Researchers at the Graduate School of Engineering at Nagoya University in Nagoya, Japan, recently reported that they developed an asymmetric catalyst that assembles spontaneously, a development that lays the groundwork for further designing functional supramolecular catalysts. Their work involved using chiral organic ion-pair catalysts assembled through a hydrogen-bonding network (1). The researchers pointed out that overall development of structurally discrete, chiral supramolecular catalysts for asymmetric organic transformations has been met with limited success. In their work, however, the researchers reported that a chiral tetraaminophosphonium cation, two phenols, and a phenoxide anion appeared to have self-assembled into a catalytically active supramolecular architecture through intermolecular hydrogen bonding. The researchers developed the catalyst for the highly enantioselective conjugate addition of acyl anion equivalents to α-, β-unsaturated ester surrogates (1).Catalytic asymmetric synthesis for nonnatural amino acids
The researchers addressed this problem by developing a scaleable catalytic asymmetric Strecker synthesis of unnatural α-amino acids. The Strecker synthesis is an approach to produce racemic α-amino acids, but catalytic asymmetric methods have been limited to small scales. The Strecker synthesis involves the reaction of an imine or imine equivalent with hydrogen cyanide followed by nitrile hydrolysis. Existing catalytic methods and the use of hazardous cyanide materials in the asymmetric Strecker reaction however, limits its application in large-scale reactions (2).
To resolve that issue, Jacobsen and his team developed a new catalytic asymmetric method for producing enantiomerically rich nonnatural amino acids using a chiral amidothiourea catalyst to control the hydrocyanation step. The researchers report that this approach is compatible with aqueous cyanide salts, which are safer than other cyanide sources, which allows the process to be run at larger scales (2).
Ligand selection in asymmetric transition-metal catalysis