Nitrogen Lewis acids as a platform for frustrated Lewis pairs

The term "Frustrated Lewis Pair (FLP)" was coined in 2007 by Stephan and Erker. In their seminal works, they reported the activation of a hydrogen molecule by a non-metallic species.^1-2 To that end, they used molecules containing both an electron-accepting boron atom, and a bulky electron-donating phosphine. In fact, despite having a significant driving force for bond formation, these two moieties do not form a classical Lewis adduct because of steric congestion. The subsequent "frustration" renders them available for various chemical bond activations.

Recently, we have demonstrated an unprecedented Lewis acidic behavior of nitrenium cations.³ It represents the first examples of nitrogen-based Lewis acids. In contrast to common Lewis acids used in FLPs, which utilize the empty orbital of a boron atom, nitrogen Lewis acids utilize an electrophilic nitrogen atom. Considering the superior electronegativity of a nitrogen atom compared with a boron atom, these molecules have the potential to activate a plethora of chemical bonds, making them an interesting subject for research.

We synthesized new inter- and intramolecular FLP candidates by combining different Lewis bases with our nitrogen Lewis acids. Our intermolecular FLPs successfully cleaved Si-H and S-S bonds (Figure). Computational studies were performed to guide a further improvement of our systems in FLP chemistry.

Figure - Bond activation by our nitrogen-based FLPs

1. G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan, Science 2006, 314, 1124-1126.
2. P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme, D. W. Stephan, Chem. Commun. 2007, 2, 5072-5072.
3. A. Pogoreltsev, Y. Tulchinsky, N. Fridman, M. Gandelman, J. Am. Chem. Soc. 2017, 139, 4062-4067.