Cationic Aluminum Compounds with Potential Relevance to Lewis Acid Catalysis

摘要

Aluminum compounds are finding increasing importance as "living" polymerization catalysts and in the Lewis acid-promoted reduction of ketones and aldehydes. In order for a compound to be useful in these applications, it must be able to coordinate a substrate such as an aldehyde, ketone, or oxirane. This implies a complex that is coordinatively unsaturated and/ or electron deficient. These qualities can be achieved by employing bulky substituents, as in AlMe(2,4,6-tri-tert-butyl-phenolate)_2, or through the use of chelating ligands, as demonstrated for SalcenAlCl (Salcen = N,N′-bis(2-hydroxy-benzylidene)-1,2-cyclohexanediamine) and porphyrinAlCl complexes. These compounds are proven catalysts in the ring-opening living polymerization of oxiranes. For this type of application, the use of cationic aluminum reagents would be ideal. Aluminum cations are naturally electron deficient, and with the choice of loosely coordinating ligands, they may behave as coordinatively unsaturated compounds. Despite the clear relevance of such compounds, however, discrete cationic aluminum complexes have hitherto been unavailable for use in this application.