From Metal-Carbyne Complexes

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

Deprotonation may be achieved by reaction tvith alkyllithiums, but has also been found in other circumstances. A reaction between CF3I and Mo =C(CH2Bu ) P(OMe)3 2Cp afforded MoI(=C=CHBu ) P(OMe)3 2Cp in a reaction suggested to proceed via electron transfer from the carbyne to CF3I, followed by abstraction of FI by the CF3 radical. A related reaction occurs with [4-FC6Fl4N2]BF4 [233]. 

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Several routes are known to Schrock s Mo- and W-alkylidene complexes that catalyze the metathesis of alkenes, including those starting from metal-carbyne complexes (see section 3.3). A practical one was discovered starting from Na2Mo04 and produced a large family of catalysts including enantiopure ones ... 

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

Alkynes can also undergo metathesis reactions catalyzed by transition-metal carbyne complexes. The intermediates in these reactions are believed to be metallacyclobutadiene species, formed from the addition of an alkyne across a metal-carbon triple bond of the carbyne (Figure 14.35). The structures of a variety of metallacyclobutadiene complexes have been determined, and some have been shown to catalyze alkyne metathesis. 

Is it possible to form a metal-carbene or a metal-carbyne complex from an alkene or an alkyne ... 

From Carbyne Complexes. Addition of HC1 across the metal-carbon triple bonds of Ru and Os d8 arylcarbyne complexes yield stable, neutral secondary alkylidene complexes ... 

It is interesting to note that the decrease in metal electron density that accompanies the change from five- to six-coordinate geometry does not have a detectable effect on the metal-carbene carbon bond length in these complexes. The metal-carbyne carbon bond in several osmium carbyne... 

The X-ray structure determination of 107 reveals that the osmium-carbon bond length is increased by 0.07 A on going from the parent carbyne complex 79 to the silver adduct 107. This may be contrasted with the weaker interaction between the metal-carbon bond and the Aul fragment in Os(CH2AuI)Cl(NO)(PPh3)2 (see Section IV,C,1). 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

Nucleophilic attack at a carbocationic carbon, similar to that observed in metal -carbynes, has been observed from CNBu additions to the //2-t/2-ace-tylide in the complex Fe2(CO)6(C=CPh)PPh2 (427). The ligand dipole, indicated from the X-ray structural determination of the product 96, makes it susceptible to additions of primary amines, generating the carbene 97. 

From a mechanistic point of view (Scheme 3), a transition metal carbyne undergoes a [2+2] cydoaddition with an alkyne to form a metallacydobutadiene as an intermediate, or possibly as a transition state. The four-membered ring then cycloreverts, expelling a di-substituted alkyne with regeneration of a catalytically active carbyne complex. In a subsequent... 

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