Vinylidene rhenium intermediate

A rhenium-catalysed regio- and stereo-selective reaction of terminal alkynes with imines forms A-alkylideneallylamines rather than the expected propargylamines. The 0-carbon of the alkynyl rhenium is believed to attack the imine carbon to give a vinylidene rhenium intermediate (Scheme 6)7 ... 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes". This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

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