A Shape Changing Rhodium Complex for Auto-Tandem Catalysis

What is it about?

Auto-tandem catalysis is the preparation of complex molecules in a single “pot” through a sequence of distinct catalytic steps using a single catalyst, without deliberate external intervention. This approach to chemical synthesis reduces waste, saves time, and improves overall efficiency. Naturally, developing such a sequence of reactions is incredibly complicated. In the auto-tandem catalytic synthesis of isobenzenes, the selective head-to-tail coupling of terminal alkynes is a particularly challenging reaction step because it occurs alongside head-to-head coupling, which forms another product called E-enynes. In a groundbreaking study, researchers from the United Kingdom have developed a procedure for the autocatalytic synthesis of unusual bicyclic isobenzenes from terminal alkynes, with a cancellation of unwanted “head-to-head coupling.” They achieved this by using a rhodium (Rh) complex of N-heterocyclic carbenes (NHCs) as a catalyst. The Rh catalyst has an NHC-based pincer-shaped ligand, which allows it to interconvert itself between two different geometries in different planes. This shape changing property enables it to catalyze the head-to-tail coupling in one conformation (meridional-geometry) and suppress the head-to-head coupling in the other (facial-geometry). They proved the success of their catalytic process through numerous lab experiments as well as extensive computational simulations. NHC-based pincer ligands could be central to developing future auto-tandem catalytic reactions.

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Photo by Pawel Czerwinski on Unsplash

Photo by Pawel Czerwinski on Unsplash

Why is it important?

The one-pot reaction sequence for the synthesis of isobenzenes from alkynes provides a solution to the orthogonal mechanistic demands of auto-tandem catalysis. Orthogonal mechanistic demands here refer to the head-to-tail and head-to-head couplings of the terminal alkynes in the reaction, both of which produce different products. By using a rhodium complex with a non-heterocyclic carbene (NHC)-based pincer ligand as a catalyst, this study demonstrates how an interconversion in catalyst geometry can promote the desired product, as well as suppress the unwanted one. There is little to no existing research that uses a similar innovative approach. The results of this study provide a simple, thermodynamic way to design future tandem catalytic reactions