Propargylic anion equivalents

Alkynic intermediates serve as important functional groups in organic synthesis. Many important reactions exploiting the unique and versatile chemistry of the carbon-carbon triple bond have been devised over the last few years. A general strategy for the synthesis of substituted alkynes involves substitution and addition reactions of propargylic anion equivalents this approach is particularly well suited for the preparation of homopropargylic alcohols (Scheme 28). 

For a review concerning the chemistry of propargylic anion equivalents, see R. Epsztein, in Comprehensive Carbanion Chemistiy , ed. E. Buncel and T. Durst, Elsevier, Amsterdam, 1984,part B, p. 107. 

Propargylic Anion Equivalent. (TMS)allene reacts with electrophiles at the C-3 position in an Se2 process analogous to electrophilic substitution reactions of allyl- andpropargylsilanes. For example, upon treatment with trimethylsilyl chlorosulfonate or sulfur trioxide-1,4-dioxane, (TMS)allene yields silyl esters of sulfonic acids (eq 3). (TMS)allene undergoes conjugate addition with a, 8-unsaturated acyl cyanides to yield 5,e-acetylenic acyl cyanides. ... 

Despite reports to the contrary early in 1980/ Danheiser and Carini have demonstrated that trimethylsilylallenes function as propargylic anion equivalents in reactions with ketones or aldehydes in the presence of titanium tetrachloride, regiospecifically forming homopropargylic alcohols. Unexpectedly, some silyl-allenes give mixtures of homopropargylic alcohols and trimethylsilylvinyl chlorides, but exposure of the crude reaction product to potassium fluoride in DMSO induces elimination from the latter to furnish exclusively the required alcohols ie.g. Scheme 113). 

Trimethylsilylallenes behave as propargylic anion equivalents during the titanium tetrachloride catalysed addition to carbonyl compounds, leading to homopropargylic carbinols. This new approach should prove useful in the synthesis of branched acetylenes which are not accessible via alkylation of acetylide anions with alkyl halides and epoxides (Scheme 23). ... 

Conjugate addition of the propargyl anion equivalent 8 to 1 proceeded with the expected >95 5 axial diastereoselectivity, to give the silyl enol ether 9. Exposure of the derived iodide 10 to catalytic [Ph PAujCl and AgBF induced smooth cyclization to the cis hydrindane 2. 

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