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... 

In path a (Scheme 4.15), the barrier for the rate-determining step is 46.5 kcal mol. In path b2 (Scheme 4.15), the barrier for the rate-determining step (the alkyne-to-vinylidene tautomerization) is only 26.4kcal mol . The significant barrier difference (about 20 kcal mol ) between paths a and b2 explains the endo-regioselectivity observed experimentally in the cydoisomerization of 4-pentyn-l-ol [117, 118]. 

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. 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

Stepwise double alkyne to vinylidene tautomerization is the key step responsible for the formation of the 77 -butadienyl iridium(in) complex [Ir K -0,C -O=G(Me)CH=CPh (77 -PhCH=CHC=CHPh)(PPh3)2]SbF6 579 346 proposed mechanism, which is illustrated in Scheme 82, involves an alkyne to vinylidene rearrangement (I —> II) followed by a hydride insertion (II — III), a second alkyne to vinylidene rearrangement (III — V), and a migratory insertion of the vinyl to the vinylidene (V — VI) resulting in the G-C bond formation. 

Until 2008, the alkyne/vinylidene tautomerization had only been observed for metal-coordinated terminal alkynes, with the corresponding monosubstituted... 

Although considerable evidence has been presented to support each of these reaction pathways, the case studies collated below demonstrate how experimental and theoretical chemistry can operate in tandem to provide mechanistic insights for the alkyne/vinylidene tautomerization. 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

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. 

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

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. 

Although theoretical calculations on the reverse vinylidene-to-q -alkyne tautomerization are limited, we can see from the examples shown in Scheme 4.10 that in order... 

In 2003, Gimeno, Bassetti and coworkers reported an unusual diastereoselective [2 + 2] cycloaddition of two C=C bonds under mild thermal conditions (Scheme 4.18) [128]. Heating the vinylidene complexes Rul leads to the bicyclic alkylidene complexes Ru2. In 2004, Sordo and coworkers investigated the mechanism of this [2 + 2] cycloaddition theoretically [25]. With model complexes in which the indenyl ligand was modeled with a Cp ligand, two different pathways (paths a and b) were studied, shown in Scheme 4.19. Path a considers a concerted process. In the stepwise pathway (path b), the vinylidene-to-alkyne tautomerization of R1 followed by... 

Keywords hydrido complex, ruthenium, tautomerization, vinylidene complex... 

The mechanism of the polarity inversion of tautomeric molecules is totally different from the orientation polarization of conventional organic dielectrics, such as camphor and poly(vinylidene fluoride), and the dielectric response of this new type of dielectric should be much faster. Furthermore, a significant contribution of the proton-tunnelling mechanism to the proton tautomerism is frequently observed. Consequently, the dielectric property derived from proton tautomerization should have a high chance of being related to quantum phenomena. 

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