Other control experiments show that there was no product observed in the absence of PA or a photocatalyst. (See supporting information for optimization details). The reaction is compatible with both electron-donating and electronwithdrawing groups on the carbonyl arene ring (products 5b-d). A mono-ester substituted cyclopropane gave product 5e in moderate yield (42%). Changing from cyclopropane bearing a diester group to a difluoro functional group failed to deliver the desired product 5f. In parallel, we observed that o-fluoroaryl substituted BCB 6b gave better yield (5g, 84% vs 5a, 72%). We then further developed the scope with BCB 6b. p-Phenyl substituted cyclopropane 1n delivered product 5h in 74% yield. However, dibenzofuran cyclopropane 1o gave only 24% of product 5i. Replacing one ester group by a chlorine or a fluorine resulted in a mixture of diastereoisomers in moderate yield, yet good diastereoselectivity (products 5j, 42% yield, dr 91:9, 5k with 51% yield and dr 81: 19). An estrone substituted cyclopropane gave 5l in 21% yield. In this case, an excellent diastereomeric ratio was observed, probably due to the high steric hindrance of the estrone group. BCBs with other aryl and ester substituents were then studied (Scheme 2E). A chlorinated BCB gave product 5m in 93% yield. The reaction tolerated also other functional groups such as CF3, an ester and OCF3, giving products 5n-5p in 55-70% yield. A methoxy group led to 30% of product 5q and 41% of starting material 1b was recovered. In most cases, full conversion of the BCB was observed due to background reactions such as polymerization or the formation of cyclobutenes. A methyl group was better tolerated and gave 5r in 75% yield. The reaction with a BCB bearing an allyl ester delivered product 5s in 52% yield. There was no [3+ 2] cycloaddition observed with this substrate, indicating that the C-C  bond on BCB is more reactive than inactivated C-C  bonds. We then performed the reaction on 1 mmol scale and 70% of 5m was isolated (Scheme 3). Product 5m was then treated with hydrazine 7, delivering 71% of product 8 for X-ray analysis. The single crystal structure of 8 is shown in Scheme 3 (CCDC number 2238001). Given the redox potentials of the [Ir] photocatalyst (E1/2 PC*/PC+ = -0.96 V vs SCE; E1/2 PC*/PC-= +0.66 V vs SCE) in comparison with reported values for 1h (E1/2 = -2.01 V vs SCE), 7 styrene (E1/2 = +1.97 V) and BCBs (E1/2 = +1.54 V vs SCE), a single electron transfer mechanism appears less probable. Based on previous studies, the formation of a ketyl radical via a proton-coupled electron transfer (PCET) process in the presence of PA could be favored by 11 kcal/mol compared to the direct electron transfer. However, it still appears thermodynamically difficult in the absent of a stoichiometric reductant. Scheme 3. Scale-up experiment and X-ray structure.