Formal intramolecular photoredox chemistry of anthraquinones in aqueous solution: photodeprotection for alcohols, aldehydes and ketones

Abstract

The formal intramolecular photoredox reaction initially discovered for the parent 2-(hydroxymethyl)anthraquinone (1) in aqueous solution has been extended to a variety of anthraquinones derivatives 6-13, to explore the generality of the reaction, and to investigate its potential utility as a photodeprotecting chromophore. In addition, the related diketone 14 was studied to investigate the need for the anthraquinone chromophore in these formal intramolecular reactions. All the anthraquinones studied (except for 9) undergo formal unimolecular photoredox reaction with a range of quantum yields (Phi = 0.02-0.7). Anthraquinones 7, 8, 10 and 11 photoreleased the corresponding alcohol, aldehyde, or ketone with good yields (80-90%), making it potentially useful for photocaging in aqueous solution. Diketone 14 undergoes an analogous photoredox reaction but only in acid (Phi = 0.003, pH < 1), to give the formal redox product diphenylisobenzofuran 32 thereby demonstrating that other aromatic diketones can react in an analogous fashion. The ionic photochemistry exhibited by these aromatic ketones is fully compatible with the recent discovery of the surprising acid-catalyzed photochemical hydration of benzophenone reported by Jacob Wirz and coworkers (M. Ramseier, P. Senn and J. Wirz, J. Phys. Chem. A, 2003, 107, 3305-3315).

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

“…20%) were observed in D 2 O compared to H 2 O. 4 All these results are consistent with the required proton transfers in the proposed mechanism.…”

“…A new type of photoreactivity was discovered by our group for anthraquinones in water, initially for the simple derivative, 2-(hydroxymethyl)anthraquinone (1), which resulted in an efficient (U ~0.8, pH 7) and clean formal intramolecular photoredox reaction, to give 2-formyl-9,10-dihydroxyanthracene (2) (eqn (2)). 3,4 In general, photogenerated dihydroxyanthracenes are unstable to oxygen and are subsequently readily oxidized to the corresponding anthraquinones. The photoredox reaction was shown to be mediated by water (no reaction was observed in aprotic solvents) and involved a unimolecular mechanism in the anthraquinone moiety.…”

“…Trapping of the carbonyl oxygen by proton from water leads to subsequent overall redox chemistry. 3,4 (1)…”

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

“…An unusual photoredox reaction was discovered for AQ compounds by Wan and coworkers 14 . We recently examined 2-(1-hydroxyethyl) 9,10-anthraquinone (HEAQ) to study its photoredox reaction mechanism in an acidic aqueous solution using time-resolved spectroscopic experiments and density functional theory (DFT) calculations 15 .…”

To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives

“…Organic photoredox reactions are one of the most rapidly expanding areas of radical chemistry in synthesis. − An efficient self-photoredox reaction, where the carbonyl is reduced to its alcohol while the alcohol at the side chain is oxidized to its carbonyl, was observed for benzophenones (BPs) and anthraquinones (AQs) in aqueous solutions. − Both 2-(1-hydroxyethyl)-anthraquinone (HEAQ) and 3-(hydroxymethyl)-benzophenone (3-BPOH) undergo efficient photoredox under moderately acidic aqueous conditions, which is proposed to take place via the carbonyl protonation process and then a subsequent deprotonation of the xylylene C–H bond. ,− Nevertheless, the mechanism has remained uncertain under nonacidic conditions, which needs to be addressed under neutral aqueous solutions. First, the origin for the experimental observations (Scheme ) that HEAQ can efficiently undergo the photoredox reaction while the related 3-BPOH and 4-BPOH compounds mainly generate the arylphenyl ketyl (ArPK) radical and do not exhibit an obvious photoredox , has not yet been clarified.…”