Based on these experiments, we propose the following mechanism for the photoredox-catalyzed radical group transfer of vinyl and alkynyl groups from siloxanes (Scheme 5). Following irradiation with visible light, the excited photocatalyst Ir(III)* undergoes single-electron reduction by Bu3N. The ground state Ir(III) photocatalyst is then regenerated through single-electron oxidation by substrate 4 or 7, resulting in fragmentation of 4 or 7 into radical 13 and I⁻. Radical 13 undergoes a rapid 5-exo cyclization to form the βsilyl radical 14, which is then oxidized into carbocation 15, presumably by the Bu3N radical cation. Ringopening leads to the silylium ion 16. Subsequent capture by the nucleophilic solvent leads to the stable intermediate 5, and simultaneously generates one equivalent of acid, which consumes one equivalent of Bu3N. Product 6 or 8 is formed after subjecting siloxane 5 to acidic conditions. Notably, this redox-neutral mechanism contrasts to the radical chain mechanism suggested by other studies. Scheme 5. Proposed mechanism By comparison, subjecting iodide 4a to BEt3 and trace levels of oxygen with H NMR monitoring results in the formation of a cyclized iodide intermediate (17, Scheme 4c). Under these conditions a conventional radical chain reaction is initiated, where cyclized radical intermediate 14 propagates the reaction by abstracting an iodide from 4a. Due to the lack of an electronacceptor in this reaction, intermediate 14 cannot undergo single-electron oxidation to cation 15. Indeed, we did not detect the presence of ring-opened vinyl species 16 or 5a in this reaction, even after 6 h. However, addition of tetrabutylammonium fluoride facilitated the formation of the ring-opened and desilylated product 6a. See the Supporting Information for the NMR spectra.