The way chemical bonds break is fairly well understood. The valence bond theory, for example, says that the electrons in symmetric σ-bonds cannot split asymmetrically. But sometimes chemists want to bend the rules. For example, Anna Tiefel and colleagues at the University of Regensburg have succeeded in breaking symmetric Se-Se bonds ‘unfairly’.

Normally, when you want to break a bond, you often use either light or heat. With asymmetric bonds, this happens heterolytically: one of the two atoms gets both electrons and you form ions. With symmetric bonds, the assumption is that if you use the same methods, you will get homolysis: the electron pair is shared equally between the atoms, resulting in radicals. This is why research into heterolysis has not really taken off for this type of bond. This is a shame, say the German chemists, because it can provide access to new substances or reactions that are otherwise difficult or impossible to carry out.

Ampholysis

So they came up with something called stimulated doublet-doublet electron transfer (SDET), in which you first break a symmetrical bond homolytically by heating it, and then use light to excite one of the two radicals formed, causing that electron to jump to the other atom, resulting in net heterolysis. They call this type of bond cleavage ampholysis.

They first tested this idea in simulations of Se-Se bonds and ‘polar-symmetric’ Se-C compounds that also normally cleave homolytically, such as allyl-SePh. When this worked, they tried it in the lab. For example, they carried out an ‘atypical 1,2-anti-addition of diselane to enoic acid’ and ‘highly chemospecific SN1 reactions of allyl- and allyl(aryl)selanes with N and C nucleophiles’. Hannah Sayre and Harsh Bhatia write in an accompanying perspective: ‘The results show that ampholysis can indeed be used for synthesis to make compounds that were previously not possible.’

This opens up the possibility of using SDET to break other (polar) symmetric bonds, such as bimetallic complexes, N-N and C-C bonds. In this way, alternative synthetic routes can be invented. Or in the words of Sayre and Bhatia: ‘The authors’ approach provides reaction intermediates that can enable new reaction mechanisms leading to the formation of previously unknown chemical groups.’

Anna Tiefel, Daniel Grenda, et al., Unimolecular net heterolysis of symmetric and homopolar σ-bonds, Nature (2024), doi:10.1038/s41586-024-07622-7 

Hannah Sayre, Harsh Bhatia, Innovative way to break chemical bonds broadens horizons for making molecules, Nature News & Views (2024), doi:10.1038/d41586-024-02437-y