Cleavage, metal carbene

Initiation involves coordination of the double bond of monomer with the transition metal (imino and OR ligands not shown), cleavage of the 7t-bond with formation of a 4-membered metallocyclobutane intermediate, followed by rearrangement to form a metal-carbene propagating center ... 

Although lesser-known than the addition transformations of metal carbenes, C-H insertion processes are among the most important asymmetric C-C bond-forming reactions available to synthetic chemists [7,10,112,113]. They result from cleavage of C-H and metal-C bonds with concurrent formation of C-C and C-H bonds (Scheme 5.5). The construction of five-membered rings is most common, but six-, four- and even eleven-membered rings [96] have been formed by C-H insertion. 

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal-carbene-olefin complexes are known see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119. 

As we saw in Section 2.2.2, an interaction between metal and the a-hydro-gen of an alkyl group is called an agostic interaction. In an extreme situation, where the interaction between the metal atom and the a-hydrogen leads to formal cleavage of the carbon-hydrogen bond, a mechanism involving a metal-carbene intermediate may be invoked. This proposal is known as the Green and Rooney mechanism, and two of the proposed catalytic intermediates are shown by 6.9 and 6.10. 

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

A a and a n C—S bond are broken during the formation of type 2 compounds. The resulting fragments function as two 3er donor ligands. Recently Patin and co-workers found that thermal activation or catalysis by electron transfer can transform a type le compound into a type 2 complex 13). This process involves not only rupture of a C—S bond, but also cleavage of an Fe—C a bond, carbon shift, and formation of a metal-carbene bond with the neighboring iron atom 28). 

The Mo(VI)/HZSM-5 catalyst exhibits excellent activity for cleavage of the C- C bond in C2H5 and thus higher CH4 selectivity can be observed. This also can be served as an evidence that the formation of CH2 carbene over Mo(VI)/HZSM-5 and that the carbene is stabilized by the formation of Mo=CH2 metal carbene. The ethane conversion over Mo(VI)/HZSM-5 thus can be schemed as follows ... 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

Interacbon of a metal alkylidene complex with an olebn forms a metallacyclobutane. Cleavage of the metallacyclobutane complex at bonds different from the original ones leads to bberabon of a new olebn and formation of a new metal carbene complex (Scheme 1.41a). This type of bansformabon is operabve in a catalybc cycle of olebn metathesis as shown in Scheme 1.42 [125]. 

It is well known that even small changes in the structures of the transition metal complexes and of the organic counterparts have a substantial effect upon the double bond cleavage reactions. That is why it worth searching for well defined transition-metal carbene- or oxo-complexes (or of their precursors) capable of providing new routes to synthesis of common and new polymers with interesting structures and properties. 

Knauss, 1970), and hydrazoic acid (Conner and Jones, 1971b) react with metal-carbene complexes to give cleavage products. 

A cyclization of 1 -(indole-2-yl)-2,3-allenols in the presence of platinum(II) chloride to give polysubstituted carbazoles via 1,2-methyl or an 1,2-aryl migration has been reported to proceed with the intermediacy of a metal carbene intermediate which induces the carbon-carbon bond cleavage via 1,2-migration to give the products. ... 

Reductive cleavage of group VI metal-carbene complexes C M CH2... 

Direct cleavage of the C-C double bond of olefins is not usually observed. The C-C double bond coordinates to transition metals first and a formal metalla-cyclopropane derivative is produced. If the C-C bond is cleaved in the metal-lacyclopropane, two metal-carbene moieties are formed on one metal center. Due to the unstability of bis(carbene) compounds, the direct and simple cleavage reaction of the C-C double bond does not easily occur (Eq. 20). 

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