Transition metal-mediated alkynylation

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

Transition Metal-Mediated Alkenylations, Arylations, and Alkynylations... 

The most general route to generate vinylidene complexes [M]=C=C(H)R is the direct activation of terminal alkynes HC=CR by a coordinatively unsaturated transition metal complex, via the generation of unstable 77 -alkyne or hydride-alkynyl intermediates which tautomerize into the thermodynamically more stable vinylidene isomers (Scheme 15) ia,ic,207 theoretical and kinetic studies on the metal-mediated alkyne-vinylidene isomerization... 

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