Propargylic substrates

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

This second class of sigmatropic rearrangements can again be subdivided here we find several classical named reactions that, again with propargylic substrates, lead to allenic products. 

Several other propargylic substrates have successfully undergone [3,3]-sigmatropic rearrangements. Examples are thiocyanates to isothiocyanates [580], the opposite conversion [581] and the equilibrium between such species as exemplified by 173 (Scheme 1.77) [582], cyanates to isocyanates [582] and the formation of isoselenocya-nates [583] and azides [584]. 

A propargyl substrate having a substituent at the propargyl position is centrally chiral and an allenic product from the SN2 substitution reaction will be axially chiral. Chirality transfer in the SN2 reaction, accordingly, may be achieved starting from an enantiomerically enriched propargyl electrophile [29]. The reactions in Scheme 3.11 are some recent examples of the center to axis chirality transfer by Pd-catalyzed SN2 reactions [41, 42]. 

Diethynylallenes, i.e. derivatives of the parent system 8 in Scheme 5.1, are obtained when the propargylic substrate already contains an additional triple bond as shown in Scheme 5.14. Here the bispropargyl carbonate 107 - for example with R = n-hexyl - is coupled with tris(isopropyl)silylacetylene to provide the protected hydrocarbon 108 in excellent yield (94%) [39, 40]. 

Scheme 3.43. Reactions of a-aminoalkylcuprates with allylic epoxides [171] and propargylic substrates (Boc = t-butoxy-carbonyl) [172].

Relatively few examples involving a phosphorous atom in the a-heteroatom-alkylcuprate have appeared [176]. Such cuprates have been treated with allylic and propargylic substrates, but have not been reported to undergo conjugate addition... 

It has been reported that quenching of the zinc intermediates with electrophiles E-X rather than simple deuteriolysis gives alkynes 33 selectively, thus providing a one-pot protocol for the introduction of a nucleophile (R3) and an electrophile (E) in the 1- and 3-positions of the propargylic substrate 28 (equation 13). 

The most widely used preparative method of allylindium(m) or propargylindium(lll) compounds is the oxidative addition of metallic indium or indium(l) halides to allylic or propargyl substrates.4 26 27 Allylic bromides and iodides serve as good allylic sources without any other activation. In the case of allylic chlorides, a proper additive such as lithium iodide is required to promote the oxidative addition. Allylic indium compounds prepared by oxidative addition of metallic indium are considered to exist as the sesquihalide structure (allyl jImXj), which has been... 

More specifically, propargylic substrates may rearrange upon treatment with bases to give the corresponding heterosubstituted acetylenes via allene intermediates (61). 

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