Disproportionation of the formed [W10O32] 5-regenerates the initial photocatalyst [W10O32] 4-and also produces W(IV) species [W10O32] 6-, which is a stronger reductant than initially produced [W10O32] 5-. The reduced forms of the photocatalyst turn the reaction mixture deep blue, which was also observed in our experiments. Considering the remarkable influence of an alkene as well as the result of D2O quenching experiment (Scheme 4, e), we believe that electron-deficient alkenes 2 act as one-electron oxidants to produce anion radical C, which recombines with β-ketoalkyl radical B to form anionic intermediate D. Upon protonation of D, product 3 is finally formed. We do not completely exclude an alternative mechanism which involves Giese-type radical addition of the nucleophilic radical B to electrophilic alkene, followed by SET from [W10O32] 5-or [W10O32] 6-to reduce the initially formed radical adduct (see Scheme S1 in the Supporting Information). -are reported by Ravelli et al. and realigned with respect to Fc + /Fc couple. To support the mechanism with additional experimental evidence, cyclic voltammetry (CV) measurements of the series of cyclopropanols and alkenes were performed. The values of peak potentials (Ep) have been compared, as all the CV experiments, including measurements for [W10O32] 5-/[W10O32] 6-and [W10O32] 4- /[W10O32] 5-pairs, were conducted under the same conditions (Figure ). Electroanalytical characteristics of TBADT have been extensively studied and provide information on the electrochemical potentials for single electron oxidation and reduction of the active catalytic species in acetonitrile and other solvents, but the values vary in the presence of water and are pH-dependent. Examination of the reaction scope (Scheme 2) showed that yields of products and the reaction rates drastically depend on nature of reacting cyclopropanols and alkenes. Such behavior could be correlated with the values of the respective oxidation and reduction potentials, as summarized in Figure . For example, 2-phenylethyl cyclopropanol 1l is the most resistant to oxidation (Ep = +1.3 V) among the three cyclopropanols tested, which agrees with its slow reaction rate (20 h) delivering 3la in a moderate 53% yield. This contrasts to fast and high yielding reaction of electron-rich 1a, which is the strongest reductant (Ep = +0.84 V). The reactions of the least susceptible for SET oxidation aliphatic cyclopropanols are accelerated or even initiated by the addition of an external oxidant (NFSI or K2S2O8). The role of the oxidant remains unclear, however substoichiometric loadings (20-50 mol%) imply that it could act as a radical initiator that induces homolytic (rather than via SET oxidation) cleavage of the O-H bond in 1 or could recover the reduced forms of TBADT catalyst formed in parasitic C-H activation processes. An analogous correlation between reactivity and single-electron oxidative properties has also been observed for alkenes: stronger oxidants, as revealed by comparison of the respective Ep values (Figure , c), are more favorable substrates in their reaction with 1a. The clear dependence on the electronic properties of the alkenes offers further support for the intermediacy of the anion radical C. It was noted that the reduction potentials measured for the radicals formed in the Giese reaction showed these are much stronger one-electron oxidants (-0.3 to -0.1 V vs Fc + /Fc) and the reaction rate in this case should be less dependent on starting alkene. Moreover, we believe that disproportionation of [W10O32] 5-delivering a stronger W(IV) reductant [W10O32] 6-is essential to enable the reduction of 2 into C.