Degenerative xanthate transfer to olefins under visible-light photocatalysis

The degenerative transfer of xanthates to olefins is enabled by the iridium-based photocatalyst [Ir{dF(CF3)ppy}2(dtbbpy)](PF6) under blue LED light irradiation. Detailed mechanistic investigations through kinetics and photophysical studies revealed that the process operates under a radical chain mechanism, which is initiated through triplet-sensitization of xanthates by the long-lived triplet state of the iridium-based photocatalyst.


Cyclic voltammetry
The cyclic voltammetry experiments were conducted with a computer-controlled Eco Chemie Autolab PGSTAT302N potentiostat in a three-electrode cell. All electrochemical measurements were carried out under an argon atmosphere and the samples were bubbled with argon before measurement. Cyclic voltammograms of 1.2 mM xanthate 1a with 0.1 M n-Bu 4 NPF 6 as the supporting electrolyte were recorded at a scan rate of 100 mV/s with a planar glassy carbon disc (1 mm diameter) as working electrode, a platinum wire as counter electrode and an Ag wire (in 0.5 M n-Bu 4 NPF 6 in MeCN) as a pseudo-reference electrode at 298 ± 2 K. Ferrocene was added to the sample solution as an internal reference (the ferrocene/ferrocenium couple appeared at E 1/2 (Fc/Fc + ) = 0.583 V vs. Ag wire). Using this ferrocene redox couple, the potential E vs. SCE (saturated calomel electrode) was calculated by the following equation, based on E 1/2 (Fc/Fc + ) = 0.400 V vs. SCE [1]: The quenching rate (k q ) was determined using Stern-Volmer kinetics.
As shown in Figure S4, when comparing the normalized TA spectra of photocatalyst 8 in the absence ( Figure 2A) and presence of xanthate 1a ( Figure 2D), an additional contribution from a broad OD band that stretches from 500 nm to 800 nm is seen for the latter which is attribute to absorption of the xanthic acid radical. In this case, the xanthic acid radical is formed from the homolytic bond cleavage of the excited triplet

Determination of quantum yield
We utilized the protocol reported by Yoon and co-workers to determine the photon flux of the blue LED [3]. All solutions were stored in the dark when not in use. A buffered phenanthroline solution was obtained by dissolving 1,10-phenanthroline (10.0 mg) and sodium acetate (2.25 g) in 0.5 M H 2 SO 4 (prepared by fresh deionized water) (10 mL total volume).

Determination of background Fe 2+ concentration
3 mL of the ferrioxalate solution was added to a 4 mL vial. Next, 0.525 mL of the phenanthroline solution was added and the mixture was stored in the dark for 1 hour.
Then the solution was transferred to a cuvette and a UV-vis spectrum was measured using UV-vis absorption spectrometer (Cary 100, Varian). The absorbance value at 510 nm was recorded. This process was repeated twice.

Determination of photon flux
Three mL of the ferrioxalate solution was added to a 4 mL vial. The vial was immediately irradiated with blue LED ( max = 469 nm) for 90 seconds and removed from the blue LED. Then, 0.525 mL of the phenanthroline solution was added to the ferrioxalate solution, and the resulting mixture was stored in the dark for 1 hour. Then the solution was transferred to a cuvette and the UV-vis spectrum was measured. The absorbance value at 510 nm was recorded. This process was repeated twice.

Calculations
The amount of Fe 2+ formed was calculated according to the following equation: where V is the volume of the sample analyzed (3.53 mL), ΔA is the difference in average absorbances (between irradiated and unirradiated ferrioxalate solutions) at 510 nm, l is the path length, and ε is the molar absorptivity at 510 nm [4].
The fraction of light absorbed by the ferrioxalate actinometer was calculated by the following equation: where A is the absorbance at 468 nm of the ferrioxalate actinometer solution prior to irradiation and addition of phenanthroline (vide infra).
The photon flux was calculated using the following equation:

Determination of fraction of light absorbed at 468 nm for the ferrioxalate solution
The absorbance at 468 nm of the ferrioxalate actinometer solution prior to irradiation and addition of phenanthroline was measured to be 0.4890742302 ( Figure S5).

Figure S5
: UV-vis absorption spectrum of ferrioxalate actinometer solution prior to irradiation and addition of phenanthroline.
The vial was fitted with a cap and sealed with parafilm. The mixture was stirred and irradiated with blue LED ( max = 469 nm) for 4 hours (14400 s). After irradiation, the vial was removed from the blue LED and the reaction mixture was was purified as detailed below (section 9.1, page S14) to provide the desired product (391 mg, 1.27 mmol) in 58% yield.
The quantum yield (Φ) was calculated using the following equation: where t is the reaction time and f is the fraction of light absorbed by 8 that was calculated using the following equation: A at 468 nm: 0.4890742302 S11 f = 1 − 10 − = 1 − 10 −0.811556876 = 0.846 where A is the absorbance at 468 nm of the 8 solution (5 mM in DMSO) (vide infra).