Alkylidynes

Alkyldene and alkylidyne compounds have also been made [200],... 

From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate. 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Thus far, all examples of C—C activation and/or C—C formation have employed alkynes, either alone or in concert with cluster-bound alkylidyne or CO. 

Fig. 22. Interconversion of vinylidene and alkylidyne at irinuclear clu.stcrs (i) Hi, R = H Mr = FeCoMo, RuCoMo. RuCoW. (ii) A -Hi, R = H, Me Mi = FeCoMo, RuCoMo. RuCoW. OsCoMo. OsCoW co-ligands omitted for clarity.

Reactions of alkynes with trinuclear group 6-group 9 clusters incorporating bridging alkylidyne ligands proceeded by coupling the C-donor ligands. 

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

B. The Synthesis and the Chemistry of Metal-Alkylidene and -Alkylidyne Functionalities... 

The attempts to obtain alkyl derivatives of 6 in the reaction with 1 or 2 equiv. of Li or Mg alkylating agents did not lead to the expected products (Scheme 24). On the other hand, when a 3 1 molar ratio was used, alkylidyne derivatives 129-131 (Fig. 12) were readily obtained (Scheme 24). The reaction went well also when... 

The alkene-W complexes used in this study are complexes 33, 166, and 168 (Schemes 5 and 31). The deprotonation of the W-alkene functionality has been carried out on 166, which has three methyl substituents at the C=C bond, in order to prevent any rearrangement of the 1-metallacyclopropene to the corresponding alkylidyne.22b The reaction of 166 with LiBu led to the 1-metallacyclopropene 169 (Scheme 32), which was isolated in quite good yield. The alkylidene 169, a very stable compound, was characterized both in solution and in the solid state.28 The h NMR spectrum shows a four-fold symmetry calixarene skeleton, while the 13C NMR spectrum contains a resonance at 271.0 ppm for the alkylidene carbon. 

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

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