Electrophilic reactions lithium acetylides

The aryl sulfoxide moiety may serve as a good leaving group in the exchange reaction. Thus, 1-haloalkenyl sulfoxide 55 undergo the exchange at —78°C to give carbenoid compounds 56 which can be trapped by electrophiles or converted to acetylenes 57 (equation 39) °. Reaction of carbenoid 58 with lithium acetylides leads to the formation of enynes 59 (equation 40). ... 

When lithium acetylides are added to acylsilanes and the reaction mixture is quenched with an electrophile, allenol silyl ethers are formed (Figure Si6.10). 

These reactions work because, although each starting material contains two carbonyl groups, one is more electrophilic and therefore more reactive towards nucleophiles (OH in the first case lithium acetylide in the second) than the other. We can order carbonyl compounds into a sequence in which it will usually be possible to react those on the left with nucleophiles in the presence of those on the right. 

Hydrolysis yields the terminal acetylene (3) carbonation yields the propargylic acid (4). Synthesis of a wide variety of acetylenes is possible by the reaction of the intermediate lithium acetylides with other electrophiles (alkyl halides, aldehydes, ketones). 

The reaction of a lithium acetylide with the electrophilic 3-methylisoxazol-5-carbonyl chloride 30 afforded the substituted isoxazole 31 in 67% yield <04TL4935>. 

Propargylic trimethylsilanes, now readily accessible by reaction of lithium acetylides with (trimethylsilyl)methyl halides or triflate, are emerging as useful synthetic intermediates. The general pattern of reactivity involves attack by an electrophile with cleavage of the carbon-silicon bond. For example, treatment of propargylic trimethylsilanes with TFA or bromine leads to allenes or 3-bromoallenes respectively, " and reactions with acetals furnish 4-alkoxy-allenes" (e.g. Scheme 101). 

SCHEME 26.22. Reaction of lithium acetylide with electrophilic trifluoromethylating reagent. 

Generally, the alkynes can be prepared by the following two types of methods (3-elimination of CF3-containing hal-ogenated alkenes and a cross-coupling of trifluoromethy-lated acetylide with various halides in the presence of palladium catalyst, as shown in Scheme 26.49. Additionally, the reaction of the lithium acetylide with various electrophiles, such as carbonyl compounds, imines, and... 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

The direct introduction of an acetylide moiety, using pyridine A-oxide (or quinoline, diazine and triazine iV-oxides) can be achieved in a comparable way, by reaction with potassium phenylacetylide reaction with the lithium salt requires addition of acetyl chloride at the end of the reaction to aromatise. At low temperature, and using i-PrMgCl, 2-metallation of pyridine iV-oxides can be achieved, and thus, the introduction of electrophiles at the 2-position. ... 

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