Alkylidene complexes Synthesis

While this compound would have to be classified as a carbene rather than an alkylidene complex, the synthesis involves an intramolecular H atom transfer, which is a recurring phenomenon in metal-alkylidene complex synthesis. 

Cantat T, Jaroschik F, Ricard L, Le Floch P, Nief F, Mezailles N (2006) Thulium alkylidene complexes synthesis. X-ray structures, and reactivity. Organometallics 25 1329-1332. doi 10.1021/om050877c... 

The synthesis and X-ray structural determination of a stable Ir111 hydride/alkylidene complex, (165), has been reported, in which the tridentate N3 ligand is TpMe2. 9 The complex undergoes reversible hydride migration onto the electrophilic carbene atom, as shown in reaction Scheme 20. 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

Although transition metal alkylidene complexes, i.e., carbene complexes containing only hydrogen or carbon-based substituents, were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. Neutral and cationic complexes of these Group 8 metals are known for both metal electron configurations d8 and d6. The synthesis, structural properties, and reactivity of these compounds are discussed in this section. 

Since the late seventies efforts were directed toward the development of well-defined catalysts that would be active without addition of additives or further modification. A wide variety of tungsten and molybdenum alkylidene complexes have been prepared. Many of them show some activity, but only few are good catalysts. The synthesis is often not straightforward and a range of synthetic procedures varying solvents, alkylating reagents, anions, and alkylidene moieties have to be tried before a desired compound will be obtained. 

Scheme 6. Synthesis of ruthenium-alkylidene complexes starting at the azolium salt without isolating the NHCs.

In situ deprotonation combined with a substitution of a phosphine ligand was reported as a convenient way for the synthesis of ruthenium-alkylidene complexes (Scheme For imidazolidin-2-ylidenes, this is the only way... 

Further investigation of the equilibrium between titanacyclobutene and titanium vinyl alkylidene complexes, as discussed in Section 2.12.6.1.4, was reported recently <2007CEJ4074>, along with the incorporation of this reactivity pattern into the synthesis of conjugated dienes, homoallylic alcohols, vinylcyclopropanes, and phosphacyclobutenes from y-chloroallyl sulfides and a source of titanocene(ll). 

Unusual bridging (/z-cyclopropylidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(iU-Co)(/z-C3H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and /ra 5-vinylidene complexes [CpCoFe](jU-CO)(jU-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent //-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

The synthesis of d° Alkylidene complexes by Schrock demonstrated that carbene complexes could be isolated that were electronically similar to those postulated to be involved in the alkene metathesis reaction see Schrock-type Carbene Complexes). Eventually, this pioneering work led to the synthesis of a class of compounds that are among the most active catalysts known for metathesis chemistry. The first observation that a d° carbene complex was involved in metathesis chemistry was when Tebbe showed that the Ti complex (2) would catalyze the degenerate metathesis of... 

Niobium and tantalum belong to the metals that are able to bind dinitrogen. The development of this area was first due to Schrock and coworkers, who in 1982 could obtain stable tantalum /X-N2 compounds in high yields by reducing Alkylidene complexes under dinitrogen at normal pressure.This route can be circumvented in a single pot synthesis by reacting the pentahalides with sUylated hydrazines (equation 12). 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

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