Lithium/magnesium acetylide

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. ... 

We see from these examples that many of the carbon nucleophiles we encountered in Chapter 10 are also nucleophiles toward aldehydes and ketones (cf. Reactions 10-104-10-108 and 10-110). As we saw in Chapter 10, the initial products in many of these cases can be converted by relatively simple procedures (hydrolysis, reduction, decarboxylation, etc.) to various other products. In the reaction with terminal acetylenes, sodium acetylides are the most common reagents (when they are used, the reaction is often called the Nef reaction), but lithium, magnesium, and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylenediamine complex, a stable, free-flowing powder that is commercially available. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. This procedure is called the Favorskii reaction, not to be confused with the Favorskii rearrangement (18-7). ... 

Sodium acetylides are the most common reagents, but lithium, magnesium and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylene diamine complex. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. 1,4-Diols can be prepared by treatment of aldehyde with dimetalloacetylenes. 

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). 

Lithioorganocuprates. Lithium bis(trimethylsilylpropynyl)-copper, prepared by addition of lithium trimethylsilylpropyne to copper(I) iodide, has been used as a nucleophilic source of the trimethylsilylpropynyl group (eq 5). Chemoselective 1,2-addition resulted in a 90% isolated yield of the desired secondary alkynyl alcohol (eq 5) use of the corresponding lithium or bromo-magnesium acetylides resulted in substantial recovery of allenic products. ... 

Hove 1. The procedure described in Ref. 1 was modified. To a solution of 2.0 mol of lithium acetylide in 1.2 1 of liquid ammonia in a 4-1 round-bottomed, three-necked flask (see Fig. 2) was added 1.5 mol of freshly distilled benzaldehyde with cooling at about -45°C. After an additional 30 min finely powdered ammonium chloride (2 mol) was introduced in 15 min. The ammonia was allowed to evaporate, then water (1.1 1) was added and the product was extracted with diethyl ether. After drying over magnesium sulfate the extract was concentrated in a water-pump vacuum. High-vacuum distillation,... 

The facility with which the transfer of acetylenic groups occurs is associated with the relative stability of the ip-hybridized carbon. This reaction is an alternative to the more common addition of magnesium or lithium salts of acetylides to aldehydes. 

While ephedrine derivatives showed some selectivity, the most promising results were obtained with cinchona alkaloids. Lithium alkoxides and lithium acetylides (n-BuLi or LiHMDS used to deprotonate both the acetylene and the alcohol) gave better results than the corresponding sodium or magnesium salts. Higher enan-tioselectivity was obtained in THF (homogeneous) than in toluene or diethyl ether (heterogeneous). 

Reaction of magnesium aUsyhdene carbenoids with Af-lithio nitrogen-containing heterocycles, lithium acetylides and hthium... 

TABLE 12. Synthesis of enyne 183 from magnesium alkylidene carbenoids 134a with lithium acetylides and electrophiles... 

Cadmium propionate, 2412 Calcium acetylide Methanol, 0582 Diazomethane Calcium sulfate, 0405 3,5-Dinitrotoluamide, 2936 Lithium tetrahydroaluminate Dioxane, 0075 Magnesium perchlorate, 4078 Magnesium Methanol, 4685 Mercury(II) thiocyanate, 0975 Silicon dioxide Hydrochloric acid, 4833 Sodium azide Heavy metals, 4753 Sodium Halocarbons (reference 10), 4790... 

Lithium acetylides and lithium thiolates have been shown to add to magnesium alkylidene carbenoids yielding conjugated enynes and vinyl sulfides.94... 

For substituted epoxides, reagents, such as dialkynyl magnesium compounds (RC=C)2Mg [56] or lithium acetylides RC=CLi, are more effective and give better yields. 

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