Without BCl3, 2 shows a band with a peak maximum, λabs, of 250 nm. Pleasingly, in the presence of BCl3 a new band appears at λabs of 300 nm and with a tail into the visible region. However, attempts at direct photoexcitation of 2 in the presence of boron trichloride with a 370 nm LED showed no reactivity and only returned unreacted starting material. Therefore, even though the Lewis acid does shift the absorption of 2 as hypothesized, this strategy did not prove productive, and investigation turned to the use of a photocatalytic DET reaction. ). The most accurate methodology used was at the BLYP/6-31G(d,p) level of theory, which predicted an ET of 62. Based on these calculations, a solution of 2 and 25 mol% of benzophenone in MeCN was irradiated with a 390 nm LED over 24 h (Table , entry 1). Pleasingly, partial conversion to the desired product (3) was observed using our standard photoreactor (see ESI). Increasing the catalyst loading gave increased product conversion over the same period but fell short of complete conversion to 3 (Table , entries 2-3). To achieve full conversion, an alternative reaction set-up that involved direct irradiation rather than the use of a photoreactor (see ESI) was employed that provided more intense irradiation (Table , entry 4-6). At this point, control reactions were carried out to confirm the necessity of benzophenone. When using the photoreactor in the absence of benzophenone, no conversion to the desired product was observed, which is consistent with our previous attempts with BCl3 (Table , entry 7). However, upon direct irradiation, a modest conversion to 3 was observed in the absence of benzophenone (Table , entry 8). This suggests that if the light is sufficiently intense, then photoirradiation at 390 nm should promote the [2+2] cycloaddition via direct excitation of 2, albeit much less efficiently. To obtain useful quantities of 3 the reaction was scaled up to a 0.5 mmol scale. Similar results were observed, with full conversion to 3 requiring 0.5 equivalents of benzophenone (Table , entries 9-11). Further increase in scale to a 1.0 mmol reaction could also be achieved using the same conditions and an NMR yield of 93% was observed (Table , entry 12). 1.0 50 100 (93) d a Conditions: 2, benzophenone, MeCN (0.1 M), N2, LED (λexc = 390 nm), rt. b From crude 1 H NMR. c Reaction performed in a photoreactor (see Figure ). d1 H NMR yield using 1,3,5-trimethoxybenzene as the internal standard.