Carbometalation

A carbometallation is any reaction where a carbon-metal bond reacts with a carbon-carbon π-bond to produce a new carbon-carbon σ-bond and a carbon-metal σ-bond. The resulting carbon-metal bond can undergo further carbometallation reactions (oligomerization or polymerization see Ziegler-Natta polymerization) or it can be reacted with a variety of electrophiles including halogenating reagents, carbonyls, oxygen, and inorganic salts to produce different organometallic reagents. Carbometallations can be performed on alkynes and alkenes to form products with high geometric purity or enantioselectivity, respectively. Some metals prefer to give the anti-addition product with high selectivity and some yield the syn-addition product. The outcome of syn and anti- addition products is determined by the mechanism of the carbometallation. Carboboration is one of the most versatile carbometallation reactions. See Carboboration. The carboalumination reaction is most commonly catalyzed by zirconocene dichloride (or related catalyst). Some carboaluminations are performed with titanocene complexes. This reaction is sometimes referred to as the Zr- catalyzed asymmetric carboalumination of alkenes (ZACA) or the Zr-catalyzed methylalumination of alkynes (ZMA). The most common trialkyl aluminum reagents for this transformation are trimethylaluminium, triethylaluminium, and sometimes triisobutylaluminium. When using trialkylaluminum reagents that have beta-hydrides, eliminations and hydroaluminum 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 aluminum. First transmetalation of a methyl from the aluminum to the zirconium occurs. Next, chloride abstraction by aluminum creates a cationic zirconium species that is closely associated with an anionic aluminum complex. This zirconium cation can coordinate an alkene or alkyne where migratory insertion of a methyl then takes place.