Cerium, transmetalation

Spiroketals and oxaspirolactones.1 An improved route to spiroketals from lactones involves use of cerium 3- or 4-cerioalkoxides such as 1, prepared from 3,3-dimethyloxetane by reductive cleavage with lithium di-r-butylbiphenylide (LDBB) followed by transmetallation. Use of the intermediate lithium 3-lithioalkoxide as the... 

Organolithiums can be converted to other types of organometallic reagents by transmetallation— simply treating with the salt of a less electropositive metal. The more electropositive lithium goes into solution as an ionic salt, while the less electropositive metal (magnesium and cerium in these examples) takes over the alkyl group. 

An example prepared by tin-lithium transmetallation is compound 637, which reacts with enolizable ketones, after transmetallation with cerium(III) chloride895. This intermediate was transformed into the corresponding vinylzinc reagent and, after palladium(O)-catalyzed cross-coupling reactions with aryl iodides, was used in the synthesis of the antitumor antibiotic rineomycinone B2 methyl ester940,941. The vinyllithium 627 has also been transformed into the corresponding vinyl iodide by stannylation followed by reaction with iodine. The arylation has been performed in this case by a palladium(0)-catalyzed... 

Lithium(l) NHC adducts are extremely rare, but some examples have been isolated as stable crystalline solids when a ftmctional group is involved for their stabilization, the amino group, for instance. Since these showed to be effective in transmetallation processes, they represent an interesting alternative to the expensive silver complexes. As an example, Arnold reported the synthesis of covalently tethered lanthanide carbene complexes (214) using adduct (213) as source ofNHC (equation 39). Using (213) and the same methodology, unusual cerium complexes (215) and (216) were isolated and characterized (equation 40). ... 

This transmetallation product, when applied to aldol couplings as shown below, exploits the stronger chelation of the cerium ion (relative to lithium) in the... 

---