The carboalumination reaction is most commonly catalyzed by
zirconocene dichloride (or related catalyst). Some carboaluminations are performed with
titanocene complexes. The most common trialkyl aluminium reagents for this transformation are
trimethylaluminium,
triethylaluminium, and sometimes
triisobutylaluminium. When using reagents that have
beta-hydrides, eliminations and hydroaluminium reactions become competing processes. The general mechanism of the ZMA reaction can be described as first the formation of the active catalytic species from the pre-catalyst zirconocene dichloride through its reaction with trimethyl aluminium. First
transmetalation of a methyl from the aluminium to the zirconium occurs. Next, chloride abstraction by aluminium creates a
cationic zirconium species that is closely associated with an anionic aluminium complex. This zirconium cation can coordinate an alkene or alkyne where
migratory insertion of a methyl then takes place. The resultant vinyl or alkyl zirconium species can undergo a reversible, but stereoretentive
transmetalation with an
organoaluminium to provide the carboalumination product and regeneration of the zirconocene dichloride catalyst. This process generally provides the syn-addition product; however, conditions exist to provide the anti-addition product though a modified mechanism.
Trimethylsilyl (TMS) protected alkynes, trimethyl
germanium alkynes, and
terminal alkynes can produce anti-carboalumination products at room temperature or elevated temperatures if a coordinating group is nearby on the
substrate. In these reactions, first syn-carboalumination takes place under the previously outlined mechanism. Then, another equivalent of aluminium that is coordinated to the directing group can displace the vinyl aluminium, inverting the geometry at the carbon where displacement takes place. This forms a
thermodynamically favorable
metallacycle to prevent subsequent inversions. Formally, this process provides anti-carboalumination products that can be quenched with electrophiles. A limitation of this methodology is that the directing group must be sufficiently close to the carbon-carbon π-bond to form a thermodynamically favorable ring or else mixtures of
geometric isomers will form. The carboalumination of alkenes to form substituted alkanes can be rendered enantioselective if
prochiral alkenes are used. In these reactions, a
chiral indenyl zirconium
catalyst is used to induce enantioselectivity. In these reactions, high enantioselectivities were obtained for several trialkyl aluminium reagents, however, the yield decreases dramatically with each additional carbon of the
alkyl chain on the trialkyl aluminium reagent. == Carbolithiation ==