Bridging carbenes

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. 

Extension to trimetallic complexes has led to the preparation of similar bridging carbene and carbyne complexes with Pt3,421 Pt2W422 and PtWFe423 metal frameworks. 

Another broad class of compounds are the bridged carbene complexes. These compounds contain two identical or two different metal centers with the carbene centers bonded to both of the metal atoms in a bridging relationship. However, these binuclear complexes generally do not show classical carbene reactivity and will therefore not be discussed further, except to mention briefly the special case of the titanium-aluminum complex (3) developed by Tebbe and Grubbs and their coworkers.101 This, and related complexes, has proven to be particularly useful in organic synthesis, although its principal importance is in reactions other than cyclopropanations. 

In 2003, Cenini and coworkers reported (tetraarylporphyrin)cobalt(II) complexes 326 as efficient catalysts (1 mol%) for cyclopropanations. In the absence of air, styrenes 321 underwent an efficient cyclopropanation with ethyl diazoacetate 322 giving cyclopropanes 324 in 65-99% yield with 3-5 1 trans/cis ratios (Fig. 77) [348]. Simple olefins and more hindered diazoesters did not react. With diazoacetate and hydrocarbons, such as cyclohexane or benzene, C-H insertion took place furnishing cyclohexyl- or phenylacetate. In line with Ikeno s proposal the cyclopropanation reaction was considerably slowed down in the presence of TEMPO, though not completely inhibited. Based on a kinetic analysis a two-electron catalytic cycle with a bridged carbene unit was formulated, however. 

Using the Isolobal Analogy Metal Complexes with Bridging Carbenes and Carbynes... 

The complications that occasionally arise in the use of diazoalkanes reflect the possible further reactions of carbene ligands, which will be dealt with subsequently, e.g. insertion into adjacent M-H or M-halide bonds and the formation of bimetallic complexes supported by bridging carbene ligands. In some cases, transition metals may catalyse reactions of diazoalkanes, leading to products which are suggestive of the reactions of free carbenes, i.e. dimerization, addition to alkenes (cyclo-propanation) and insertion into C-H bonds (Figure 5.9). In such cases, however, the actual mechanism does not involve free carbenes but rather transient diazoalkane/carbene complexes. This is supported by the obser-... 

Suitably constructed bridged carbenes exhibit a tendency for structural rearrange ment and undergo the so-called foiled methylene reaction 141 142) with formation of bicyclo[3.3.0]octane derivatives. The behavior of P6143) and 97144) exemplifies such conversions. Carbonium ion analogies to such reactions are known. ... 

Alkylidyne Derivatives and Those with Bridging Carbene or Carbyne Ligands... 

One of the most valuable synthetic routes to bridging carbenes involves the use of diazomethane (equation 37) and related free carbene precursors the product of equation (37) has a type (11) structure. 

Like CO, carbenes can act as bridging ligands, usually accompanied by a metal-metal bond (11). In bridging, carbenes lose some of their unsaturation, and therefore the very high reactivity of their mononuclear analogs. Fischer methylenes are very reactive and barely isolable, while bridging methylenes are well known and relatively stable. 

Mononuclear cis-biscarbene complexes of Cr, Mo and W were obtained from M(CO)metallates with [EtjO][BF4]. Binuclear biscarbene complexes with a bridging carbene ligand are synthesized from (CO)jM (M = Cr, W) and Li2[p-C H4]-[Et30][BF4]. ... 

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