Propargylic electrophiles, transformations

The introduction of a double bond between the triple bond and the leaving group of a propargyl electrophile leads to enyne electrophiles (e.g., 198) which would give access to vinylallenes 199 if the attack of the nucleophile takes place at the triple bond in an Sn2" (l,5)-substitution reaction (Scheme 48). Besides the regioselectivity, two types of stereoselectivity have to be considered in this transformation, that is, the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. In the event, the reaction of enyne acetates 198 with various lithium cuprates proceeds... 

Homopropargylic alcohols as well as propargylic epoxides and pentynols readily form cyclic ruthenium alkoxycarbenes upon intramolecular nucleophilic addition of the OH group to the electrophilic a-carbon of ruthenium-vinylidene species. Their oxidation in the presence of N-hydroxysuccinimide leads to the formation of penta-lactones. The best catalytic system reported until now for this transformation of but-3-ynols is based on RuCl(C5H5)(cod), tris(2-furyl)phosphine, NaHCOs as a base, in the presence of nBu4NBr or nBu4NPp6, and N-hydroxysuccinimide as the oxidant in DMF-water at 95 °C (Scheme 8.11) [22]. 

Allenylsilveifl) compounds are prepared in situ by deprotonation of the allenic hydrocarbon with n-butyllithium and subsequent treatment with silver bromide (equation 5). Allenylsilver compounds afford allenic derivatives when treated with a variety of electrophiles, without significant isomerization to the propargyl system (equation Reaction of the allenylsilver compound with carbon disulfide results in an interesting transformation to p, y-unsaturated-7-dithiolactones presumably by way of silver salts (Scheme 11). - ... 

The addition of sulfur nucleophiles to alkynes is a less developed transformation. However, Yamamoto described the attack of the sulfur atom of aryl thioethers to afford benzothiophenes (equation 31). More recently, it has been showed that propargylic thioethers or thioacetals undergo migration to give carbenes that cyclize in hydroarylation processes and thiocarbamates that evolve by propargylic rearrangement. Thiosilanes can perform as both sulfur nucleophiles and silicon electrophiles in intramolecular reactions to afford benzothiophenes (equation 32). ... 

The transformation of propargyl vinyl ethers into 2f/-pyrans is also possible with electrophilic iodine or bromine as demonstrated recendy by Xin and co-workers (Scheme 22) (15TL401). Firstly, propargylic alcohols 65 and bis-acceptor-substituted alkynes 66 are converted in situ into propargyl vinyl ethers.While no transition metal-catalysts are involved, the cychzation is induced by the initial formation of a halonium ion. The halonium ion is then attacked by the internal vinyl group and, after deprotonation, the product 67 is formed. The halogenated 2H-pyrans 67 were obtained in good yields under mild conditions. 

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