Ruthenium vinylidenes protonation

Scheme 10.14 rationalizes the divergent behavior of the two catalytic systems in these selective transformations of pent-l-yn-ols. The presence of phosphine ligands promotes the formation of ruthenium vinylidene species which are key intermediates in both reactions. The more electron-rich (p-MeOC6Fl4)3P phosphine favors the formation of a cyclic oxacarbene complex which leads to the lactone after attack of the N-hydroxysuccinimide anion on the carbenic carbon. In contrast, the more labile electron-poor (p-FC6H4)3P) phosphine is exchanged with the N-hydroxysuccinimide anion and makes possible the formation of an anionic ruthenium intermediate which liberates the cyclic enol ether after protonation. 

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

An intermolecular version of this rearrangement involving dissociation of the acidic proton on Ca of the slipped acetylene, followed by reprotonation of an intermediate acetylide (discussed in Section VI,C), must also be considered as a potential route to the cationic ruthenium vinylidene species (Scheme 7). Unfortunately, to date this mechanism has not been addressed experimentally or theoretically. 

The formation of complexes 109 has been shown to proceed via a vinylidene ruthenium intermediate (112), which has been indirectly isolated by protonation of an acetylide-ruthenium complex (112). Arene ruthenium vinylidene complexes 113 appear to be much more reactive than their isoelectronic (C5H5)(R3P)2Ru=C=CHR+ complexes (63,66). 

In the ruthenium series, more success has been reported, particularly with RuP2Cp (Cp = Cp < [225],q -C9H7[115,226-232], Tp [130]). This can be rationalized in terms of a high contribution from the vinylidene to the stmcture of the alke-nylcarbyne formed by protonation of the allenylidene (Equation 1.9) ... 

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. 

For ruthenium catalysts a detailed study of [(PP3)RuH2] proposes a bis(alkynyl) complex as the real catalyst. The catalytic key step involves the protonation of an alkynyl ligand by external PhC=CH, allowing subsequent C-C bond formation between cis vinylidene and alkynyl groups [9]. 

The vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

Scheme 8.24 Preparation of cationic chelating ruthenium(II) carbenes 89 by protonation of neutral ruthenium(II) vinylidenes 87 via ruthenium carbynes 88 as intermediates (L = P Pr3 R = H, Me, Ph ...

Cyano-derivatives can be readily obtained by a ruthenium-catalyzed addition of various hydrazines to terminal alkynes [89] in which the cyano carbon atom arises from the terminal alkyne carbon atom. The tris(pyrazolyl)borate (Tp) complex RuCl(Tp)(PPh3)2 (1 mol%) was found to be the most active catalyst, and N,N-dimethylhydrazine (5 equiv.) the best nitrogen source. The proposed mechanism involves the nucleophilic attack of the nitrogen nucleophile on the a-carbon of a vinylidene intermediate (Scheme 8.27). Proton migration in the resulting a-hydrazi-nocarbene, followed by deamination, would give the nitrile derivative and regenerate the catalytic species. 

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. 

Switching the cubic nonlinearity of ruthenium alkynyl complexes by a protmi-ation/deprotonation sequence (via a vinylidene complex) was demraistrated by fs Z-scan studies at 800 nm several years ago [41]. Recently, protic and electrochemical switching were demonstrated in the ruthenium alkynyl cruciform complex 11 for which distinct linear optical and NLO behavior were noted for the vinylidene complex and the Ru(II) and Ru(III) alkynyl complexes [42]. Because the oxidation/ reduction and protonation/deprotonation procedures are independent, this system corresponds to switching by orthogonal stimuli. 

The proposed mechanism begins with the dissociation of the chloride to afford the starting Ru precatalyst, which upon coordination with the corresponding aUcyne would give rise to the key vinylidene intermediate A (Scheme 15). Nucleophilic attack by the pendant alcohol to the vinylidene with concurrent removal of a proton by the amine would provide alkenyl ruthenium species B, which after protonolysis... 

The mechanism of this reaction was investigated in details by isolation of intermediates, deuterium-labelling experiments and DFT calculations [123], The postulated catalytic cycle involves first protonation of a ruthenium(n)-alkyne species to give a Ru(IV)-vinylidene intermediate via a Ru(IV)-vinyl species. The nucleophilic addition of water to the a-carbon of the vinylidene ligand followed by reductive elimination affords the aldehyde (Scheme 33). 

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