Protonation of γ‐Butyrolactone and γ‐Butyrolactam

Abstract

Invited for this month's cover is the group of Andreas Kornath at the Ludwig‐Maximilian University of Munich (Germany). The cover picture shows the crystal structures of protonated γ‐butyrolactone ([(CH₂)₃OCOH][AsF₆]) and γ‐butyrolactam ([(CH₂)₃NHCOH][AsF₆]). Both salts were synthesized by reacting the neutral compounds in the superacidic system HF/AsF₅ at low temperature. Interestingly, the X‐ray structure analyses revealed not only structural parameters of the salts, but also the existent of C⋅⋅⋅F contacts in both species. Quantum chemical calculations were performed to investigate the nature of these contacts. Read the full text of their Full Paper at 10.1002/open.202000220.

“…Both, the γ‐butyrolactone and the γ‐butyrolactam motifs are very common in natural products. Their respective protonated species are common reaction intermediates which are however not well known and characterized so far… Find out more about the story behind the front cover research at 10.1002/open.202000220.

What prompted you to investigate this topic/problem?

Both, the γ‐butyrolactone and the γ‐butyrolactam motifs are very common in natural products. In total syntheses some synthesis routes include the respective protonated species, which were however not well known and characterized so far. We were interested in investigating these two protonated species to get insights into the structural and electronical properties.

What aspects of this project do you find most exciting?

The most exciting aspect was the comparison of both protonated species. As the reaction behavior of both compounds in the superacidic systems turned out to be very similar, we had to take a closer look on the differences. By means of quantum chemical calculations, we could explain the different formation of C⋅⋅⋅F contacts in the two crystal structures of protonated γ‐butyrolactone and protonated γ‐butyrolactam.

Did you expect a very different outcome? If so, what was your initial guess?

As γ‐butyrolactone and γ‐butyrolactam bear two potential basic centers, the question was to what extend it is possible to protonate each species. Furthermore, a ring opening reaction was possible, and the question was, if it could occur in superacidic media. Especially because an acid‐catalyzed ring opening reaction was reported for γ‐butyrolactone.

What was the biggest surprise (on the way to the results presented in this paper)?

The biggest surprise was, that only the formation of monoprotonated species were obtained and that we never observed a diprotonation or ring opening reaction.

S. Beck, M. Feller, L. Spies, K. J. Dietrich, C. Jessen, K. Stierstorfer, A. J. Kornath, ChemistryOpen 2021, 10, 3.