Aluminum and gallium complexes as homogeneous catalysts for reduction/oxidation reactions

The roles of Al(III) and Ga(III) complexes in the homogeneous catalysis of oxidation/reduction reactions is reviewed. Free metal salts and discrete complexes have primarily been used to catalyze hydride transfer between organic molecules, the epoxidation of alkenes, and the activation of allylic and aliphatic C-H bonds. The complexation of more highly coordinating ligands can improve the chemo-, regio-, and/or stereoselectivity of the reactions catalyzed by the free Al(III) and Ga(III) salts. The Al(III) and Ga(III) metal centers can accelerate the reactions by activating one or both of the redox-active starting materials and/or bringing the reagents together to orient them in a closer approximation of the reaction's transition state. Complexation of a redox-active ligand can enable Al(III) and Ga(III) complexes to gain or lose electrons directly. The reactivities of complexes with non-innocent ligands can approximate those of transition metal complexes in that they can be converted into metal-based oxidants and reductants.