Insoluble Catalyst Works Wonders

A growing number of chemists are turning to water as an environmentally friendly alternative to standard organic solvents—water is abundant, inexpensive, nonflammable, and nontoxic. In one example, Shū Kobaya­shi and coworkers at the University of Tokyo have been exploring the use of nonimmobilized chiral heterogeneous catalysts in water. Although the catalysts are insoluble in water, they nevertheless can mediate reactions between lipophilic reagents. The Tokyo researchers have now paired copper acetylacetonate with a chiral bipyridine ligand for the enantioselective addition of silylboronates to a range of α,β-unsaturated acceptor molecules (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b11418). The products feature a chiral silyl group, which is a convenient placeholder for a C–C or C–O bond that can be formed in later steps. The new reactions only proceed in water and don’t work in organic solvents when the catalyst or reactants are soluble. The team believes water plays a prominent role in constructing and stabilizing sterically confined, rigid transition states and accelerating subsequent protonation leading to the high yields and enantioselectivities.