Markovnikov addition fragmentation

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

Recently, the cyclization of pent-4-yn-l-ols and but-3-yn-l-ols via anti-Markovnikov addition of the hydroxy group to the terminal carbon of the triple bond with a ruthenium catalyst in THF at 80°C and no other additive has been reported. All types of acetylenic alcohols, purely aliphatic and including a phenylacetylene fragment have been cycloisomerized in excellent yields. The catalyst is a cationic ruthenium(II) complex has depicted in Scheme 29 [114]. 

The mechanism begins the same way as the Hg-mediated nucleophilic addition to alkenes. In the first step, an electrophilic 7r complex forms between the alkyne and Hg(II). Water attacks one of the C atoms of the ir complex in Markovnikov fashion to give a 2-hydroxy- l-alkenylmercuryfH) compound, an enol, which is pro-tonated to give a carbocation. Fragmentative loss of Hg(II) then occurs to give a metal-free enol, which tautomerizes to give the ketone product. 

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