The Singlet Oxygen Oxidation of Chlorpromazine and Some Phenothiazine Derivatives. Products and Reaction Mechanisms

Crucial Roles of Leaving Group and Open-Shell Cation in Photoreaction of (Coumarin-4-yl)methyl Derivatives

Photoreactions of (coumarin-4-yl)methyl derivatives have been extensively studied in many fields of chemistry, including organic synthesis and photoinduced drug delivery systems. The identification of the reaction intermediates involved in the photoreactions is crucial not only for elucidating the reaction mechanism but also for the application of the photoreactions. In this study, the photoreactions of 7-diethylamino(coumarin-4-yl)methyl thioester 1a [-SC(O)CH3], thionoester 1b [-OC(S)CH3], and ester 1c [-OC(O)CH3] were investigated to clarify the intermediary species and their chemical behavior. While a radical pair [i.e., 7-diethylamino(coumarin-4-yl)methyl radical and CH3C(O)S•] plays an important role in the photoreactions of 1a and 1b, an ion pair [i.e., 7-diethylamino(coumarin-4-yl)methyl cation, and CH3CO2-] was the key in the photoreaction of 1c. 18O-isotope-labeling of 1c revealed a negligible recombination process within the ion pair. The unprecedented observation was rationalized by the open-shell character of the 7-diethylamino(coumarin-4-yl)methyl cation, whose formation was confirmed through product analysis and transient absorption spectroscopy.

Reaction-Environment-Dependent Photoaddition Reactions of N-Phenyl Amino Acid Esters Possessing a Silyl Group with Fullerene C60: Selective Formation of Aminomethyl-1,2-dihydrofullerenes vs Fulleropyrrolidines

The current study investigates SET-promoted photoaddition reactions of the silyl-group-containing N-phenylglycinates and N-phenylalaninates, N-((trimethylsilyl)methyl)-N-phenyl-substituted glycinates and alaninates, respectively, with fullerene C60 to explore how the types of amino acid esters (AAEs) and molecular oxygen affect the photoaddition reaction efficiencies and chemoselectivity of in situ formed radical cations of AAEs. The results showed that under deoxygenated (N2-purged) conditions, photoreactions of N-phenylglycinates with C60 produced aminomethyl-1,2-dihydrofullerenes through the addition of α-amino radicals arising by sequential SET and desilylation processes from initially formed secondary anilines to C60. In oxygenated conditions, photoreactions of N-phenylglycinates with C60, albeit less efficient, took place to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to C60 assisted by in situ formed 1O2. The same types of photoproducts were observed with N-phenylalaninates, though the reactions were less efficient. The use of methylene blue (MB) as a photosensitizer in the photoreactions under oxygenated conditions was especially effective in enhancing the efficiency of fulleropyrrolidine formation. These results demonstrate that photoaddition reactions of silyl-tether-containing N-phenyl AAEs with C60 can be governed by the reaction conditions and the presence or absence of a photosensitizer employed.

Effects of the Chalcogenide Identity in N‐Aryl Phenochalcogenazine Photoredox Catalysts

Phenochalcogenazines such as phenoxazines and phenothiazines have been widely employed as photoredox catalysts (PCs) in small molecule and polymer synthesis. However, the effect of the chalcogenide in these catalysts has not been fully investigated. In this work, a series of four phenochalcogenazines is synthesized to understand how the chalcogenide impacts catalyst properties and performance. Increasing the size of the chalcogenide is found to distort the PC structure, ultimately impacting the properties of each PC. For example, larger chalcogenides destabilize the PC radical cation, possibly resulting in catalyst degradation. In addition, PCs with larger chalcogenides experience increased reorganization during electron transfer, leading to slower electron transfer. Ultimately, catalyst performance is evaluated in organocatalyzed atom transfer radical polymerization and a photooxidation reaction for C(sp²)−N coupling. Results from these experiments highlight that a balance of PC properties is most beneficial for catalysis, including a long‐lived excited state, a stable radical cation, and a low reorganization energy.

Photoreaction of 4-(Bromomethyl)-7-(diethylamino)coumarin: Generation of a Radical and Cation Triplet Diradical during the C-Br Bond Cleavage

7-Diethylamino-4-methyl coumarin (DEACM) derivatives are widely used as photolabile protecting groups. In this study, the photolysis of DEACM-Br with Br as the leaving group was investigated. The main reaction path was found to be the generation of radical D via homolytic C-Br bond cleavage. Interestingly, products derived from C-T, the triplet state of the carbocation intermediate (i.e., 7-(diethylamino)-4-methyl cation (C)), were identified, thereby confirming the existence of C-T for the first time.

Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60 with α-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C₆₀ with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C₆₀ with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C₆₀. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of ³C₆₀* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C₆₀. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C₆₀ with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

Microscope laser assisted photooxidative activation of bioorthogonal ClickOx probes

A photoactivatable fluorogenic tetrazine-rhodaphenothiazine probe was synthesized and studied in light-assisted, bioorthogonal labeling schemes. Experimental results revealed that the bioorthogonally conjugated probe efficiently sensitizes ¹O₂ generation upon illumination with green or orange light and undergoes self-oxidation leading to an intensely fluorescent sulfoxide product. An added value of the present probe is that it is also suitable for STED super-resolution microscopy using a 660 nm depletion laser.

Photooxygenation of 2-propargylfurans: a path to structurally diverse nitrogen-containing 5-membered rings

The photooxygenation of 2-propargylfurans enabled access to original nitrogen-containing cyclopentenones and related compounds in a one-pot fashion. By employing readily-available substrates such as furans and amines, we succeeded in achieving a high degree of molecular complexity. Relying on the introduction of an alkyne moiety and tailored substrates, this transformation reveals a new facet for reaction sequences featuring the photooxygenation of furans.

Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism

As an important class of soil minerals and a key constituent of colloidal particles in surface aquifers, smectite clays can strongly retain tetracyclines due to their large surface areas and high cation exchange capacities. However, the research on phototransformation of tetracyclines at smectite clay surfaces is rarely studied. Here, the phototransformation kinetics of tetracycline preadsorbed on two model smectite clays (hectorite and montmorillonite) exchanged with Na⁺, K⁺, or Ca²⁺ suspended in aqueous solution under simulated sunlight was compared with that of tetracycline dissolved in water using batch experiments. Adsorption on clays accelerated tetracycline phototransformation (half-lives shortened by 1.1-5.3 times), with the most significant effects observed for Na⁺-exchanged clays. Regardless of the presence or absence of clay, the phototransformation of tetracycline was facilitated by increasing pH from 4 to 7. Inhibition or enhancement of photolysis-induced reactive species combined with their measurement using scavenger/probe chemicals indicate that the facilitated production of self-photosensitized singlet oxygen (¹O₂) was the key factor contributing to the clay-enhanced phototransformation of tetracycline. As evidenced by the red shifts and the increased molar absorptivity in the UV-vis absorption spectra, the complexation of tetracycline with the negatively charged (Lewis base) sites on clay siloxane surfaces led to formation of the alkalized form, which has larger light absorption rate and is more readily to be oxidized compared to tetracycline in aqueous solution at equivalent pH. Our findings indicate a previously unrecognized, important phototransformation mechanism of tetracyclines catalyzed by smectite clays.

Photooxidation mechanism of levomepromazine in different solvents

Unwanted photoinduced responses are well-known adverse effects of most promazine drugs, including levomepromazine (LPZ, Levoprome(®) or Nozinan(®)). This drug is indicated in psychiatry primarily for the treatment of schizophrenia and other schizoaffective disorders. Levomepromazine's particular sedative properties make it especially fit for use in psychiatric intensive care. Nevertheless, it is photolabile under UV-A and UV-B light in aerobic conditions resulting in the formation of its sulfoxide. The LPZ photochemistry in acetonitrile (MeCN) is completely different from that in methanol (MeOH) and phosphate buffer solutions (PBS, pH = 7.4). The major photoproduct in PBS and MeOH under aerobic conditions is levomepromazine sulfoxide (LPZSO), although the amount is considerably higher in the aqueous environment. The corresponding main photoproduct in MeCN could not be characterized. The destruction quantum yields of LPZ in PBS, MeOH and MeCN are 0.13, 0.02 and <10(-3), respectively. It is further demonstrated that LPZSO does not form by the reaction of singlet oxygen with ground-state LPZ. This oxidation product is actually produced by the reaction of the cation radical of LPZ (LPZ·(+)) with molecular oxygen. This cation radical in turn, is produced by an electron transfer process between the (3) LPZ* and ground-state molecular oxygen.