Carbene complexes of molybdenum

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

Similarly, thermolysis of cyclopropyl(5-phenyl-4-pentynoxy)carbene complexes of molybdenum and tungsten gave via intramolecular coupling the corresponding 3,4,5,7-tetrahydro-5-phenylcyclohepta[ ]pyran-6(2//)-one (equation inter-... 

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

When solutions of pentacarbonyl-alkoxy(organyl)carbene complexes of chromium, molybdenum or tungsten are treated at low temperatures with BCI3, BBr3 or BI3, the... 

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

The reactions are stereospecific.35 The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), molybdenum ) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state.31... 

We must however keep in mind that some of the above reactions may not be simple reactions at the silicon atom, since transition metal complexes show multicenter reactivity (metal atom, ligands) as exemplified in the chemistry of triphenylgermyl-carbene complexes of cobalt carbonyl (253). Thus, displacements of a silyl ligand may result from a multistep process and a thorough examination of these reactions has to be made. An example can be drawn from molybdenum-germanium chemistry (247). As shown in Scheme 59, germanium is displaced from complex 167 by HO with retention of configuration. Actually,... 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Catalyst. Two types of catalysts were used in the present study. The one is the so-called Ziegler type catalyst composed of a tungsten, molybdenum or rhenium compound (A component) and an organometallic compound (B component). The other type of catalyst is composed of a carbonyl-carbene complex of tungsten and a Lewis acid(24). 

Carbene complexes of tungsten, molybdenum or rhenium initiate the ring-opening polymerization of cycloolefins to polyalkenamers. Thus, Ph2C= W(CO)5 (1) polymerizes cyclobutene, 1-methylcyclo-butene, cyclopentene, cycloheptene, cyclooctene and norbornene while Ph(MeO)C=W(CO)5 (2) polymerizes cyclobutene and norbornene to highly stereospecific polyalkenamers [28]. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

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