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Concept
Carbometalation
Summary
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.
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Related concepts (2)
Organozinc chemistry
Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds. Organozinc compounds were among the first organometallic compounds made. They are less reactive than many other analogous organometallic reagents, such as Grignard and organolithium reagents. In 1848 Edward Frankland prepared the first organozinc compound, diethylzinc, by heating ethyl iodide in the presence of zinc metal.
Organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and sometimes broadened to include metalloids like boron, silicon, and selenium, as well. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide (metal carbonyls), cyanide, or carbide, are generally considered to be organometallic as well.