Metals carbon—lead bonds

While the alkoxymetallation process has typically been affected by highly electrophilic metal salts, high-valent metal species generated by an oxidative addition have also been used to activate alkynes through the formation of 7r-complexes. In such cases, the metal-carbon emerging from the attack of an oxygen nucleophile may enter a reaction manifold that leads to an additional C-G bond formation rather than a simple protic quench. This approach, pioneered by Arcadi and Cacci, has proved to be a powerful strategy for the synthesis of structurally diverse substituted... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

As with metal-carbon monoxide bonds, the MP2/6-3IG model does not lead to results of the same calibre as those from density functional models (except local density models). The model actually shows the opposite behavior as 6-3IG, in that bond lengths are consistently shorter than experimental values, sometimes significantly so. In view of its poor performance and the considerable cost of MP2 models (relative to density functional models), there seems little reason to employ them for structural investigations on organometallics. 

The pair of electrons in the sp1 orbital may be donated to a metal lo form a u bond, and an empty p. orbital is present to accept it electron density. Filled d orbitals of (he metal may donate electrons to the p. orbital, to give a metal-carbon double bond, and electrons from filled p orbitals of the oxygen atom may also be donated to form a carbon-oxygen double bond (Fig. 15.20). Resonance form 15.20b appears to be dominant and, although the M—C bond is shorter than expected fora single bond, it is too long for an M—C double bond, leading to the conclusion that the bond order is between one and two. 

Sulfur dioxide insertion is not limited to metal-carbon a bonds, although M—C is by far the most common substrate involved. Reactions have been reported which lead to insertion of SOa metal-carbon TT (or polyhapto) bonds 26, 102, 130-132), as well as transition metal-transition metal 112), transition metal-Group IVB metal 14, 19), and metal-oxygen 9,58) linkages. Moreover, reaction (8) (97) where M = Rh... 

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

Protonation of organometallic complexes containing metal-carbon [Pg.388]

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

Some of the substances that lead to 1,1 insertion reactions into metal-carbon ff bonds are carbon monoxide, CO alkyl- or arylisocyanide, CNR, carbenes CR2 sulfur dioxide, SO2 and nitrogen oxide NO. Most studied are CO, CNR, and SO2 therefore a more detailed account of them is presented. 

Lead compounds are often involved in radical reactions, because of the weakness of the carbon/lead bond, and the fact that lead has two accessible oxidation states. In the case of lead, though, because the oxidation states are plus two and plus four, the transition between them requires the addition or removal of two electrons. However, radical reactions usually require the involvement of only a single electron. Thus, one would expect transition metals that have readily occupied oxidation states that only differ by a single unit to facilitate radical reactions more easily than lead. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

These and other results are best explained, at least when the reaction is carried out in liquid SOj, by the mechanism shown in Figure 12.2. Initial 5 2 attack by the electrophilic sulfur of SOj inverts the configuration at carbon and forms an ion pair. Collapse of the ion pair to form an 0-bound sulfinate occurs reversibly, and the more stable S-bound sulfinate is eventually formed. The cation [CpM(L)(L )] is slow to invert, leading to retention of stereochemistry at the metal. Sulfur electrophiles that are similar to SOj, such as N-sulfinyl-sulfonamides (RS02)NS0 and sulfur bis(sulfonylimide)s (RSOjN)jS, also insert into transition metal-carbon a-bonds with inversion of stereochemistry at carbon, - probably by a mechanism analogous to that shown in Figure 12.2. 

The reaction of dicarbonyl(t 5-cyclopentadienyl)-carbyne complexes Cp(CO)2-n(PMe3)nMsC-R (M = Mo, W n = 0, 1 R = Me, Ph, Tol) with azides, N3R (R = CO2CH3, CH2CO2CH3), results in a [3+2]-cycloaddition to the electron rich metal carbon triple bond, leading to neutral 1-metalla-2,3,4-triazole complexes in high yields [43,44]. 

---