Chiral Counterion Enables Asymmetric C–C Coupling

A ruthenium catalyst equipped with a chiral phosphate counterion promotes stereoselective C–C coupling of alcohols and aldehydes to butadiene, providing a direct route for converting basic feedstocks to chiral building blocks, report chemists from the University of Texas, Austin (Science, DOI: 10.1126/science.1219274). Led by chemistry professor Michael J. Krische, the group used H₂Ru(CO)[P(C₆H₅)₃]₃ with a 1,1′-bi-2-naphthol-derived phosphate counterion to combine butadiene with benzyl alcohol or benzaldehyde to form (1R,2R)-2-methyl-1-phenylbut-3-en-1-ol with high yield and enantiomer and diastereomer ratios. The researchers believe that alcohol dehydrogenation triggers butadiene hydrometalation to produce a π-crotylruthenium complex and an aldehyde. The bulky chiral counterion drives partitioning between the (E)- and (Z)-σ-crotylruthenium isomers, favoring stereospecific carbonyl addition from the E isomer. The resulting homoallylic ruthenium alkoxide exchanges with a reactant alcohol to release the product and close the catalytic cycle. The approach avoids the generation of stoichiometric by-products and use of cryogenic conditions, making it more amenable to process-scale use than related methods for asymmetric C–C bond formation, the authors say.