In the case of [3+2] cycloaddition with BCBs, the role of PA may be to activate either the carbonyl group on the cyclopropanes or the one on the BCBs. Yoon and co-workers have shown that coordination of Lewis and Brønsted acids to the carbonyl group can lower the triplet energy of aryl olefin ketones. Hydrogen bonding also has a significant impact on the formation of triplet 1,3 diradical intermediates, as reported by Bach. In experiments performed in the dark, we observed conversion of BCBs, indicating that PA could also play a role in activating the BCB partners. An activation of the BCBs would be more in agreement with the fact that PA was not needed for the other reaction partners. Further mechanistic studies should be performed to better understand this activating effect. In conclusion, we have developed a photocatalyzed method to promote homolytic cleavage of aromatic carbonyl cyclopropanes in the absence of metal coordination. Both ethynylarenes and styrenes can be used for the [3+2] cycloaddition. Furthermore, the catalytic system can be applied for the synthesis of bicyclo[3.1.1]heptanes via [3+2] cycloaddition in presence of diphenyl hydrogen phosphate (PA) as co-catalyst. The obtained products are useful bioisosteres of meta-substituted arenes. Based on reported redox and triplet energy values, a single molecule photoexcitation of one of the reaction partners appears improbable. Preliminary observations led us to hypothesize the photoactivation of a pre-formed - interaction complex. Further mechanism investigations are currently ongoing in our laboratory to support this hypothesis. Due to the high demand for structurally diverse ring systems and bioisosteres in drug discovery, we believe that our study will be of high interest for both synthetic and medicinal chemists.