Self-assembly of heterobimetallic complexes and reactive nucleophiles: A general strategy for the activation of asymmetric reactions promoted by heterobimetallic catalysts

Heterobimetallic asymmetric catalysts, such as the lanthanum-lithium-binaphthol complex (LaLi-BINOL), the aluminum-lithium-binaphthol complex (AlLi-BINOL), and a newly prepared gallium-sodium-binaphthol complex (GaNa-BINOL), have been self-assembled with reactive nucleophiles, such as lithium nitronates and sodium malonates, to generate more efficient catalysts than the parent heterobimetallic catalysts. For example, by the combined use of LaLi-BINOL (1 mol %; contains one H2O molecule) and BuLi (0.9 mol%) as the catalyst system, asymmetric nitroaldol reactions are greatly accelerated in all cases without a decrease in the optical purity of the nitroaldol products. Kinetic analyses have also been carried out on the GaNa-BIhTOL-catalyzed Michael reaction of dibenzyl malonate with cyclohexenone, with or without NaOtBu. The calculated rate constants show that the combined use of GaNa-BINOL and NaOtBu as the catalyst gives reaction rates that are about 50 times faster than with GaNa-BINOL alone. This activation method should be useful for other asymmetric reactions catalyzed by heterobimetallic complexes.