Grignard reagents alkynide

Alcohols are obtained from epoxides by acid-catalysed cleavage of H2O or base-catalysed cleavage by Grignard reagents (RMgX, RLi), metal acet-alides or alkynides (RC=CM), metal hydroxides (KOH or NaOH) and LiAlH4 (see Section 5.5.4). 

Terminal alkynes are acidic, and the end hydrogen can be removed as a proton by strong bases (e.g. organolithiums, Grignard reagents and NaNH2) to form metal acetylides and alkynides. They are strong nucleophiles and bases, and are protonated in the presence of water and acids. Therefore, metal acetylides and alkynides must be protected from water and acids. 

Besides electrophilic addition, terminal alkynes also perform acid-base type reaction due to acidic nature of the terminal hydrogen. The formation of acetylides and alkynides (alkynyl Grignard reagent and aUcylnyllithium) are important reactions of terminal alkynes (see Section 4.5.3). Acetylides and alkynides undergo nucleophilic addition with aldehydes and ketones to produce alcohols (see Section 5.3.2). 

Grignard reagents, organolithium compounds, and sodium alkynides react with formaldehyde to produce primary alcohols, all other aldehydes to produce secondary alcohols, and ketones to produce tertiary alcohols. 

Thiazolines activated with an equivalent of BF3 readily react with a wide range of organometals, giving tranj-4,5-disubstituted thiazoles stereoselectively. Alkyllithiums, Grignard reagents, lidiium alkynides, nitronates, ester and ketone enolates have been employed as the nucleophile. Stereocontrolled construction of three contiguous asymmetric centers is performed with a lithiated isothiocyanatoacetate, and the product is successfully transformed to (+)-biotin (Scheme 27), ° ... 

Lithium alkynides in tetrahydrofuran or dioxane often give substitution products with secondary haloalkanes, while alkynide Grignard reagents do not usually react with haloalkanes except in the presence of other metals such as cobalt and copper. Substitution of iodine or bromine for chlorine in the halo-alkane often leads to an increased yield of the alkylation product and alkanesulfonates may give greater yields than haloalkanes. Scheme 1 illustrates examples of alkylation of haloalkanes and alkyl sulfates with alkynides of Group I metals. 

Alkynide ions react with carbonyl groups in much the same way as Grignard reagents do. We recall that these ions are effective nucleophiles that will displace a hahde ion ftom an alkyl halide to give an alkylated alkyne. The alkynides are prepared in an acid-base reaction with acetylene or a terminal alkyne using sodium amide in ammonia. If a carbonyl compound is then added to the reagent, an alcohol forms after acid work-up. If the alkynide is derived ftom acetylene, an acetylenic alcohol forms. 

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