TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Theoretical insights into the effect of solvents on the [4+2] cycloaddition of singlet oxygen to substituted anthracenes BT - A change from a stepwise process to a concerted process JF - Journal of physical organic chemistry N2 - The [4 + 2] cycloadditions of singlet oxygen to 9,10-diphenylanthracene (1) and the meta and para isomers of 9,10-dipyridylanthracene (2m/p) and 9,10-methoxyphenylanthracene (3m/p) have been studied by density functional calculations in the gas phase at the UB3LYP/6-31G* level and for the first time in solvents at the conductor-like polarizable continuum model (CPCM) UM062X/6-31G* level. The differences in calculated transition state (TS) energies derived from this method are in line with experimentally observed reactivity orders in solution. For the gas-phase reaction, the first TS of the stepwise pathway (TS1) has biradical character, and its energy lies below the energy of the TS of the concerted path (TSconc). In contrast, in the solvent acetonitrile, TS1 resembles a zwitterion and lies significantly higher than the TSconc. Thus, a concerted mechanism applies in solvents, and the energy gap between the TS of the two processes decreases with decreasing polarity. A change from a pyridyl against a methoxyphenyl substituent in the para position causes a maximal reduction of the activation barrier by approximately 1.7 kcal/mol, resulting in a fivefold increased reactivity. KW - anthracenes KW - DFT calculations KW - endoperoxides KW - photooxygenation KW - singlet oxygen KW - solvent effect Y1 - 2019 U6 - https://doi.org/10.1002/poc.3951 SN - 0894-3230 SN - 1099-1395 VL - 32 IS - 7 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Bauch, Marcel A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates JF - Molecules : a journal of synthetic chemistry and natural product chemistry N2 - 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. KW - singlet oxygen KW - photooxygenation KW - naphthalenes KW - carbohydrates KW - stereoselectivity KW - auxiliary control KW - [4+2] cycloaddition Y1 - 2021 U6 - https://doi.org/10.3390/molecules26040804 SN - 1420-3049 VL - 16 IS - 4 PB - MDPI CY - Basel ER - TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Reversible Photooxygenation of Alkynylanthracenes chemical generation of singlet oxygen under very mild conditions JF - Chemistry - a European journal KW - anthracenes KW - oxidation KW - peroxides KW - reversibility KW - singlet oxygen Y1 - 2011 U6 - https://doi.org/10.1002/chem.201102230 SN - 0947-6539 VL - 17 IS - 49 SP - 13661 EP - 13664 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Release of Singlet Oxygen from Aromatic Endoperoxides by Chemical Triggers JF - Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition N2 - 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. KW - anthracenes KW - donor-acceptor systems KW - mild reaction conditions KW - selective oxidations KW - singlet oxygen Y1 - 2018 U6 - https://doi.org/10.1002/anie.201806881 SN - 1433-7851 SN - 1521-3773 VL - 57 IS - 39 SP - 12971 EP - 12975 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Bauch, Marcel A1 - Krtitschka, Angela A1 - Linker, Torsten T1 - Photooxygenation of oxygen-substituted naphthalenes JF - Journal of physical organic chemistry N2 - The reaction of oxygen-substituted naphthalenes with singlet oxygen (O-1(2)) has been investigated, and labile endoperoxides have been isolated and characterized at -78 degrees C for the first time. Low-temperature kinetics by UV spectroscopy revealed that alkoxy and silyloxy substituents remarkably increase the rate of photooxygenations compared to 1,4-dimethylnaphthalene, whereas acyloxy-substituted acenes are inert towards O-1(2). The reactivities nicely correlate with HOMO energies and free activation energies, which we determined by density functional theory calculations. The lability of the isolated endoperoxides is due to their very fast back reaction to the corresponding naphthalenes even at -20 degrees C under release of O-1(2), making them to superior sources of this reactive species under very mild conditions. Finally, a carbohydrate-substituted naphthalene has been synthesized, which reacts reversibly with O-1(2) and might be applied for enantioselective oxidations in future work. KW - kinetics KW - labile peroxides KW - low-temperature experiments KW - naphthalenes KW - singlet oxygen Y1 - 2017 U6 - https://doi.org/10.1002/poc.3734 SN - 0894-3230 SN - 1099-1395 VL - 30 SP - 6803 EP - 6813 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Photooxygenation of naphthalene JF - ChemPhotoChem N2 - The photooxygenation of naphthalene to the corresponding endoperoxide (EPO) under various conditions is described. Substantial conversion is only observed at -10 degrees C and after more than two days, indicating that the [4+2] cycloaddition of singlet oxygen to this acene proceeds much more slowly than corresponding reactions of substituted naphthalenes, a rate constant of k = 5.4 +/- 0.3 M(-1)s(-1) was determined by competition kinetics. Another problem is the thermal lability and photochemical cleavage of the naphthalene EPO. We investigated the mechanism of this radical process depending on the light source and sensitizer in comparison to known cyclohexadiene EPO. Thus, bisepoxides and keto epoxides are formed after homolysis of the O-O bond by irradiation with sodium lamps or blue LEDs and subsequent cyclization. This process is accelerated by the sensitizers methylene blue and 9,10-dicyanoanthracene, indicating an electron transfer mechanism. Finally, the cleavage of the peroxidic bond is inhibited with red LEDs, and photooxygenation under such conditions affords 20 % EPO. Thus, we could demonstrate that contrary to literature statements singlet oxygen does indeed react with naphthalene. KW - naphthalene KW - photooxygenation KW - reaction mechanisms KW - reactive KW - intermediates KW - singlet oxygen Y1 - 2021 U6 - https://doi.org/10.1002/cptc.202100097 VL - 5 IS - 11 SP - 1004 EP - 1008 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Klaper, Matthias A1 - Linker, Torsten T1 - New Singlet Oxygen Donors Based on Naphthalenes: Synthesis, Physical Chemical Data, and Improved Stability JF - Chemistry - a European journal N2 - Singlet oxygen donors are of current interest for medical applications, but suffer from a short half-life leading to low singlet oxygen yields and problems with storage. We have synthesized more than 25new singlet oxygen donors based on differently substituted naphthalenes in only a few steps. The influence of functional groups on the reaction rate of the photooxygenations, thermolysis, half-life, and singlet oxygen yield has been thoroughly studied. We determined various thermodynamic data and compared them with density functional calculations. Interestingly, remarkable stabilities of functional groups during the photooxygenations and stabilizing effects for some endoperoxides during the thermolysis have been found. Furthermore, we give evidence for a partly concerted and partly stepwise thermolysis mechanism leading to singlet and triplet oxygen, respectively. Our results might be interesting for dark oxygenations and future applications in medicine. KW - density functional calculations KW - oxygenation KW - peroxides KW - photodynamic therapy KW - singlet oxygen Y1 - 2015 U6 - https://doi.org/10.1002/chem.201500146 SN - 0947-6539 SN - 1521-3765 VL - 21 IS - 23 SP - 8569 EP - 8577 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Fudickar, Werner A1 - Linker, Torsten T1 - Imaging by sensitized oxygenations of photochromic anthracene films BT - Examination of effects that improve performance and reversibility JF - Chemistry - a European journal N2 - The aliphatic anthracene compound 1 and the oligomeric anthracene 2 were synthesized. Thin films of 1 and 2 mixed with the sensitizers tetraphenylporphyrin (TPP) and methylene blue (MB) were irradiated with visible light in air. Upon formation of singlet oxygen, the anthracene units were converted quantitatively to the corresponding endoperoxides. Heating of the irradiated samples afforded the parent anthracenes with high yields. Here, we demonstrate that the kinetics and reversibility of this reaction strongly depend on the microenvironment of the anthracene groups in the two compounds. The photooxidation of thin films of I is accompanied by interesting changes in the morphology of the film and allows the first application of 1 as a nondestructive negative-tone photo-resist for lithography and as an oxidizing ink. The morphology of 2 remained unchanged after photooxidation as a result of the stabilizing oligomer backbone. This stabilizing effect significantly improves the photochromic performance of 2. The reversibility of the photooxidation is very high (> 90%) for oligomeric films of 2 after several cycles of irradiation and beating. Decomposition of the anthracene and a loss of the activity of the sensitizer diminish slightly the performance of the monomeric species. KW - anthracenes KW - lithography KW - photochromism KW - singlet oxygen KW - thin films Y1 - 2006 U6 - https://doi.org/10.1002/chem.200600387 SN - 0947-6539 VL - 12 SP - 9276 EP - 9283 PB - WILEY‐VCH CY - Weinheim ER - TY - JOUR A1 - Klaper, Matthias A1 - Linker, Torsten T1 - Evidence for an oxygen anthracene sandwich complex JF - Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition KW - arenes KW - peroxides KW - sandwich complexes KW - singlet oxygen KW - interactions Y1 - 2013 U6 - https://doi.org/10.1002/anie.201304768 SN - 1433-7851 SN - 1521-3773 VL - 52 IS - 45 SP - 11896 EP - 11899 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Fudickar, Werner A1 - Bauch, Marcel A1 - Ihmels, Heiko A1 - Linker, Torsten T1 - DNA-triggered enhancement of singlet oxygen production by pyridinium alkynylanthracenes JF - Chemistry - a European journal N2 - There is an ongoing interest in O-1(2) sensitizers, whose activity is selectively controlled by their interaction with DNA. To this end, we synthesized three isomeric pyridinium alkynylanthracenes 2 o-p and a water-soluble trapping reagent for O-1(2). In water and in the absence of DNA, these dyes show a poor efficiency to sensitize the photooxygenation of the trapping reagent as they decompose due to electron transfer processes. In contrast, in the presence of DNA O-1(2) is generated from the excited DNA-bound ligand. The interactions of 2 o-p with DNA were investigated by thermal DNA melting studies, UV/vis and fluorescence spectroscopy, and linear and circular dichroism spectroscopy. Our studies revealed an intercalative binding with an orientation of the long pyridyl-alkynyl axis parallel to the main axis of the DNA base pairs. In the presence of poly(dA : dT), all three isomers show an enhanced formation of singlet oxygen, as indicated by the reaction of the latter with the trapping reagent. With green light irradiation of isomer 2 o in poly(dA : dT), the conversion rate of the trapping reagent is enhanced by a factor >10. The formation of O-1(2) was confirmed by control experiments under anaerobic conditions, in deuterated solvents, or by addition of O-1(2) quenchers. When bound to poly(dG : dC), the opposite effect was observed only for isomers 2 o and 2 m, namely the trapping reagent reacted significantly slower. Overall, we showed that pyridinium alkynylanthracenes are very useful intercalators, that exhibit an enhanced photochemical O-1(2) generation in the DNA-bound state. KW - Anthracene KW - DNA KW - intercalations KW - photochemistry KW - singlet oxygen Y1 - 2021 U6 - https://doi.org/10.1002/chem.202101918 SN - 1521-3765 VL - 27 IS - 54 SP - 13591 EP - 13604 PB - Wiley-VCH CY - Weinheim ER -