Visible‐Light‐Triggered Quantitative Oxidation of 9,10‐Dihydroanthracene to Anthraquinone by O₂ under Mild Conditions

Abstract

The development of mild and efficient processes for the selective oxygenation of organic compounds by molecular oxygen (O2) is key for the synthesis of oxygenates. This paper discloses an atom‐efficient synthesis protocol for the photo‐oxygenation of 9,10‐dihydroanthracene (DHA) by O2 to anthraquinone (AQ), which could achieve quantitative AQ yield (100 %) without any extra catalysts or additives under ambient temperature and pressure. A yield of 86.4 % AQ was obtained even in an air atmosphere. Furthermore, this protocol showed good compatibility for the photo‐oxidation of several other compounds with similar structures to DHA. From a series of control experiments, free‐radical quenching, and electron paramagnetic resonance spin‐trapping results, the photo‐oxygenation of DHA was probably initiated by its photoexcited state DHA*, and the latter could activate O2 to a superoxide anion radical (O2.−) through the transfer of its electron. Subsequently, this photo‐oxidation was gradually dominated by the oxygenated product AQ as an active photocatalyst obtained from the oxidation of DHA by O2.−, and was accelerated with the rapid accumulation of AQ. The present photo‐oxidation protocol is a good example of selective oxygenation based on the photoexcited substrate self‐activated O2, which complies well with green chemistry ideals.

Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton‐Coupled Electron Transfer

“…In particular, the J sub / J 0 parameter was shown to decrease progressively from xanthene ( J sub / J 0 =3.45±0.5; BDFE=67.9 kcal mol −1 ) to γ‐terpinene ( J sub / J 0 =2.4±0.7; BDFE=67.9–68.4 kcal mol −1 ) and to dihydroanthracene ( J sub / J 0 =1.4±0.6; BDFE=69.8), while flattening to values close to 1 for more challenging substrates, in line with the threshold limit given by the BDFE of 80.5±2.3 kcal mol −1 for the N−H bond in QNC (purple dashed vertical line in Figure 6B). [61] It is worth mentioning that the abstraction of a hydrogen atom bound to carbon is associated with slow kinetics and to significant activation energy when compared to a hydrogen atom bound to heteroatoms, [62,63] in particular when the C−H group is allylic or benzylic: [59] this might explain the low photoelectrochemical response with dihydroanthracene and fluorene, albeit characterized by C−H with BDFE being below the 80.5 Kcal mol −1 threshold. No photoelectrochemical response ( J / J 0 ≈1) was observed in the presence of tert ‐butylbenzene and of dimethylxanthene, electron rich substrates but without reactive C−H benzylic/allylic moieties, supporting that a hydrogen abstraction mechanism is operating in the C−H activation, rather than a single electron oxidation of the substrate. In the case of γ‐terpinene and xanthene substrates, a low photoelectrochemical response ( J / J 0 in the range 1.2–1.4 and 1.0–1.2 for γ‐terpinene and xanthene, respectively) was observed with SnO 2 electrodes sensitized with a QNC−C16 derivative where nitrogen atoms are alkylated with −(CH 2 ) 15 CH 3 pendants, thus proving the key role of the N−H group in QNC to enable the C−H activation (Figures S17, S18). Oxidation of γ‐terpinene and xanthene substrates through a single electron transfer process involving the QNC dye is energetically unfavorable, on the basis of the redox potentials determined by cyclic voltammetry: anodic waves are observed at E >1.1 V and at E >0.95 V vs. Fc + /Fc for γ‐terpinene and xanthene, respectively, while the anodic peak potential of QNC in QNC|SnO 2 electrodes in acetonitrile is 0.75 V vs. Fc + /Fc (Figure S19).…”

“…[59,60] In particular, the J sub /J 0 parameter was shown to decrease progressively from xanthene (J sub / J 0 = 3.45 � 0.5; BDFE = 67.9 kcal mol À 1 ) to γ-terpinene (J sub / J 0 = 2.4 � 0.7; BDFE = 67.9-68.4 kcal mol À 1 ) and to dihydroanthracene (J sub /J 0 = 1.4 � 0.6; BDFE = 69.8), while flattening to values close to 1 for more challenging substrates, in line with the threshold limit given by the BDFE of 80.5 � 2.3 kcal mol À 1 for the NÀ H bond in QNC (purple dashed vertical line in Figure 6B). [61] It is worth mentioning that the abstraction of a hydrogen atom bound to carbon is associated with slow kinetics and to significant activation energy when compared to a hydrogen atom bound to heteroatoms, [62,63] in particular when the CÀ H group is allylic or benzylic: [59] this might explain the low photoelectrochemical response with dihydroanthracene and fluorene, albeit characterized by CÀ H with BDFE being below the 80.5 Kcal mol À 1 threshold. (iii) No photoelectrochemical response (J/J 0 � 1) was observed in the presence of tert-butylbenzene and of dimethylxanthene, electron rich substrates but without reactive CÀ H benzylic/allylic moieties, supporting that a hydrogen abstraction mechanism is operating in the CÀ H activation, rather than a single electron oxidation of the substrate.…”

Cavity-Shape-Dependent Divergent Chemical Reaction inside Aqueous Pd6L4 Cages

“…This reaction is industrially important as it was used for the commercial production of anthraquinone from coal tar. 55a Recently, it had been reported that this oxidation can be performed under mild conditions in organic solvents like acetone; 56 however, a report of the reaction in aqueous medium is not known.…”

Nanocrystals perovskites photocatalyzed singlet oxygen generation for light-driven organic reactions

“…The reaction catalyzed by C4 did not give the same performaces since low yields and low chamoselectivity characterized the outcome. All the compounds reported in Scheme 6 are known in literature [19,20,[59][60][61][62][63][64][65][66].…”