LICKOR superbase

Alkali metal alkyls, particularly n-butyl lithium, are the most frequently used reagents to form metallated intermediates.246 247 In certain cases (di- and triphenyl-methane, acetylene and 1-alkynes, cyclopentadiene) alkali metals can be directly applied. Grignard reagents are used to form magnesium acetylides and cyclopenta-dienyl complexes.248 Organolithium compounds with a bulky alkoxide, most notably M-BuLi-ferf-BuOK in THF/hexane mixture, known as the Lochmann-Schlosser reagent or LICKOR superbase, are more active and versatile reagents.249-252... 

The LICKOR superbase was shown to be an effective and chemoselective reagent to obtain sterically demanding alkylbenzenes from methylbenzenes.445 When a methyl group is flanked by two neighboring methyl groups, it adds only one molecule of ethylene ... 

Selective o,o -dimetallation of diphenylacetylene was successfully performed with the LICKOR superbase.328 The dimetallo product thus formed reacts smoothly with various electrophiles to give o.o -disubstituted derivatives in good yields ... 

C-H Transformation at Allylic Positions with the LICKOR Superbase... 

Extended allylic systems can be formed by deprotonation of dienes such as 142,144 and 147 with s-BuLi. The dienyllithiums 143, 146 and 148 adopt an extended W conformation, and react to give 1,3-butadienes 146 and 149 with retention of double bond geometry.19 The equivalent species 150 formed by deprotonation with LiCKOR superbases (see section 2.6) adopt a U -shaped configuration. 

The combination of an alkyllithium with a metal aUcoxide provides a marked increase in the basicity of the organolithium . The most widely used of these superbases is the one obtained from BnLi and KOBu-t, known as LiCKOR (Li—C + KOR) °. The exact natnre of the prodncts obtained by snperbase deprotonations—whether they are organolithinms, organopotassiums, or a mixtnre of both—is debatable, as is the precise nature of the superbase itself. For example, while prolonged mixing of alkyllithium and... 

The violence of superbasic slurries towards functionalized organic molecules means that they are at their most effective with simple hydrocarbons they also tolerate ethers and fluoro substituents. LiCKOR will deprotonate allyUc, benzylic, vinylic, aromatic and cyclopropane C—H bonds with no additional assistance. From benzene, for example, it forms a mixture of mono and dimetallated compounds 617 and 618 (Scheme 241) . ( Li/K indicates metallation with a structurally ill-defined mixture of lithium and potassium.)... 

The expedient and regioselective metalation of unprotected biphenyl-2-, -3-, and -4-carboxylic acids has been reported.59 Unprotected biphenyl-2-carboxylic acid has been cleanly metalated with. sex-butyllithium at the position adjacent to the carboxylate and can then be subjected to site-selective electrophilic substitution (Scheme 8). The remote C(2 )-position has been attacked by the superbasic mixture of n-BuLi and t-BuOK (LICKOR) in THF or benzene. The resulting dianion cyclizes to give the fluorenone skeleton. The mechanism of the metalation of homologous compounds, 2-(pyridin-3-yl)benzoic acid derivatives, with strong bases has also been discussed.60... 

Two general procedures are used to perform LICKOR deprotonations. Alkyllithium and potassium alkoxide can be combined to form the superbase before alkene addition, or the substrate mixed with one component of the superbase before addition of the other component. For LiC, w-BuLi, s-BuLi, and t-BuLi have all been used, whereas the hindered KOR component is most commonly KOt-Bu or sometimes potassium t-pentoxide. Although LICKOR generation is rapid even at low temperatures, substrate deprotonation can be sluggish. For this reason, an incubation period with the substrate at 0 °C or room temperature is often part of a LICKOR procedure. Such extended reaction times are also useful when torsional equilibration of the allylic carbanion is intended. Afterwards, the reaction mixture is cooled back to low temperature before addition of the electrophile. This is typically accompanied by a distinct color change from the vivid red of the allylic carbanion to yellow or decolorized reaction mixtures. 

As shown in Equation (62), 1,2-/fe(mcthylthio)bcnzene 338 was dimetallated using butyllithium or superbasic mixture of butyllithium/potassium r/-butoxide (LICKOR), and then quenched with dichlorodimethylsilane to obtain 1,5,3-benzodithiasilepin 339 <1999T14069>. 

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