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. 2014 Feb 7;79(3):1427-36.
doi: 10.1021/jo500031g. Epub 2014 Jan 27.

Redox mediators in visible light photocatalysis: photocatalytic radical thiol-ene additions

Elizabeth L Tyson  1 , Zachary L Niemeyer, Tehshik P Yoon
Affiliations

Redox mediators in visible light photocatalysis: photocatalytic radical thiol-ene additions

Elizabeth L Tyson et al. J Org Chem. .

Abstract

Synthetically useful radical thiol-ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol-ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

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Figures

Scheme 1
Scheme 1. Photocatalytic Radical Thiol–Ene Reaction
Scheme 2
Scheme 2. Proposed Mechanism of Direct Photocatalytic Thiol–Ene Coupling
Scheme 3
Scheme 3. H-Atom Shuttle Hypothesis
Figure 1
Figure 1
Relationship of N–H BDE to thiol–ene yield.
Scheme 4
Scheme 4. Brønsted Base Hypothesis
Figure 2
Figure 2
Relationship of pKa to thiol–ene yield.
Scheme 5
Scheme 5. Redox Mediator Hypothesis
Figure 3
Figure 3
Relationship of Eox to thiol–ene yield.
Scheme 6
Scheme 6. Redox Mediated Radical Thiol–Ene Reaction
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