Vinylidene complexes deprotonation

Closely related to the a-addition of nucleophiles is the P-deprotonation of electrophilic carbyne complexes. In many of the examples reported [143,530,531] the resulting vinylidene complexes could not be isolated but were generated in situ and either oxidized to yield stable carbene complexes [532] or used as intermediates for the preparation of other carbyne complexes [527]. Cationic carbyne complexes can be rather strong acids and undergo quick deprotonation to vinylidene complexes with weak bases [143]. An interesting example of the use of anionic vinylidene complexes as synthetic intermediates is sketched in Figure 3.24. 

Fig. 3.24. Generation of nucleophilic vinylidene complexes by deprotonation of carbyne complexes [530].

In 1979, Rudler ef al. reported another example of the presence of a vinylidene complex during the reaction of pentacarbonyl[methoxy(methyl)carbene] tungsten 5 with MeLi followed by acidification with TFA [4]. It was proposed that the vinylidene complex 7 was generated by deprotonation of the a-proton of the carbene complex followed by elimination of methoxide and reaction with the dimethylcarbene complex 8, the addition-elimination product of MeLi with the starting carbene complex, to give the dinuclear complex 6 (Scheme 5.2). 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Those vinylidene complexes which are not readily deprotonated by bases undergo attack at the a-carbon by anions such as H-, MeO-, NH2 to give vinyl derivatives... 

Although the reaction of copper acetylides with transition metal halides has been successfully applied to the preparation of a variety of transition metal acetylides (64), the generation of copper-complexed derivatives is not unprecedented (65). A simpler and more general route to ruthenium acetylide complexes involves the deprotonation of ruthenium vinylidene complexes as described in Section VI,C. 

The monosubstituted vinylidene complexes are readily deprotonated with a variety of mild bases (e.g., MeO-, C032 ), and this reaction constitutes the most convenient route to ruthenium acetylide complexes. Experimentally the deprotonation is most easily achieved by passing the vinylidene complex through basic alumina. Addition of a noncomplexing acid (e.g., HPF6) to the acetylide results in the reformation of the vinylidene complex [Eq. (66)]. Reaction of 1 and terminal alkynes such as phenylacetylene in methanol followed by the addition of an excess of... 

As detailed above, alkyne complexes can be prepared from vinylidene complexes by deprotonation. The reverse... 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

A range of vinylidene complexes [ReX(=C=CFlR)(dppe)2] (X = F, Cl) has been prepared by deprotonation of the corresponding carbynes [ReX(=C-CH2R)(dppe)2]+. The mechanism of the protonation of these complexes has shown that, depending on the reaction conditions, protonation occurs either directly at the vinylidene /3-carbon or at the metal, followed by migration to the referred /3-carbon. ... 

Vinylidene and carbyne complexes also offer a rich chemistry following electron transfer, and a comprehensive review on that topic is available in the literature.As an example, oxidation of vinylidene complexes generates highly reactive radical cations. These may undergo a host of different follow-up reactions and they are summarised in Scheme 6.10. Possible follow reactions include dimerisation by direct Cp—Cp homocoupling to dinuclear butanediyli-dyne complexes M " =C—CRH—CRH—deprotonation to 17 valence-electron alkynyl radicals M —C=CR, which subsequently dimerise to the corresponding bis(vinylidenes) (= 1,3-butadiene-1,4-diylidene derivatives) M = C = CR-CR = C = C Mf and CH-bond homolysis. The latter... 

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. 

Protonation of metal-acetylide complexes affords the corresponding vinylidene complexes e.g. 20 and 99, Figure 1.48). Proceeding from 20 to 99 leads to a lowering of (3 values, by a factor of five. As the vinylidene complexes can be easily deprotonated to give back to the alkynyl precursors, and this sequence can be repeated, these complex pairs can provide an interesting protically switchable NLO system. 

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