To confirm the formation of an EDA complex between 1a and 2a, a UV-Vis spectroscopic study has been conducted (Figure , a). As commonly observed for weakly associated EDA complexes, a UV absorption band for a mixture of 2a and 1a (blue line in Figure , a) experienced a bathochromic shift with respect to the individual substrate 1a (gray line in Figure ). The excessive absorption in the range of ca. 370-400 nm can be more clearly visualized by subtracting the spectrum of 1a from the spectrum of the mixture (yellow line). To obtain additional evidence for the complex formation and to evaluate the association constant, a 1 H NMR titration experiment was performed (Figure ). Clear and gradual changes in the chemical shifts of the two diagnostic protons Ha and Hb of 2a were observed, shifting towards the highfield region of the 1 H NMR spectrum upon the addition of 1a (ranging from 1 to 95 equivalents). Fitting the experimental titration curve (see Figure in the Supporting Information) yielded the value of the association constant Ka = 0.16 M -1 . Scheme 3. EDA complex-mediated cross-coupling of aryl cyclopropanols with electrophilic alkenes. Reaction conditions: Alkene 2 (0.2 mmol), cyclopropanol 1 (1.5 equiv.), CH3CN (0.2 M), 370 nm Kessil LED, under Ar, r.t. Yields were determined by H NMR analysis of the crude reaction mixture against triphenylmethane as an internal standard. These results suggest that the EDA interaction is relatively weak, even between electron-rich cyclopropanol 1a and highly electron-deficient olefin 2a, though sufficient to engage these molecules into a photoinduced charge transfer process. Furthermore, the presence of an electron-rich aromatic moiety in 1a is probably responsible for the formation of the respective EDA complex, making it unlikely to be formed for aliphatic cyclopropanols. Consequently, electron transfer between the substrates is either accelerated or enabled by TBADT acting as a photoredox catalyst, and the catalytic process should likely generate the same key intermediates A and C. The reaction should start with a SET oxidation of cyclopropanol by photoexcited TBADT leading to the formation of radical cation A and its further rearrangement into β-keto alkyl radical B. The generation of B was supported by performing the reaction in the presence of TEMPO (2,2,6,6-tetramethyl piperidine-N-oxyl) as a radical scavenger. TEMPO interrupted the formation of 3aa as the TEMPO-trapped adduct 5 was formed instead (83% 1 H NMR yield, Scheme 4, a). In addition, the reaction with a disubstituted cyclopropanol 1s smoothly afforded 3sa (75% yield, Scheme 4, b), indicating that the ring opening occurred exclusively at the most substituted C1-C2 bond of the cyclopropane ring and therefore proceeded via a more stable secondary alkyl radical. After the addition of β-ketoalkyl radical B to the double bond of an alkene, HAT from solvent, cyclopropanol or other source might occur and result in the formation of product 3. Consequently, deuterium labelling experiments were performed to trace the origin of the δ-hydrogen in 3aa. First, D atom was not abstracted from acetonitrile-d3 (Scheme 4, c) which makes HAT from this solvent and from dichloromethane with a similar C-H bond dissociation energy of 96 kcal mol -1 unlikely. However, the reaction of benzylidene malononitrile 2a with deuterium-labeled cyclopropanol 1a-d (87% D) in dry dichloromethane resulted in 3aa-d with 69% D incorporation at δ-position, indicating that OH-group of 1a is the source of the δhydrogen (Scheme 4, d). More interestingly, the reaction of 1a-d was much slower than that of 1a, resulting in only 44% yield after 20 h of irradiation. The remarkable difference in the reaction rates along with lower deuterium incorporation in the product 3aa-d (69% D), in comparison to the content of deuterium in the starting material 1a-d (87% D), were clear consequences of a primary kinetic isotope effect (KIE) associated with the breaking of O-H bond in 1a. Moreover, the KIE was an indication that oxidation of 1a is the ratelimiting step of the whole process. However, it remained unclear whether a hetero-(SET followed by deprotonation) or a homolytic (HAT) cleavage of the O-H bond in 1a was involved. When cyclopropanol 1a underwent the reaction with dimethyl fumarate 2f in acetonitrile/D2O mixture (Scheme 4, e), rather fast (4 h) reaction occurred and produced 3af-d (98% D incorporation) in 87% yield. In this specific experiment, 3af served as a more reliable mechanistic probe than the previously used malononitrile adduct 3aa, as the latter compound experienced fast exchange of its more labile δ-H to deuterium with D2O (see section 4.3.3 in the Supporting Information). Based on these results, as well as considering high BDE of the O-H bond in alcohols and especially O-D bond in D2O (125 kcal mol -1 ), we concluded that the heterolytic cleavage of O-H bond and proton transfer are more likely than HAT. The conclusion was also bolstered by an observation that HAT processes occurred slowly under the standard reaction conditions, even in the case of relatively weak C-H bonds. For example, when ketone 6 was subjected to the reaction with benzylidene malononitrile 2a under the standard conditions (Scheme 4, f), HAT from the methoxy group occurred and the formation of product 7 was confirmed by H NMR and HRMS analysis, but in low 13% yield after 16 h. Furthermore, TBADT did not provoke any notable C-H activation side processes at sensitive benzylic sites during investigation of the substrate scope (for example, in the synthesis of compounds 3la and 3ma). A plausible mechanism, which stays in accord with the collected experimental data, is shown in Figure . Photoexcitation of TBADT followed by intersystem crossing produces the triplet excited state *[W10O32] 4-. The photoexcited W(VI) species oxidizes cyclopropanol 1 via SET, resulting in cation radical A.