Lithium compounds, organolithium acetylide

The rate of RLi additions to methyl isothiocyanate seems to be strongly dependent upon the basicity of RLi [9]. Whereas phenyllithium reacts rapidly at — 90 °C, the addition of the much more weakly basic 2-thiazolyllithium (pK of 1,3-thiazole is about 29) has to be carried out at —30 to —50 °C and the reaction with lithium acetylides RC=CLi (pK acetylenes is 26 or lower) requires 0 to 20 °C (all reactions were carried out at the same 0.5 to 1 mol/1 concentrations). These temperatures indicate the conditions for other organolithium compounds and also the chance of success for other reactions. A rough estimation of the pK values of RH has to be made first, taking into account stabilizing effects of substituents. Side reactions have not yet been observed in the conversions of isothiocyanates with lithium compounds. 

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... 

Such compounds, derived from substituted acetylenic substrates (excluding simple acetylides), form oligomeric structures involving bridging lithium atoms and possible interactions of the lithium atoms with the acetylenic it system. The Erlangen group has prepared and characterized a number of organolithium compounds derived from S-substituted-2,2,8,8-tetramethyl-3-6-nonadiyne (43). 

An acid-base reaction can also be used to prepare sp hybridized organolithium compounds. Treatment of a terminal alkyne with CH Li affords a lithium acetylide. Equilibrium favors the products because the sp hybridized C-H bond of the terminal alkyne is more acidic than the sp hybridized conjugate acid, CH4, that is formed. 

Another excellent example demonstrating the utility of organolithium compounds in the formation of C-glycosides from modified carbohydrates was reported by Koll, et al.38 This study, shown in Scheme 2.3.13, involves the addition of a lithium acetylide to a C-glycosyl oxetane. The result was the extension of a pre-existing C-glycosyl side chain. 

Another route to carbon-carbon bond formation is via iodine-induced rearrangement of adducts of trisubstituted boron compounds and organolithium reagents. The adducts of trialkylboranes with lithium acetylides give an akylated acetylene. The mechanism involves electrophilic attack by iodine on the acetylenic... 

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