Transmetalation to other metals

Transmetalations of zirconacyclopentadienes to other metals have to be found and the products can be expected to show some characteristic carbon—carbon bond formation. For example, evidence for the transmetalation of zirconacyclopentadienes to Co has been described (Eq. 2.32) [33]. The most attractive feature of the transmetalation of zirconacyclopentadienes is the formation of dimetallo-diene species, the reactivities of which are completely dependent on the metal. 

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Transmetallation to other Metals for Addition and Coupling Reactions... 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

In this chapter we focus on the selective synthesis of aromatic hydrocarbons and heterocycles from unsaturated molecules by using zirconocenes and titanocenes via transmetallation to other metals, such as Cu, Ni, and Li. Although there are several examples of aromatic ring formations directly from metallacycles, we emphasize the cooperative effects in particular, transmetallation from group 4 metals to the other metal species. The coupling reaction of a Cp ligand with the diene moiety in a... 

Additions of metallacyclopentadienes to carbon—carbon triple bonds are rare, and only a few examples are known (Eq. 2.45) [37]. The 1,1-addition of zirconacyclopentadienes is quite different from other carbon—carbon bond-forming reactions described in this chapter. This reaction does not require transmetalation of zirconacyclopentadienes to other metals. Thus, in the absence of any added metal halide, zirconacyclopentadienes react with propynoates to give cyclopentadiene derivatives. This reaction requires the use of at least 2 equivalents of the propynoate (Eq. 2.46). 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

The transmetallation of lithiated heterocycles has been described as a method to provide NHC complexes, and other Fischer-type carbene complexes [161]. The method proceeds via the alkylation of an alkylimidazole with BuLi to generate the lithiated azole, which can transmetallate to the metal complex. Further reaction with an acid or an alkylating agent would provide the desired NHC - M complex (Scheme 47). 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Carreira and Kruger reported facile transmetallation of silicon enolates to other soft metal enolates including Gu derivatives.499 They reasoned that the use of soft metal fluoride complexes enabled silyl metal transmetallation with catalytic use of a soft metal source. The concept is illustrated in Scheme 103. Normal Lewis acid-catalyzed reactions of silicon enolates with aldehydes proceed via activation of aldehydes by carbonyl oxygen coordination to Lewis acids, as shown in the upper equation of Scheme 103. A key step for catalytic turnover is the desilyation of 233 by the... 

Addition reactions depend on the metal used for the transmetalation of the zirconacyclo-pentadiene. After transmetalation to copper, an addition reaction occurs to the carbon-carbon double bond or to a carbon—carbon triple bond bearing electron-withdrawing groups (Michael addition reactions). On the other hand, transmetalation to Ni allows the use of carbon—carbon triple bonds bearing electron-donating groups. 

Transmetallation with metal halides (e.g., Re,311 TiCl4,309 ZrCl4,312 315 or HfCl4314) provides a route to other metallocenes, for example, Equation (118).314... 

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