Lithium alkynylides

Under similar conditions (diethyl ether or THF as solvent) the reaction of lithium alkynylides with methanesulfonyl chloride (CH35O2CI) gave the corresponding alkynyl sulfones in low to moderate yields. 

Compared to many other types of synthetic intermediates, acetylides, RC=CM (M = Li, Na, K), show a moderate reactivity towards alkyl halides in the usual organic solvents E O and THF and in liquid ammonia [2], In this respect acetylides resemble enolates >C=COM. In the absence of dipolar aprodc co-solvents (DMSO or HMPT), lithium alkynylides, RC=CLi, react sluggishly in Et O or THF with most alkyl halides [2]. In liquid ammonia the alkylation of alkali acetylides with the lower (up to C-5) alkyl bromides or iodides proceeds at a satisfactory rate [5]. A certain amount of DMSO added to the reaction mixture increases the solubility of halides with a longer carbon chain. A second effect of the addition of this co-solvent is that the temperature of the reaction mixture can gradually rise as more ammonia evaporates. In this way, the reaction can proceed gradually over the range from -33 C (b.p. NH3) to room temperature. Specific alkylation on the acetylenic carbon takes place if an equivalent amount of an alkyl halide is added to dilithiated propargvl alcohol in liquid ammonia... 

For many alkylation reactions of alkynylides in liquid ammonia Li+ is preferred to both Na+ and K+ as counter-ion, since lithium alkynylides generally have a better solubility. Furthermore, the principal side-reaction, dehydrohalogenation of the alkyl halide, occurs to a lesser extent with lithium compounds. 

In the procedures described below, lithium alkynylides are used. When R is CH3 or higher alkyl, the use of this cation is clearly preferred for several reasons. In particular the sodium alkynylides are poorly soluble in liquid ammonia. The second reason for choosing the lithium compounds is that the reaction of NaOCH with (an excess of) oxirane affords appreciable amounts of the subsequent product HOCCH2CH2OCH2CH2OH. 

DMSO-Promoted Reaction of Lithium Alkynylides with Oxirane... 

Compared to the reaction in exp. 30, the conversion of 2,3-dibromotetrahydrofuran with propynylmagnesium bromide can be carried out at higher temperatures, so that the reaction time can be shortened to 2 h at 50 C. When propynyUithium is used instead of the Griguard reagent, a very impure product is obtained in low yield. Elimination possibly predominates with the more strongly basic lithium alkynylides (compare exp. 28). 

The general scheme for preparation in the laboratory has a number of alternatives, the choice of a particular method being determined by the availability of the starting acetylene, r CsCH, the desired scale of the preparation (e.g. a few millimoles, 100 mmolar, 1 molar or more) and a number of other factors. The most versatile method, suitable for working on scales varying from a few mmoles to -0.5 mole, is the reaction of a lithium alkynylide with a carbonyl compound in THF(-hexane) ... 

General Procedure for the Coupling of lithium Alkynylides with Aldehydes... 

For syntheses on a relatively small scale, lithium alkynylides are the most convenient starting compounds they can be used as such or transformed into other organametallic intermediates. 

Acetylenic Esters from Lithium Alkynylides and Alkyl Chloroformates... 

Di(l-alkynyl)sulfides are formed in excellent yields, if (freshly distilled) sulfur dichloride is added to a solution or suspension of a lithium alkynylide in Et20 [106). In an analogous manner, di(l-alkynyl)sulfoxides are formed from thionyl chloride and an alkynyllithium [106], while sulfinyl chlorides R S(=0)G may be used to prepare the sulfoxides RC=CS(=0)R Preliminary experiments suggest that interaction between alkynyllithium and sulfonyl chlorides R S02C1 can give both sulfones ROCSOjR and chlofo-alkynes RCsCCl [2). 

Mahyisulfonyl chloride CH3SO2CI and tosyl chloride give low to moderate yields of ROCSO2CH3 and RC=CS02-Aryl. Part of the sulfonyl chloride reacts with the lithium alkynylide to give a chloroalkyne. A more succesful method to prepare acetylenic sulfones involves Oxidation of acetylene sulfides with peiacids. 

The halogenations with CI2, Br2 and I2 are carried out most conveniently with the lithium alkynylides [2,119], Subsequent addition of Q2 and Br2 to the triple bond can, in many cases, be avoided by a sufficiently slow addition of the solution of the halogen to a strongly cooled solution or suspension of the lithiated alkyne in E12O or THF. 

Allenes, H2C=C=CHR (R = alkyl), are metallated regioselectively at the terminal C-atom by butyllithium in THF ai low temperatures [186], Above 0 C, the metallated allenes rearrange to lithium alkynylides ... 

The LiBr is added to solubilize part of the propynyllithium. If no LiBr is added, the coupling with ketones is slower and pan of the ketone is converted into the enolate. In the cases of soluble lithium alkynylides, the addition of LiBr is not necessary. 

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