Cyclopentadiene, reactions with metal atoms

Novel organometallics have been prepared via metal atom synthesis since the 1970s. Especially interesting have been diene reactions with early transition metal atoms. Cyclopentadiene on cocondensation with metal atoms reacts with Cr through a bis(cyclopentadienyl)chromiumdihydride intermediate to yield chromocene. For Mo and W, the intermediate species can be isolated. Labeling studies with Cp H and Cp D indicate that oxidative addition of the C-H bond to M occurs stepwise (- Cp MH CP2MH2) (see Oxidative Addition). 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

The interaction of metal atoms with cyclopentadiene is characterized by a tendency toward formation of a stable electronic configuration for the metal. Thus, cyclopentadiene undergoes the following reactions on condensation with metal vapors at -196°C (21, 115, 127, 136) ... 

Although the foregoing reactions are quite facile, they are not competitive with the very well-established conventional methods for synthesis of metallocenes. Cyclopentadiene, nevertheless, can be a useful substrate in metal atom synthesis as part of a mixed-ligand system (6, 110, 136, 140), e.g.,... 

Diarenemanganese cations have not been formed by the metal atom route, but condensation of arenes, cyclopentadiene, and manganese gives low yields of (arene) Mn(C5H5). As cyclopentadiene does not react with manganese atoms, a primary manganese-arene reaction is postulated (131, 140) (see Section III,A,3). 

The reactions of cycloheptatriene with transition metal atoms are similar to those of cyclopentadiene. In both cases, the reactions can involve extensive migration of hydrogen in the formation of the final product (9, 10, 133, 136) ... 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

The reaction of M3(CO)12 with both open-chain and cyclic poly-alkenes has attracted some attention, especially in the case of Ru3(CO)i2. In most of the examples reported, the organic fragment bonds to the metal framework in such a way as to interact with more than one of the three metal atoms (68-77). There are some exceptions to this general statement, however. One is the reaction of Ru3(CO)j 2 with cyclopentadiene, in which a mononuclear complex is obtained (78). In other cases, tetranuclear and hexanuclear compounds are obtained (79 81). Cluster breakdown has also been observed in the case of a rhodium complex upon reaction with ethylene (55) as shown in Fig. 3. 

Metal-atom vapor synthesis techniques have been successfully applied to the preparation of metaUacarbaboranes, that is, reaction type M - - C -I- B. A disadvantage is that product distribution tends to be nonspecific, for example, when Co metal is vaporized by electrical heating and cocondensed at -196 °C with cyclopentadiene, B5H9, and the alkyne MeCsCMe, l-(CpCo)-2,3-Me2-2,3-C2B4H4, l,7-(CpCo)2-... 

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