Organic aluminum is a series of compounds containing carbon and aluminum bonds. As an important part of the metal organics, organoaluminium compound shows some unique properties in many reactions. The behavior of organoaluminium compounds can be understood based on the polarity of the aluminum-carbon bond and the high Lewis acidity of the three-coordinated species.
In general, the organoaluminium compounds can be simply divided into two types according to their structure: one type is the compounds with a chain structure such as RAlX2, R2AlX and R3Al; and the other one is the compounds contain cyclic structure. Organoaluminium compounds have high chemical reactivity, and can react with a variety of electrophilic reagents, which have important application prospects in synthesis chemistry.
Applications
Electrophile: The aluminum-carbon bond is polarized, so the carbon is highly basic and can react with acids to produce alkanes. In this reaction, not just simple inorganic acids, other different kinds of acids can also participate in reaction. Amine will produce amide derivatives when reacting with carbon dioxide and trialkyl aluminum compounds. Similarly, the reaction between the trialkylaluminum compound and carbon dioxide has been used to synthesize alcohols, alkenes, or ketones.
Alkene polymerization: In industry, it is well-known that organoaluminum compounds are used as catalysts for the polymerization of olefins, such as the catalyst methylalumoxane, which is commonly called MAO. It can be separated as a white pyrophoric solid and used to activate precatalysts of olefin polymerization. MAO is well-known as a cocatalyst for homogeneous catalytic olefin polymerization. In traditional Ziegler-Natta catalysis, supported titanium trichloride is activated by treatment with trimethylaluminum (TMA). TMA weakly activates homogeneous catalysts, such as zirconium dichloride. Kaminsky discovered that metallocene dichloride can be activated by MAO in the mid-1970s. MAO provides two functions during activation process. First, alkylation of the metal chloride precatalyst species gives the Ti/Zr-methyl intermediates. Second, it abstracts the ligand from the methylated precatalyst to form an electrophilic, coordinatively unsaturated catalyst that can undergo ethylene insertion. The activated catalyst is an ion pair between the cationic catalyst and a weakly basic MAO-derived anion. MAO also acts as a scavenger for protic impurities.
Lewis acid: Organoaluminum compounds are hard acids and can form aluminum-centered tetrahedral adducts with Lewis bases such as pyridine, tetrahydrofuran and tertiary amines.
Fig 1. Vinyl polymerization of norbornene by bis(salicylaldiminate)copper(II)/methylalumoxane catalysts.
Reference
- Carlini C.; et al. Vinyl polymerization of norbornene by bis(salicylaldiminate)copper (II)/methylalumoxane catalysts[J]. Organometallics, 2006, 25(15): 3659-3664.
