The optimized conditions used two readily available 467 nm LED Kessil lamps, Cs2CO3 as base and DMF as reaction solvent (see the Supporting Information page S13 for a detailed description of the reaction set-up). Iridium photocatalyst [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 ([Ir]; see Scheme 2) provided appropriate redox potentials to oxidize the carboxylate and reduce the Giese adduct. Other photocatalysts such as [Ru(bpy)3](PF6)2 or (Mes-Acr)(ClO4) formed little (<5%) or no product (entries 2-3). Only slight reductions in yield were observed when reducing photocatalyst loading to 0.5 mol% (entry 4), performing the reaction at lower temperatures (28 °C, fan controlled; entry 5), in different solvents (MeCN, 1,4-dioxane; entries 6-7), lower concentration (entry 8), with variable amounts of ethyl acrylate (entries 9-10) or with an organic base (DBU; entry 11). Control experiments demonstrated the requirement for photocatalyst, light and base (entries 12-14). We examined deviations in reaction conditions that are often encountered between laboratories by using a sensitivity screen as described by Glorius (Figure ). The reaction was shown to be broadly insensitive to small changes in the optimized conditions, thus facilitating implementation of the decarboxylative oxetane-functionalization protocol (see Supplementary Table for details). Only high concentrations (0.25 instead of 0.2 M) and high levels of oxygen (i.e. under air) were identified as potential pitfalls for the reaction, factors that can be readily controlled. Notably, given the known sensitivity of photochemical reactions to increased scale, 32 the input of 1 could be increased to 5 times the standard scale (0.2 to 1.0 mmol) by simply increasing the size of the reaction vial with no impact on yield (58% of 2a). Repurification of 1 (to 99.9% pure instead of 98.9% by quantitative 1 H NMR) further increased the yield of 2a (67%). With optimized and reliable conditions in hand, the scope of the reaction was probed (Scheme 2). A diverse set of novel 3,3-disubstituted oxetane products was obtained varying both the radical acceptor and the oxetane acid precursor. Simple acrylates were successful coupling partners and generated the products in good yields (2a, 2b). Vinyl ketones, sulfones, phosphonates and nitriles could also be employed, albeit in lower yields (2c-2f). Medicinally interesting amide derivatives were synthesized in moderate yields (2g-2i). ). d Isolated in 70% purity. e Yield based on alkene (0.20 mmol scale). f Using TIPS-protected oxetane acid.