3- propynal, nucleophilic addition

Bhanu and Scheinmann " have shown that the dilithium salt of propyne alkylates firstly on the methyl group and then on the alkyne. The process occurs sequentially and two different electrophiles can be used. The reaction gave superior yields when the second electrophile underwent nucleophilic addition rather than substitution (Scheme 20). 

Intramolecular cyclopropanations of pendant alkenes are more favorable. Heteroatom-substituted 2-aza- and 2-oxabicyclo[3.1.0]hexanes, together with 2-oxabicyclo[4.1.0] heptanes, can be prepared from chromium and tungsten Fischer carbenes having a tethered alkene chain. An interesting carbene formation via a cationic alkylidene intermediate, nucleophilic addition see Nucleophilic Addition Rules for Predicting Direction), mA intramolecular cyclopropanation is shown in Scheme 59. An intramolecular cyclopropanation via reaction of alkenyl Fischer carbene complex (28) and propyne was used in a formal synthesis of carabrone (Scheme 60). 

Acetylide ion alkylation is limited to primary alkyl bromides and iodides, RCHgX, for reasons that will be discussed in detail in Chapter 11. In addition to their reactivity as nucleophiles, acetylide ions are sufficiently strong bases that they cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product cyclohexylpropyne. 

Tandem chain extension at both the 1-alkyne and propargylic positions of 1-alkynes by sequential (1) alkylation and (2) hydroxy-alkylation (1,2-addition) of 1,3-dilithiated alkynes provides an attractive, one-pot preparation of functionally disubstituted alkynes. The required 1,3-dilithiated species are obtained by lithiation of 1-alkynes at both the 1- and 3- positions with two equivalents of n-BuLi in the presence of TMEDA (tetramethylethylenediamine). The two-stage chain extension depicted below for propyne involves initial alkylation of the dilithiated propyne with the less reactive electrophile, n-BuBr, at the more nucleophilic propargylic position. Subsequent hydroxyalkylation at the less nucleophilic alkynylide position with the more electrophilic formaldehyde furnishes the a-hydroxyalkyne. ... 

The other major synthetic use of alkyne anions is their reaction with ketones and aldehydes to give an alkynyl alcohol via nucleophilic acyl addition. The lithium salt of 1-propyne, for example, reacted with aldehyde 40 to give alcohol 41 as part of Smith s synthesis of (+)-acutiphycin.50 The reaction is selective for ketones and aldehydes in the presence of acid derivatives, if the acetylide is not present in large excess. l... 

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