Reactions Affording Vinylidenes

The following are some examples of reactions which have produced vinylidene complexes but are either not of general application or have not been further developed. Oxygen atom transfer occurs in reactions of NbH (t] — OC — CPh2) Cp 2 with nitriles or isonitriles to give isocyanates and Nb(=C=CPh2)Cp 2 [260]. Metathesis of Ph(R) C=C=NPh (R = Me, aryl) with W(=CHPh)(CO)5 proceeds via W C(NPh=CHPh)=C (R)Ph (CO)s, which is converted to W =C=C(R)Ph (CO)5 by treatment with BF3. OMe2 [261]. 

Unusual iron-porphyrin vinylidene complexes were obtained from DDT [l,l-bis(4-chlorophenyl)-2,2,2-tricMoroethane] and Fe(tpp) [tpp = meso-tetraphenylporphinato (2-)] in the presence ofa reducing agent [10a, 264]. The derived N,N -vinylene-bridged porphyrin reacts with metal carbonyls [Fe3(CO)i2, Ru3(CO)i2] to break one or both N—C bonds with insertion of the vinylidene into an M—N bond. While the iron complex was formed in 90% yield, the reaction with Ru3(CO)i2 afforded three products, the vinylidene being formed in only 40% yield [265]. 

Alkylation of Ru C=C(C6Hg) (PPh3)2Cp with BrCH2C02Me affords [Ru =C=C (C6Hg)(CH2C02Me) (PPh3)Cp]+ [199]. The reaction between Ru(NCMe)2 

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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 ... 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

Photochemical desulfurization of thiocarbonates occurs with [Fe(CO)j] to give mononuclear as well as trinuclear (type 8) carbene complexes (24). A similar reaction affords desulfurization of diphenylcyclopropenethione (22). The thioketene cluster 13 reacts quantitatively, but in a more complicated way, with a thioketene to form a dinuclear vinylidene complex (14). 

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. 

It should be pointed out that the electrophilic [2 + 2] cycloaddition of CH2=0 and fluoroolefins carried out in HF is rather limited in the scope. For example, reaction of formaldehyde with hexafluoropropylene exclusively affords CF3CF(CF3)CH20H, and in reaction with vinylidene fluoride, the ether (CF3CH2CH2)20 is formed as the major product. More on the electrophilic reactions of fluoroolefins in anhydrous HF can be found in detailed review. ... 

The homobimetallic, ethylene-ruthenium complex 15, which contains three chloro bridges, was readily obtained from the reaction of [RuCl2(/ -cymene)]2 with 1 atm of ethylene [34]. In 2009, Demonceau and Delaude [34] showed that complex 15 could be a useful precursor to allow subsequent access to the diruthenium vinylidene complex 16, allenylidene complex 17, and indenylidene complex 18 (Scheme 14.8). Upon reaction with propargylic alcohol, complex 15 afforded vinylidene complex 16, which converted into the allenylidene complex 17 in the presence of molecular sieves [34]. As shown in the acid-promoted intramolecular rearrangement of mononuclear ruthenium allenylidene complexes [19, 20, 32], the addition of a stoichiometric amount of TsOH to complex 17 at -50 °C led to the indenylidene binuclear complex 18 [34]. Complex 18 has been well... 

The electrophilicity of the Gc,-atom of the vinylidenes [Ru =G=G(Me)R (77 -G9H7)(dppm)][OTf] (R = Ph, Bu ") also allows the intramolecular nucleophilic attack of a methanide group generated in situ by deprotonation of the methylene unit of the /i (P,P)-coordinated dppm ligand. The reaction affords the alkenyl metallacycles 73... 

Pyridines can be functionalized by a range of metal complexes, notably ruthenium analogs. Ruthenium vinylidene complexes promote the reaction of pyridines with silylalkynes in both a regio- and stereoselective manner, affording 2-styrylpyridines (Equation (78)). 

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

The alternative building scheme C2 + Q was used by Petasis and Hu [89], who reacted various aldehydes and ketones with alkenyltitanocene derivatives 172 to obtain the corresponding allenes 173 in high chemical yields (Scheme 2.54). The reaction probably proceeds via titanocene vinylidene complexes, which can also be trapped with alkynes and isocyanides to afford allenylketene imines [90],... 

The transformation predominates in Group 9 (Rh, Ir) chemistry. Reactions of RhCl(L)2 2 with 1-alkynes give the q -alkyne complexes which slowly convert to the hydrido(alkynyl)s at room temperature. The latter are sensitive to air and not often isolated. Addition of pyridine affords RhHCl(C=CR)(py)(L)2, which readily lose pyridine in hydrocarbon solvents to give square-planar fran.s-RhCl(=C=CHR)(L)2. Alternatively, the Cp complexes Rh(=C=CHR)(L)Cp can be obtained by reaction of the chloro complexes vdth TlCp. In the iridium series, heating for 36h in refluxing toluene afforded the vinylidenes in 80-90% yields. Table 1.2 lists several examples of reactions in which the q -alkyne complexes have been detected. 

While protonation affords the vinylidene expected by H migration from the original 1-alkyne, use of other electrophiles provides a convenient route to disubsti-tuted vinylidenes. The stereospecificity of this reaction with Re(C=CR)(NO)(PPh3)... 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

Reactions with dioxygen generally afford the corresponding metal carbonyl derivatives, with loss of organic aldehyde or acid. This reaction can be expressed as an analog of multiple bond metathesis and corresponds to oxidation of vinylidene to CO [46]. Oxidation of OsHCl(=C=CHPh)(L)2 affords the styryl complex 0sC102( -CH=CHPh)(L)2 [243]. 

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