Magnesium Acetylide

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

The process is initiated by a double Grignard reaction of 4-pentynoic acid 153, first with 3-butenyl magnesium bromide and subsequently magnesium acetylide followed by silylation of the formed ter-tiaiy hydroxyl function. The cobalt induced polycy-clization leads directly to the fenestrane 157 interestingly, the reaction halts at the stage of 156 when employing the unprotected alcohol. 

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 alkynyl aryl tellurides are more conveniently prepared by means of the lithium acetylide,173 although magnesium acetylides can be used as well80 (Scheme 59). 

A mixed Li /Mg+ aggregate corresponding to (213) is formed with either phenyl or methyl carbanions. An unusual lithium/magnesium acetylide is formed with stoichiometry Li2[(PhCsC)3Mg(TMEDA)]2 and is depicted as (214). The same authors also report the ion pair characterized as the mixed benzyllithium/magnesium TMEDA complex (215). ° A different mixed lithium/magnesium aggregate depicted as (216) is found for the THF-solvated anion of tris(trimethylsi-lyl)methyl carbanion. ... 

---