Photooxygenation of oxygen-substituted naphthalenes

Aromatic Endoperoxides

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides, and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices. Improvement of the existing methods and development of new methods for the synthesis of endoperoxides is necessary as result of the demand to improve existing and to create new applications for these valuable materials. On the other hand, prevention of endoperoxide formation is crucial to inhibit irreversible oxidative degradation of aromatic hydrocarbons and to extend their lifetimes as useful organic semiconductors.

Mechanism for the photodegradation of 9,10-dibutoxyanthracene in the presence of air

The photoreactivity of anthracene has been previously verified for a range of its derivatives. 9,10-Dibutoxyanthracene is commonly used as an electron transfer sensitizer for photopolymerization because of its favorable optical properties. This study experimentally demonstrated that 9,10-dibutoxyanthracene produces an endoperoxide species upon reaction with the oxygen present in air. A secondary decomposition product formed during the photodecomposition of the endoperoxide species was also isolated and identified. The proposed reaction pathway is supported by singlet oxygen scavenger studies and calculations of the singlet–triplet transition energies. Our findings suggest that 9,10-dibutoxyanthracene can be used as a photo-induced oxygen scavenger.

Chemically Triggered Release of Singlet Oxygen from Bisphenalenyl Endoperoxides with a Brønsted Acid

Aromatic endoperoxides have emerged as intriguing stimulus-responsive materials for molecular oxygen (O₂) storage and delivery but are currently limited in their application because they require heat to trigger O₂ release. Here we present the first example of acid-triggered singlet oxygen (¹O₂) release that does not require external heating by treating bisphenalenyl endoperoxides (EPOs) with trifluoroacetic acid. Mechanistic studies reveal that diprotonation of EPOs leads to a >10-fold increase in cycloreversion rates by lowering the energy of activation (ΔE_a) by as much as 71.1 kJ mol^-1. Remarkably, acid-catalyzed ¹O₂ release is even demonstrated at room temperature. Chemical trapping experiments indicate that reactive ¹O₂ is present during acid-triggered release, which is promising for the development of these molecular materials for metal-free, on-demand ¹O₂ delivery.

Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.

Release of Singlet Oxygen from Aromatic Endoperoxides by Chemical Triggers

The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus.