Vinyllithium compounds

Alkenyllithium compounds are intermediates in the Shapiro reaction, which is discussed in Section 5.7.2. The reaction can be run in such a way that the organolithium compound is generated in high yield and subsequently allowed to react with a variety of electrophiles.64 This method provides a route to vinyllithium compounds starting from a ketone. 

Triphenyl- [1] and trimethylvinylsilane [2] as well as l,l-bis(trimethylsilyl)ethylene [3] are known to react with lithium metal in THF yielding 1,4-dilithiobutane derivatives by a dimerizing Schlenk addition. Interestingly, by using hexane as the solvent trimethylvinylsilane 1 does not yield the dimer product 2 but a 1 1 -mixture of the corresponding vinyllithium compound 3 and the lithioalkyne 4... 

Treatment of a functionalized vinyllithium compound with two equivalents of CuBr (see Scheme 1.17) afforded an aggregate, the structure of which was established by an X-ray crystal structure determination [41, 42] (see Fig. 1.18A). 

By the analogous reaction, the doubly lithiated sulphur substituted vinyllithium compound 126 was generated by Brandsma, Maercker and coworkers, starting from tellura-cycle 125. An interesting pathway to functionalized heteroatom-substituted polylithium compounds has been introduced by this method (Scheme 46). 

The dilithiated vinyllithium compound 1,2-dilithioethylene (132) was synthesized by Maercker and coworkers, using the mercury-lithium exchange reaction . Bis(chloromercurio)ethylene (131) was reacted with four equivalents of f-butyllithium... 

The stereospecific construction of the trisubstituted double bond of the side chain at C-1 of carbazomadurins A (253) and B (254) was achieved using Negishi s zirconium-catalyzed carboalumination of alkynes 758 and 763, respectively. Reaction of 5-methyl-l-hexyne (758) with trimethylalane in the presence of zirconocene dichloride, followed by the addition of iodine, afforded the vinyl iodide 759 with the desired E-configuration of the double bond. Halogen-metal exchange with ferf-butyllithium, and reaction of the intermediate vinyllithium compound with tributyltin chloride, provided the vinylstannane 751a (603) (Scheme 5.79). 

Aryllithium compounds are converted smoothly into the corresponding fluoroaromatics upon treatment with N-F reagents and difluorodiazene (Table 8)/ Vinyllithium compounds are also converted into the corresponding vinyl fluorides upon treatment with N-F reagents (Table... 

Two equivalents of a strong base such as n-butyllithium abstract first the tosylhydrazone proton and then the less acidic a-carbonyl proton, leaving a dianion A. The A eliminates first p-toluenesulfinate and then N2. The resulting vinyllithium compound B on protonation gives alkene (Scheme 4.54). 

Numerous functionalized aryl- ", alkyl- , and vinyllithium " compounds have been used as acylating reagents for the preparation of carbene complexes mainly of Cr and W. 

The cyclization of a 4-substituted-5-hexenyllithium proceeds also by a 5> n-addition of the CHj-Li moiety to the triple bond (Scheme 7-11). By contrast, vinyllithium compounds bearing aryl or silyl substituents on the lithiated sp carbon are known to undergo facile cis-trans isomerization [15]. 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

Thus, besides two additions of lithium and two consecutive lithium hydride eliminations a [ 1,4]-proton shift has occurred. In order to prove the proposed reaction mechanism all three intermediates, i.e., the vinyllithium compound (E)- A, R = Ph, and the trilithium compounds 25 and 26, were synthesized independently. 

R = SiMea.Interestingly, the vinyllithium compound is not stable under these conditions but adds to the starting material with formation of 39, a dimeric monolithium derivative (Scheme 9). The formation of 39 by lithium hydride elimination from 4 could be excluded experimentally. 

In order to study this unusual dimerization in detail 49 [40] was synthesized and subjected to the same reaction conditions. The cumyl group inhibits the Grovenstein-Zimmerman rearrangement as in this case the formation of an anion at a tertiary center would be encountered, so only cleavage to the vinyllithium compound 18 is possible. Indeed, after reacting 49 with lithium metal for 4 h at room temperature and subsequent work-up with dimethyl sulfate the dimer 51 is isolated in 82 % yield, with t-butylbenzene (53) as the other product in 84 % yield (Scheme 11). 

When quenching the reaction mixture already after 1.5 hours, 20 % of the starting vinylsilane 49 is still found, besides 11 % of 52 derived from the intermediate vinyllithium compound 18, R = SiMes, and only 64 % of 51 and 61 % of 53, respectively, are observed. This result clearly proves the proposed mechanism. 

This interesting dimerization is without precedent in the literature. Recent results in our laboratory have shown that 2,2-diphenylvinyllithium dimerizes as well This reaction must be anticipated as a true dimerization (no lithium is consumed) of a vinyllithium compound (Scheme 12), catalytic amounts of the metal are sufficient, the reaction is catalyzed by electron transfer [18]... 

The addition of lithium metal to vinylsilanes is a suitable approach to vinyllithium compounds as the addition of lithium metal is usually followed by lithium hydride elimination, especially when performing the reaction in less polar solvents like toluene. So far, the formation of vicinal or geminal dilithiovinylsilanes by two consecutive lithium addition and lithium hydride elimination sequences is... 

The reaction of a tosylhydrazone with at least 2 equiv. of an alkyllithium reagent in ether or hexane generates the dianion (Shapiro reaction) this gives, after loss of Ts" and N2, the vinyllithium compound, which can be trapped by a variety of electrophiles, e.g. H", CO2, DMF (Scheme 27). The less-substituted alkene is formed predominantly under these conditions. 

J5. A literature review on vinyllithium compounds and their use in synthetic applications is given in ref. 114... 

Another fragmentation reaction especially useful for the synthesis of vinyllithium compounds is the well-known Shapiro reaction We were successful in preparing 2,3-dilithio-l,3-butadiene 166 by a double Shapiro reaction although in 12% yield only The main reaction product was 2-butyne. 

Table 9. Fluorination of Vinyllithium Compounds with N-F Reagents...

---