Presidential Green Chemistry Challenge: 2001 Academic Award
Professor Chao-Jun Li of Tulane University
Quasi-Nature Catalysis: Developing Transition Metal Catalysis in Air and Water
Innovation and Benefits: Professor Li developed a novel method to carry out a variety of important chemical reactions that had previously required both an oxygen-free atmosphere and hazardous organic solvents. His reactions use metal catalysts and run in open containers of water. His method is inherently safer, requires fewer process steps, operates at lower temperatures, and generates less waste
Summary of Technology: The use of transition metals for catalyzing reactions is of growing importance in modern organic chemistry. These catalyses are widely used in the synthesis of pharmaceuticals, fine chemicals, petrochemicals, agricultural chemicals, polymers, and plastics. Of particular importance is the formation of C–C, C–O, C–N, and C–H bonds. Traditionally, the use of an inert gas atmosphere and the exclusion of moisture have been essential in both organometallic chemistry and transition-metal catalysis. The catalytic actions of transition metals in ambient atmosphere have played key roles in various enzymatic reactions including biocatalysis, biodegradation, photosynthesis, nitrogen fixation, and digestions, as well as the evolution of bioorganisms. Unlike traditionally used transition-metal catalysts, these "natural" catalytic reactions occur under aqueous conditions in an air atmosphere.
The research of Professor Chao-Jun Li has focused on the development of numerous transition-metal-catalyzed reactions both in air and water. Specifically, Professor Li has developed a novel [3+2] cycloaddition reaction to generate 5-membered carbocycles in water; a synthesis of β-hydroxyl esters in water; a chemoselective alkylation and pinacol coupling reaction mediated by manganese in water; and a novel alkylation of 1,3-dicarbonyl-type compounds in water. His work has enabled rhodium-catalyzed carbonyl addition and rhodium-catalyzed conjugate addition reactions to be carried out in air and water for the first time. A highly efficient, zinc-mediated Ullman-type coupling reaction catalyzed by palladium in water has also been designed. This reaction is conducted at room temperature under an atmosphere of air. In addition, a number of Barbier-Grignard-type reactions in water have been developed; these novel synthetic methodologies are applicable to the synthesis of a variety of useful chemicals and compounds. Some of these reactions demonstrate unprecedented chemoselectivity that eliminates byproduct formation and product separation. Application of these new methodologies to natural product synthesis, including polyhydroxylated natural products, medium-sized rings, and macrocyclic compounds, yields shorter reaction sequences.
Transition-metal-catalyzed reactions in water and air offer many advantages. Water is readily available and inexpensive; it is not flammable, explosive, or toxic. Consequently, aqueous-based production processes are inherently safer with regard to accident potential. Using water as a reaction solvent can save synthetic steps by avoiding protection and deprotection processes that affect overall synthetic efficiency and contribute to solvent emission. Product isolation may be facilitated by simple phase separation rather than energy-intensive and organic-emitting processes involving distillation of organic solvent. The temperature of reactions performed in aqueous media is also easier to control since water has such a high heat capacity. The open-air feature offers convenience in operations of chemical synthesis involving small-scale combinatorial synthesis, large-scale manufacturing, and catalyst recycling. As such, Professor Li's work in developing transition-metal-mediated and -catalyzed reactions in air and water offers an attractive alternative to the inert atmosphere and organic solvents traditionally used in synthesis.