Palladium is the undisputed champion when it comes to catalyst metals, enabling chemists to carry out a versatile array of cross-coupling reactions. Bruce H. Lipshutz and his group at the University of California, Santa Barbara, have a goal to take this versatility further by seeing how green and efficient they can make palladium catalysis.
In their latest endeavor, the Lipshutz group and its collaborators have devised a new structurally optimized phosphine ligand that helps facilitate palladium-catalyzed cross-couplings with only a parts-per-million level of palladium, which is one to two orders of magnitude lower than that typically required (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201510570).
Chemists have concerns about the cost and availability of palladium and about the amount of residual palladium left after synthesis of a product requiring FDA approval. Lipshutz has been chipping away at these concerns in recent years by developing an aqueous nanomicelle reaction system that minimizes the amount of palladium needed. The lipophilic “nanoreactors” allow common organic reactions to be run in water at room temperature with repeated in-flask recycling of the nanomicelles and catalyst.
The new ligand, which is called HandaPhos after its creator, postdoc Sachin Handa, gives the researchers the ability to carry out Suzuki-Miyaura reactions in the nanomicelles using complex aryl coupling partners with less palladium than ever before, below 0.1 mol % compared with previous lows of about 1 mol %.
“Lipshutz is a magician who uses his micelles as a wand to make the impossible possible,” says Thomas J. Colacot, global R&D manager for homogeneous catalysis at Johnson Matthey Catalysis & Chiral Technologies. “The group’s work is having a great impact on our understanding of how Nature uses enzymes and trace amounts of metals to make biaryls, many of which are the essence of natural products.”