
"In addition to its conceptual and mechanistic importance, this method also has a synthetic benefit," explains lead author Roman Kleinmans. "This is because we can use it to build polycyclic, three-dimensional carbon scaffolds that have been difficult or impossible to access. Such three-dimensional architectures are fascinating and play an increasingly important role in medicinal chemistry."

The Münster team uses this laboratory set-up to perform photocycloaddition. © University of Münster - Roman Kleinmans
The chemistry in detail For more than a century, so-called [2+2] photocycloadditions have been studied and developed. Research has focused specifically on [2π+2π]-systems in which, for example, two double bonds react to form two new single bonds giving a four-membered ring product. The team from Münster has now achieved a breakthrough by realising this type of reaction using a single bond ("2π+2σ").

The new unconventional photocycloaddition; top: schematic representation of the reaction equation with a ’butterfly-like’ starting compound (2?); bottom: structures of two reaction products (far right as 3D representation/crystal structure analysis). © University of Münster - Roman Kleinmans
The visible light driven "triplet-triplet energy transfer catalysis" allows the reaction to be carried out as mildly as possible, so that irradiation of the reaction with harsh UV light, which is commonly used in chemistry of this type, was not needed. In this mechanism, the catalyst absorbs energy from the irradiated light and transfers it to a suitable substrate. The catalyst returns to the ground state (it is regenerated), and the corresponding molecule is left in an energetically excited state (triplet state). The excited molecule is then able to react with the single bond. "We used a simple organic photosensitizer for this, namely thioxanthone, dispensing with rare and expensive transition metal-based catalysts," emphasizes Frank Glorius. To further understand the molecular mechanism of the reaction in more detail, the chemists carried out computational calculations using "density functional theory".