Long-range intramolecular photoredox reaction via coupled charge and proton transfer of triplet excited anthraquinones mediated by water

Abstract

The formal intramolecular photoredox reaction initially discovered for the parent 2-(hydroxymethyl)anthraquinone (1) has been extended to include analogs 3-6 in which the oxidizable benzyl alcohol group is significantly further away from the anthraquinone moiety. All of 3-6 undergo a clean and efficient formal intramolecular photoredox reaction in water catalyzed by acid (Phi = 0.1-0.6), in which the alcohol is oxidized to aldehyde and the anthraquinone is reduced to dihydroxyanthracene. The need for water, observation of acid catalysis, unimolecularity of reaction in anthraquinone, AM1 calculations and LFP studies support a mechanism involving a highly polarized triplet excited state in which the electron density of the distal phenyl moieties is transferred to the central anthraquinone ring, which is subsequently trapped adiabatically by protonation at the anthraquinone carbonyl oxygen and deprotonation at the benzyl C-H.

Time-Resolved Spectroscopic Study on the Photoredox Reaction of 2-(p-Hydroxymethyl)phenylAnthraquinone

“…Hence, it is proposed that species 3 was probed and directly observed in the fs-TA and ns-TA spectra. This assignment is consistent with Wan and coworkers study on a similar AQ compound in that the enol intermediate had a characteristic absorbance at 520 nm, and the decay of this species ultimately leads to a weak broad absorption in the 400 nm region and attributed to the photoredox product 18 .

Figure 10 The ns-TA spectra of PPAQ in pH 2 MeCN-H 2 O (1:1) recorded with 266 nm excitation are shown.

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“…It is noteworthy that AQ compounds with a benzyl alcohol moiety much further away from the AQ ketone groups like 2-( p -hydroxymethyl)phenylanthraquinone (PPAQ) can also undergo a photoredox reaction in aqueous solutions with a fairly high quantum yield ( Φ ~ 0.5, pH 1 MeCN-H 2 O) but not as high as was found for HEAQ under similar conditions 18 . Final product analysis results clearly demonstrated that the insertion of an additional phenyl group has modulated the reaction efficiency.…”

“…Second, the intramolecular photoredox reaction in aqueous solutions for PPAQ was first proposed to take place through a concerted route (see Fig. 2 ) of an initial excited state electron migration from the aromatic ring of the benzyl alcohol to the AQ coupled by trapping with solvent protons 18 . This concerted reaction mechanism is different from the two-step route found in our previous time-resolved spectroscopic studies on HEAQ 15 .…”

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Figure 2 A proposed concerted reaction mechanism of the photoredox reaction of PPAQ in acid aqueous solutions based on that of ref. 18 .

Figure 3 A proposed two-step reaction mechanism of the photoredox reaction of PPAQ in acidic aqueous solutions based on the results of a previously reported mechanism study on HEAQ is shown.

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“… A proposed concerted reaction mechanism of the photoredox reaction of PPAQ in acid aqueous solutions based on that of ref. 18 . …”