Unsaturated alkyllithiums

Nucleophiles such as alkyllithium, or the anion derived from 2-nitropropane, readily add to y-hydroxy-a,/1-unsaturated sulfones (equations 69 and 70)59. Oxidation followed by elimination of f-butylsulfinic acid leads to the formation of dienones (equation 70). 

An interesting new method for the conversion of [I, y-epoxy sulfones (82) to cyclo-alkenones (85) has been developed61. It includes the addition of alkyllithium to y-hydroxy-a,/ -unsaturated sulfones generated from 82 and the alkylation of sulfonyl carbanion thus formed. Oxidation of the resulting y-hydroxy sulfone to 84 followed by elimination of benzenesulfinic acid gives the desired product 85 in good yields (equation 72)61. 

Treatment of the potentially electrophilic Z-xfi-unsaturated iron-acyl complexes, such as 1, with alkyllithium species or lithium amides generates extended enolate species such as 2 products arising from 1,2- or 1,4-addition to the enone functionality are rarely observed. Subsequent reaction of 2 with electrophiles results in regiocontrolled stereoselective alkylation at the a-position to provide j8,y-unsaturated products 3. The origin of this selective y-deproto-nation is suggested to be precoordination of the base to the acyl carbonyl oxygen (see structures A), followed by proton abstraction while the enone moiety exists in the s-cis conformation23536. 

Representative examples of enolates produced by 1,4-addition of alkyllithium reagents to E-a,/ -unsaturated iron-acyl complexes are presented in Table 4. 

Methods for the preparation of the requisite -2,/l-unsaturaied iron-acyl complexes are presented in Section D.1.3.4.2.5., but it is noted here that several examples of 1,4-alkylative preparations of a-enolates from in situ generated fi-complexes, such as 9, have been reported in which the alkyllithium base employed acts both as a base to produce the unsaturated species via elimination and subsequently, as a nucleophile to afford alkylation products 10 and 1244. 

General methods for the preparation of a.jS-unsaturated iron-acyl complexes are deferred to Section D 1.3.4.2.5.1.1. examples of the alkylation of enolates prepared via Michael additions to ii-0 ,/ -unsaturated complexes prepared in situ are included here. Typical reaction conditions for these one-pot processes involve the presence of an excess of alkyllithium or lithium amide which first acts as base to promote elimination of alkoxide from a /f-alkoxy complex to generate the -a,)S-unsaturated complex which then suffers 1,4-nucleophilic addition by another molecule of alkyllithium or lithium amide. The resulting enolate species is then quenched with an electrophile in the usual fashion. The following table details the use of butyllithium and lithium benzylamide for these processes44,46. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

Alkyllithium compounds as well as polymer-lithium associate not only with themselves but also with other alkalimetal alkyls and alkoxides. In a polymerization initiated with combinations of alkyllithiums and alkalimetal alkoxides, dynamic tautomeric equilibria between carbon-metal bonds and oxygen-metal bonds exist and lead to propagation centers having the characteristics of both metals, usually somewhere in between. This way, one can prepare copolymers of various randomness and various vinyl unsaturation. This reaction is quite general as one can also use sodium, rubidium or cesium compounds to get different effects. 

Lithium metal or alkyllithium derivatives react with dihalocyclopropanes to provide the corresponding lithiohalocyclopropanes I which are stable at temperatures around —100 °C. These metalated species are easily trapped with electrophiles (R—X) like methyl or ethyl iodide, trimethylstannyl chloride, tri-methylsilyl chloride etc. In the case of the unsaturated bicyclic substrate II a double bond migration is observed, which in the presence of excess starting bromide is accompanied by isomerization of the axo-lithio intermediate III to its endo-isomer IV [58],... 

The preference for conjugate addition exhibited by a,/J-unsaturated acylphosphoranes even toward the addition of such hard nucleophiles as alkyllithiums is exploited for the preparation of 2-alkylcyclopropylacyl compounds from y-halogenated acylphosphoranes and alkyllithiums (equation 30). 1-Aminocyclopropanecarboxylic acid derivatives have... 

Electron-poor alkenes are in general too susceptible to direct attack by alkyllithiums to be useful as traps in cyclisation reactions. For example, the unsaturated r-butyl esters 173 cyclise successfully to give four- or five-membered rings 174, but six-membered rings form in only very low yield, with the major side reaction being direct attack of BuLi on the unsaturated ester.84... 

Unsaturated ketones and amides are not successful traps for alkyllithiums, and the only other activated alkene to have successfully trapped an alkyllithium is the unsaturated phosphorane 175.85 This trap works remarkably well with both vinyl and alkyl iodides, giving good yields of ring sizes 3-6, and is not perturbed even by a second substituent on the Michael acceptor, as in 176. 

Cyclisations of alkyllithiums onto E unsaturated phosphoranes or r-butyl esters are less stereoselective than the corresponding cyclisations onto unactivated alkenes, presumably because the additional stabilisation allows the transition state to become looser .86 With Z enoates 177, however, trans selectivity is high because of congestion in the transition state leading to the cis isomer of 178. With an alkoxy substituent (177, R = OMe), the stereoselectivity reverses, possibly due to Li-coordination. 

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