New Catalysts and Mechanistic Studies

Electron-deficient (having less than 18 electrons) and coordinatively unsaturated (containing less than 6, usually only 4 or 5 ligands) metal carbene complexes are highly effective in metathesis.191-194 

They are highly efficient initiators since they are all converted to propagating species at the very early stage of the reaction. Theoretical treatments,198 and the use of advanced analytical techniques, for example, electrospray ionization tandem mass spectrometry199 have provided useful information about the nature and properties of intermediates involved in metathesis. 

A surprising new observation made by Rooney and coworkers shows that complex 12 generates radical anions when treated with jr-acceptors even dienes and simple alkenes.213 

Metathesis of alkynes is a much less general reaction and has not yet found the attention that alkene metathesis has received.158-160 Nonetheless, a few observations have shown that it has the potential of useful applications emerging as a new synthetic tool.214-216 

Mechanistic studies revealed that alkyne metathesis and ring-opening metathesis polymerization of cycloalkynes proceed via metal carbyne complexes,217 218 which is also supported by theoretical studies.219 The polymerization of PhC=CMe with NbCIs or TaCIs yields a polymer that degrades to oligomers as a result of secondary metathesis reaction. A stable polymer, however, may be synthesized with TaCIs and Ph4Sn as a cocatalyst.220 

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