Roles of Bases in Transition-Metal Catalyzed Organic Reactions

Bases, either organic bases or inorganic ones, are very often added in transition-metal catalyzed organic reactions to promote the catalytic reaction efficiency and increase the yields of products. As a common practice for most published papers, a reaction condition screening table is given, listing a number of bases and the respective yield of products. However, no discussion or a little in some cases is provided on why one base works well to give a high yield formation of the product, but other bases afford no or low yields of the product. Furthermore, in many cases a certain base works well for one reaction but may not work for another. Indeed, for a complicated reaction mixture containing several different components, it is very difficult to analyze and understand the roles of certain bases. Yet there are sporadic discussions and rationalization on the roles of bases in the literature. The role of bases has been reported to be straightforward in some cases, for examples, to abstract protons or to neutralize acids in the reaction system, but very complicated in many other cases. The roles of bases may be affected by several factors, including basicity, solubility, ionization ability, solvent, aggregation state, the size of the metal cations, Lewis acidity of the metal cations, the HSAB theory, the size of the counter anions; the coordination ability of the counter anions, etc. In addition to abstract protons or to neutralize acids in the reaction system, a base may activate the catalysts and facilitate the regeneration of reactive catalytic species. The roles of the metal cation in a base may mainly influence the solubility of the base in organic solvents, or interact with substrates or solvents. The roles of the counter anion in a base may mainly contribute to the coordination with a metal center and subsequent stabilize the complex. The major differences between inorganic bases and organic bases include their solubility and bulkiness. Metal contaminants in bases may also have innegligible effect on the reactions, since in many cases a large excess amount of bases are added. This review is written based on the limited knowledge, focusing on commonly used inorganic bases such as LiOBu-t, NaOBu-t, KOBu-t, LiOAc, NaOAc, KOAc, LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3, Cs2CO3, KF, CsF and organic bases such as DBU and Et3N.