Vinylidene from alkynyl complexes

From Vinylidene Complexes Generated from Alkynyl Complexes... 

These alkynyl complexes can be protonated to afford vinylidene complexes, which can in turn be deprotonated to give the starting alkynyl complex, reactions that are spectroscopically quantitative. The tabulated data also provide the opportunity to assess the effect of this protonation, in proceeding from alkynyl complex to vinylidene derivative. One would perhaps expect that replacing the electron-rich ruthenium donor in the alkynyl complexes with a (formally) cationic ruthenium centre in the vinylidene complexes would result in a significant decrease in nonlinearity. 

A series of general headings can be applied to the synthetic routes used for generating vinylidene complexes (i) reactions of alkynes with labile and/or coordinatively unsaturated species, (ii) reactions with alkynes in the presence of a halide abstracting agent, (iii) formation from alkynyl complexes, and (iv) formation from carbyne complexes. 

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

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

Table 1.3 Some metal vinylidene complexes, L M=C=CRR, obtained from alkynyl-metal systems.

Just as vinylidene-type resonance forms constitute one extreme description for the oxidised forms of alkynyl complexes (Scheme 6.4), cumulenic structures may result from the oxidation of oligoynediyl-bridged dimetal complexes (Schemes 6.5 to 6.8). Interesting examples come from the work of Sato, who... 

Lin and co-workers have shown that the alkynyl complex TpRu(C=CPh)(PPh3)2 (prepared from TpRuCl(PPh3)2 and HC = CPh and NaOMe) reacts with ICH2CN to afford the cationic vinylidene salt [TpRu(=C = CPhCH2CN)(PPh3)2]I, which... 

Carbenes can sometimes be made from organic carbene precursors, such as diazo compounds, from 1,1-diphenylcyclopropene (Eq. 11.9), or from rearrangement of an alkynyl complex, as in the first step of Eq. 11.7 to form a vinylidene. NHC (11.8) syntheses are discussed in Section 4.3. 

The authors reported also one example that demonstrated the suitability of the method also for the synthesis of enol lactones starting from alkynyl carboxylic acids. Also in this case, due to the mechanism involving a Rh-vinylidene complex, the approach is limited to terminal alkynes. 

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). 

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]. 

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