49). ... 

The products obtained from the reaction of (chloromethyl)trimethylsilane with organolithium reagents depend very much on the structure of the lithium compound. While lithium 2,2,6,6-tetramethylpiperidide initiates an a-elimination as described above, the treatment with sec-butyllithium leads to the formation of chloro(trimethylsilyl)methyllithium (11). This reagent cyclopropanates an electron-deficient alkene through sequential Michael addition and intramolecular ring closure (MIRC reaction), for example, the formation of cyclopropane 12. 

Such nucleophilic additions of organolithiums to olefins are of limited use in laboratories. Nevertheless, there are certain categories of alkenes and alkynes that undergo ready addition [15]. A secondary organolithium reagent is easily transformed to the more stable primary reagent via the rapid equilibration [16]. 

G.ii. Intramolecular Addition to Alkenes and Alkynes. Organolithium reagents can react with alkenes or alkynes in an intramolecular reaction, although both are relatively inert to the analogous intermolecular reaction. Ward treated alkynyl bromide 263 with n-butyllithium and observed a 60% yield of... 

Organolithium reagents are known to add to alkenes. This generates a carbon-carbon bond and a new organolithium species. Addition to styrene generates a benzyllithium species that normally adds to a second molecule of styene and so on to undergo polymerization, although under suitable conditions the benzyllithium... 

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