Alkyne/vinylidene tautomerization

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. 

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. 

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. 

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... 

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. 

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). 

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. 

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... 

Equations 13.10-13.12 show three examples of the synthesis of vinylidene complexes by reactions of metal-acetylide complexes with acid or base. The molybdenum(II) acetylide complex in Equation 13.10 reacts with acid to protonate the p-carbon and generate a cationic vinylidene complex. In this case, the vinylidene complex is thermodynamically unstable. Warming to 0 °C leads to rearrangement of this species to the tautomeric alkyne complex. In contrast, the more electron-rich molybdenum-acetylide complex in Equation 13.11 containing three phosphite donors generates a vinylidene complex upon addition of a proton from alumina to the 3-carbon of the acetylide. The vinylidene form of the complex is apparently more stable than the alkyne complex in this case. 

DFT calculation revealed the origin of the (Z)-selectivity of the anft -Markovnikov hydroalkoxylation of terminal alkynes (122), catalysed by the rhodium(I) 8-quinolinolato carbonyl chelate (123). The reaction is likely to commence by the formation of the // -complex PhC=CH[Rh], which tautomerizes via a 1,2-hydrogen shift to generate the Rh(I) vinylidene complex PhCH=C=[Rh]. Methanol, as an oxygen nucleophile, then attacks the Ca, and via the transition state (124), which is 1.2kcalmol lower in energy than its stereoisomer, thus giving the (Z)-vinyl ether (125). An improvement in the (Z)-selectivity in the related Rh(I)-catalysed 0... 

Alkyne clusters of 1- and 2-types (R = H) and the tautomeric vinylidene cluster, Fe3(GO)9(/U3-G=CH2), were examined by DFT calculations. Two-electron reduction of 1 (R = Et 46e) consisting of two consecutive one-electron... 

It was also possible to achieve cross-dimerization with (Z)-selectivity of two types of alkynes that possess significantly different properties with respect to the tautomerization between alkyne and vinylidene ligands. Cross-dimerization of arylacetylenes and silylacetylenes was reported to proceed on using vinylidene-ruthenium complexes bearing bulky and basic trialkyl phosphine ligands in the presence of methylpyrrolidine (Scheme 27) [140]. 

---