Lewis Pair Gets A Handle Just SO On Sulfur Monoxide

It’s one of those quirks of the periodic table: Molecular oxygen, O₂, is stable, ubiquitous in Earth’s atmosphere, and often used as an oxidant in chemical reactions. But sulfur monoxide, SO, just one element removed, is unstable and has a fleeting existence. Chemists have long been searching for ways to stabilize SO to make it useful as a small molecule for organic synthesis, such as complexing it with transition metals, but to little avail. Douglas W. Stephan, Lauren E. Longobardi, and Vanessa Wolter at the University of Toronto have now used frustrated Lewis pairs to tame SO (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201409969). Frustrated Lewis pairs are acid-base pairs dressed with bulky substituents that prevent them from coming close enough together to completely form a neutral adduct. The resulting reactive species can function like a metal catalyst to activate small molecules such as H₂, CO₂, CO, and N₂O. The Toronto chemists show that the combination of a bulky phosphine and bulky borane can capture N-sulfinylamines to form an unusual seven-membered heterocycle containing six different elements, including an SO unit (shown). In a set of test reactions, the researchers used the new chemical to transfer SO to a phosphine, rhodium complex, or N-heterocyclic carbene, a first step in using SO as a reagent.