Acetylene-vinylidene tautomerization

A few theoretical studies have been carried out to clarify the mechanism of the metal-assisted acetylene to vinylidene tautomerization. The first work done by Silvestre and Hoffmann using extended Hiickel calculations on the complex... 

The alkyne-to-vinylidene tautomerization processes on various transition metal centers have also been discussed. Three different pathways for the formation of vinylidene from p -acetylene on electron-rich transition metals were the most theoretically studied. Most studies suggested that the favorable pathway proceeded via an intermediate with an agostic interaction between the metal center and one C—H bond followed by a 1,2 hydrogen shift (the bl+b2 pathway shown in Scheme 4.5). The reverse process, the vinylidene-to-p -acetylene tautomerization, was also discussed. It was found that complexes with electron-poor metal centers were able to mediate the reverse process. 

Fig. 11 (A) Acetylene—vinylidene isomerization. (B) Metal-induced acetylene to vinyli-dene tautomerization.

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

Tautomerization between Ti -acetylene and vinylidene on transition metal centers... 

Tautomerization Between T) -Acetylene and Vinylidene on Transition Metal Centers... 

Conversion of the unsubstituted acetylene to vinylidene has been widely investigated both experimentally [66-68] and theoretically [69-71]. These studies showed that when metals were not involved, the formation of vinylidene from free acetylene (Scheme 4.4) was very endothermic (44—47 kcal mol ). Since most transition metal fragments can stabilize a vinylidene ligand, the tautomerization of r -acetylene to vinylidene on transition metal centers becomes feasible. Recently, Clot and Eisen-stein have thoroughly reviewed theoretical studies on various tautomerization pathways [44]. For the completeness of this chapter, we here briefly summarize the relevant theoretical flndings. 

---