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. 2016 Jan 11;55(2):539-44.
doi: 10.1002/anie.201508060. Epub 2015 Nov 27.

Activation of Electron-Deficient Quinones through Hydrogen-Bond-Donor-Coupled Electron Transfer

Amanda K Turek  1 , David J Hardee  1 , Andrew M Ullman  1 , Daniel G Nocera  2 , Eric N Jacobsen  3
Affiliations

Activation of Electron-Deficient Quinones through Hydrogen-Bond-Donor-Coupled Electron Transfer

Amanda K Turek et al. Angew Chem Int Ed Engl. .

Abstract

Quinones are important organic oxidants in a variety of synthetic and biological contexts, and they are susceptible to activation towards electron transfer through hydrogen bonding. Whereas this effect of hydrogen bond donors (HBDs) has been observed for Lewis basic, weakly oxidizing quinones, comparable activation is not readily achieved when more reactive and synthetically useful electron-deficient quinones are used. We have successfully employed HBD-coupled electron transfer as a strategy to activate electron-deficient quinones. A systematic investigation of HBDs has led to the discovery that certain dicationic HBDs have an exceptionally large effect on the rate and thermodynamics of electron transfer. We further demonstrate that these HBDs can be used as catalysts in a quinone-mediated model synthetic transformation.

Keywords: catalysis; electron transfer; hydrogen bonding; oxidation; quinones.

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Figures

Figure 1
Figure 1
Effect of quinone structure on oxidizing ability and Lewis basicity.
Figure 2
Figure 2
Series of HBDs and additives examined in this study.
Figure 3
Figure 3
Experimental CV data and comparison with simulation. CVs (0.1 V/s) recorded for 0.5 mM Q in 0.1 M nBu4NBArF24/CH2Cl2 (glovebox) in the presence of increasing (a) [1], (b) [2], (c) [3], (d) [4].
Figure 4
Figure 4
Initial rate constants (kobs) vs. [HBD]2 or [HBD] for ET from ferrocene derivatives to Q in CH2Cl2 at 25 °C under N2. (a) Second-order plots for 1 (10–1.0 mM), Q (1.0 mM), bromoferrocene (BrFc) (1.0 mM); and 2 (10–1.0 mM), Q (1.0 mM), 1,1’-dibromoferrocene (Br2Fc) (1.0 mM); (b) First-order plot for 3 (5.0–0.5 mM), Q (0.5 mM), 1,1’-dimethylferrocene (Me2Fc) (0.5 mM).
Scheme 1
Scheme 1
(a) Square scheme describing pathways for HBD-coupled ET to quinones and their associated equilibrium constants. (b) Extended square scheme accounting for two binding events.
Scheme 2
Scheme 2
(a) Proposed oxidative lactonization mechanism and yields obtained at 24 hours. (b) Kinetic isotope effect experiment.
See this image and copyright information in PMC

References

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