Vinyllithiums exchange

This type of metallic exchange is used much less often than 12-32 and 12-33. It is an equilibrium reaction and is useful only if the equilibrium lies in the desired direction. Usually the goal is to prepare a lithium compound that is not prepared easily in other ways, for example, a vinylic or an allylic lithium, most commonly from an organotin substrate. Examples are the preparation of vinyllithium from phenyl-lithium and tetravinyltin and the formation of a-dialkylamino organolithium compounds from the corresponding organotin compounds ... 

In terms of their synthetic use, (E)-vinyl tellurides 56 easily undergo Li/Te exchange upon treatment with wBuLi at — 78 °C (Scheme 4.32). The resulting vinyllithium has con-... 

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyUithiums. These results are in sharp contrast to the earlier tin-lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (ii)-vinyllithiums. ... 

Lithium-tellurium exchange generation of vinyllithium (typical procedure). Method A. To a solution of butyl vinyl telluride (0.211 g, 1 mmol) in THF (4 mL) at -78°C under N2 is added dropwise a solution of n-BuLi (1.5 M in hexane, 0.67 mL, 1 mmol). After stirring for 40 min at -78°C, benzaldehyde (0.016 g, 1 mmol) is added. The mixture is allowed to react at room temperature for 30 min, diluted with EtOAc (40 mL) and washed with brine. 

Being less basic than the saturated analogs, vinyllithium as all other acyclic or cyclic 1-alkenyllithiums can be prepared from iodo or bromo and sometimes even chloro precursors using butyllithium or fert-butyUithium (Tables 12 and 13). Hetero-substituents such as dialkylamino, alkoxy and silyloxy groups or halogen atoms again accelerate the exchange process considerably (Table 14). This holds for 0-lithiated hydroxy or carboxy functions as well (Table 15). 

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). 

The /S-bromodihydropyran (652) on halogen-metal exchange with f-butyllithium affords a useful organometallic (653) that can function as the equivalent of a 5-hydroxy aldehyde enolate (Scheme 151) (79TL67). The vinyllithium species adds to ketones and aldehydes at -110 °C in good yield and also reacts in a conjugate mode with a,/S-unsaturated carbonyl compounds in the presence of copper(I) ions. On hydrolysis and oxidation, the dihydropyran component (654) can be transformed into a substituted 5-lactone (655). 

The synthesis of 3 was initiated by reaction of wBuLi with the protected cyclopentenone 2 generating the corresponding vinyllithium reagent by halogen-metal exchange. Subsequent condensation with (S)-(-)-menthyl para-toluenesulfinate (13) provides the enantiodefined sulfoxide substituent in 3.5 Since thermal equilibration of chiral sulfoxides at room temperature is slow, the large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into carbon compounds. 

Tin-lithium exchange has been widely used in synthesis, particularly to form stereodefined organolithiums (section 5.2.1). For example, Sinay used a radical reaction to displace the sulfone from the glycoside 161, substituting for a BUjSn- group. Tin-lithium exchange of 162 gave a vinyllithium 163 which reacted with electrophiles to yield products such as 164.132... 

Tellurium-lithium exchange proceeds in much the same way as selenium-lithium exchange but is rarely used.144 Vinyltellurides such as 179 can be converted stereospecifically into vinyllithiums 180,145 and alkyltelluro-heterocycles have been used as precursors to lithio-species.146... 

The stereochemistry of the tin-lithium exchange of non-a-heterosubstitued stannanes has been studied only to a limited extent because the reaction itself typically lacks the driving force that favours the formation of stabilised a-heteroorganolithiums, aryllithiums or vinyllithiums. The examples that do exist lead to either (a) cyclopropyl or cyclobutyllithiums or (b) benzyllithiums. 

The faster halogen-metal exchange and controlled conformation in vinyl and aryl systems meant that aryllithiums and vinyllithiums can be cyclised remarkably efficiently onto cyclic vinylogous amides to give 179 and 180. 180 was used in a synthesis of (+)-indolizidine 209D.87... 

The first alkyne cyclisations, from 377, 379 and 381, predate the early alkene cyclisations by a couple of years these three date from 1966173 and 1967,174 and illustrate the favourability of both exo and endo-dig cyclisation. All three generate benzylic vinyllithiums (378, 380 and 382), and both aryl (377, 379) and alkyl halides (381) are successful starting materials. Similar organomagnesium cyclisations were described at about the same time.175 However, it is not clear in these reactions how much of the product is due to participation of radicals in the mechanism - alkylbromides undergo halogen-metal exchange with alkyllithiums via radical intermediates (chapter 3).176 If it really is an anionic cyclisation, cyclisation to 378 is remarkable in being endo. Endo-dig anionic cyclisations are discussed below. 

Cascade reactions using alkyne cyclisations have not been explored, and the isolated example of 416 is unique in a number of ways.145 For a start, 5-exo-dig cyclisation onto the alkyne to give 417 must be sufficiently fast that it competes with halogen-metal exchange of the second iodine atom, which instead traps the product vinyllithium 417, giving the bicyclic product 418 in 47% yield. 

Waak 13-15) was the first to attempt anionic initiation by means of an unsaturated lithiumorganic compound, namely vinyllithium. This reagent can be obtained by an exchange reaction between tetravinylzinc and butyllithium ... 

To our knowledge, the first example reported of a tellurium-lithium exchange reaction leading to a vinyllithium was the reaction of 2,5-diphenyltellurophene 165 with -butyllithium, giving l,4-dilithium-l,4-diphenylbuta-l,3-diene 166, which was trapped with several electrophiles leading to the corresponding disubstituted dienes 167 with retention of configuration (Scheme 95).254... 

Kauffmann reported the first tellurium-lithium exchange reaction of a vinylic telluride with an organolithium compound.255 Phenyl vinyl telluride 168 was deprotonated by lithium dicyclohexylamide (LDCA) in THF, and the resulting vinyl anion 169 was reacted with chlorotrimethylsilane to give telluride 170. Vinylsilane telluroacetal 170 was then reacted with phenyllithium to give the corresponding vinyllithium, which was captured with chlorotrimethylsilane to give the bis-silylated ethane 171 (Scheme 96).255... 

Trimethylsilyloxy)vinyllithium 548 has been prepared by tin-lithium exchange from the vinylstannane 552, which is generated from acetyltri-n-butyltin 551 (Scheme 149)830. This vinyllithium suffers a reverse Brook rearrangement to generate the alkoxide 553 used for the synthesis of acylsilanes831. 

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