Vinylidene 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].

The deprotonation of carbyne complexes is the formal reverse of the addition of a proton to the vinylidene (Equation 1.10, Table 1.4) ... 

Double deprotonation of Ru(=C=CH2)(PPh3)2Cp with LiBu affords the lithio-alkynyl Ru(C=CLi)(PPh3)2Cp [281]. Reversal of the intramolecular reductive elimination leading to vinylidene is found on treatment of trans-IrCl(=C=CH R)(L)2 with CO to give IrH(CO)(C=CR)(L)2 [154]. 

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]. 

Double cyclization of iodoenynes is proposed to occur through a Rh(I)-acetylide intermediate 106, which is in equilibrium with vinylidene lOS (Scheme 9.18). Organic base deprotonates the metal center in the course of nucleophilic displacement and removes HI from the reaction medium. Once alkenylidene complex 107 is generated, it undergoes [2 + 2]-cycloaddition and subsequent breakdown to release cycloisomerized product 110 in the same fashion as that discussed previously (Scheme 9.4). Deuterium labeling studies support this mechanism. 

Deprotonation of some molybdenum-carbyne complexes affords vinylidene derivatives suitable reagents are aryldiazonium salts, trifluoroiodo-methane 34), or n-butyllithium (55) ... 

Treatment of some iron-acyl complexes with trifluoromethanesul-phonic anhydride (TfzO) affords vinylidene derivatives directly (5 7,38). The reaction is envisaged as a nucleophilic attack on TfzO by the acyl, followed by deprotonation to the vinyl ether complex. A combination of an excellent leaving group (TfO-) with a good electron-releasing substituent on the same carbon atom facilitates the subsequent formation of the vinylidene ... 

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... 

Deprotonation of the alkyne group of propargyl halides or sulfonates can also lead to elimination and formation of a vinylidene. Interestingly, these derivatives react with alcoholates, not yielding enol ethers via O-alkylation but undergoing C-H bond insertion instead (Scheme 5.52). 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

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... 

Addition of internal alkynes to (t)5-C5H5)(PR3)2RuCI does not lead to the formation of the corresponding disubstituted vinylidene (68). The failure of this reaction could reflect the relative difficulty of a 1,2-alkyl shift for internal alkynes as compared to the 1,2-proton shift for the successful rearrangement of terminal alkynes (Scheme 9). Alternatively, if the deprotonation-reprotonation route is important in the rearrangement of terminal alkynes (vide supra), then clearly internal alkynes would not undergo a similar isomerization. 

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