Vinylidene from 1-alkynes

From Vinylidene Complexes Generated from Alkynes... 

Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes.

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. 

Scheme 5.6 Generation of Generation of vinylidene complexes directly from alkynes.

All metal vinylidenes described herein are derived from alkynes. While alkyne-to-vinylidene interconversion typically occurs via the 1,2-shift of a hydrogen atom, the corresponding migration of heavier main group heteroatoms is also possible. 

The best example of chemical activation in cluster chemistry is the use of Me3NO which results in CO replacement under mild conditions (129). In an interesting example taken from alkyne-cluster chemistry, when Me3NO is used dry a monosubstituted cluster derivative is obtained, but with damp Me3NO a tetranuclear vinylidene complex is isolated (130, 131), as illustrated in Eqs. (4) and (5), respectively. 

It is noteworthy that computational and experimental studies have shown that the formation of ruthenium-vinylidenes from terminal alkynes and ruthenium hydride complexes also proceeds via the formation of t -vinyl intermediate (Scheme 8.4) [14]. Thus, in this case the vinylidene ligand is not formed directly from the alkyne, and its /3-hydrogen atom arises from the hydrido ligand. 

Scheme 1 General pathways for the formation of metal vinylidenes from terminal alkynes...

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

Since Bruce s pioneering work in the area of ruthenium vinylidene chemistry (1), it has been well known that isomerization of a terminal alkyne to a vinylidene on a metal center is not only favorable but also effects a reversal in the reactivity of the carbon atoms. However, hydration catalysis was not possible, because alkyl migration from a proposed acyl intermediate led to an... 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

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). 

The acetylene coordinates trans to the least o electron donor group, chlorine. Coordination of the C-H bond is a less favorable alternative to coordination of the n system. The o C-H complex is 17.1 kcal.mol 1 less stable than the rc-alkyne complex (Figure 5). From this c C-H intermediate the 1,2 shift is possible with a relatively small activation barrier (+15.5 kcaLmol 1) to yield the vinylidene complex. However this mechanism is in contradiction with the labeling experiment. 

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. 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

This reaction is quite different from the other P-H addition reactions in that it involves external nucleophilic attack of HPPh2 on the vinylidene ligand as shown in Scheme 13. The ZIE ratio depends on the structures of the substrate and the catalyst. Ru-Cp" (Cp =77 -CsMes) species selectively forms the Z isomer while Ru-Cp (Cp r -CsHs) favors the E isomer. Since the key intermediate is the vinylidene species that has an electrophilic carbon, the reaction is applicable to other alkynes that are vinylidene precursors. Thus, phenylacetylene also reacts similarly to give Ph2PCH=CHPh ZIE=93I7), while internal alkynes are totally unreactive. 

---