Photodecomposition of Thienylsulfonyl Azides: Generation and Spectroscopic Characterization of Triplet Thienylsulfonyl Nitrenes and 3-Thienylnitrene

Sulfonyl Nitrene and Amidyl Radical: Structure and Reactivity

Photocatalytic generation of nitrenes and radicals can be used to tune or even control their reactivity. Photocatalytic activation of sulfonyl azides leads to the elimination of N₂ and the resulting reactive species initiate C−H activations and amide formation reactions. Here, we present reactive radicals that are generated from sulfonyl azides: sulfonyl nitrene radical anion, sulfonyl nitrene and sulfonyl amidyl radical, and test their gas phase reactivity in C−H activation reactions. The sulfonyl nitrene radical anion is the least reactive and its reactivity is governed by the proton coupled electron transfer mechanism. In contrast, sulfonyl nitrene and sulfonyl amidyl radicals react via hydrogen atom transfer pathways. These reactivities and detailed characterization of the radicals with vibrational spectroscopy and with DFT calculations provide information necessary for taking control over the reactivity of these intermediates.

Switching on prodrugs using radiotherapy

Chemotherapy is a powerful tool in the armoury against cancer, but it is fraught with problems due to its global systemic toxicity. Here we report the proof of concept of a chemistry-based strategy, whereby gamma/X-ray irradiation mediates the activation of a cancer prodrug, thereby enabling simultaneous chemo-radiotherapy with radiotherapy locally activating a prodrug. In an initial demonstration, we show the activation of a fluorescent probe using this approach. Expanding on this, we show how sulfonyl azide- and phenyl azide-caged prodrugs of pazopanib and doxorubicin can be liberated using clinically relevant doses of ionizing radiation. This strategy is different to conventional chemo-radiotherapy radiation, where chemo-sensitization of the cancer takes place so that subsequent radiotherapy is more effective. This approach could enable site-directed chemotherapy, rather than systemic chemotherapy, with 'real time' drug decaging at the tumour site. As such, it opens up a new era in targeted and directed chemotherapy.

3-Nitrene-2-formylthiophene and 3-Nitrene-2-formylfuran: Matrix Isolation, Conformation, and Rearrangement Reactions

Two new heteroarylnitrenes, 3-nitrene-2-formylthiophene (15/15') and 3-nitrene-2-formylfuran (16/16'), in the triplet ground state have been generated in solid Ar (10.0 K) and N₂ (15.0 K) matrices by the 266 nm laser photolysis of 3-azido-2-formylthiophene (13) and 3-azido-2-formylfuran (14), respectively. According to the characterization with matrix-isolation IR spectroscopy and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level, both nitrenes exhibit two conformations depending on the orientation of the formyl groups. Upon subsequent green-light irradiation (532 nm), both the nitrenes 15/15' and 16/16' undergo ring closure to form 3,2-thienoisoxazole (17) and 3,2-furoisoxazole (18), respectively. Traces of 3-imino-4,5-dihydrothiophene-2-ketene (19), formally formed through the intramolecular 1,4-H shift in the corresponding nitrenes 15/15', have been also identified among the laser photolysis products of the azide 13. In sharp contrast to the photochemistry, the high-vacuum flash pyrolysis (HVFP) of the azide 13 at ca. 1000 K mainly yields imino ketene in two conformations 19/19' together with traces of isoxazole 17. In addition to the reversible conformational interconversion in the imino ketene 19 ↔ 19', the photoisomerization from isoxazole 17 to imino ketene 19 has also been observed. The HVFP of the azide 14 at ca. 1000 K results in complete dissociation to HCN, C₂H₂, CO, CO₂, H₂O, and N₂. Unlike the recently disclosed hydrogen-atom tunneling (HAT) in the transformation from the structurally related 2-formyl phenylnitrene (2) to imino ketene 3 in a cryogenic Ar-matrix, the absence of HAT in nitrenes 15 and 16 can be reasonably explained by the higher barrier heights and also larger barrier widths in the isomerization reactions.

Insights into the Photodecomposition of Azidomethyl Methyl Sulfide: A S2/S1 Conical Intersection on Nitrene Potential Energy Surfaces Leading to the Formation of S-Methyl-N-sulfenylmethanimine

UV photodecomposition of azidomethyl methyl sulfide (AMMS) yields a transient S-methylthiaziridine which rapidly evolves to S-methyl-N-sulfenylmethanimine at 10 K. This species was detected by infrared matrix isolation spectroscopy. The mechanism of the photoreaction of AMMS has been investigated by a combined approach, using low-temperature matrix isolation FTIR spectroscopy in conjunction with two theoretical methods, namely, complete active space self-consistent field and multiconfigurational second-order perturbation. The key step of the reaction is governed by a S₂/S₁ conical intersection localized in the neighborhood of the singlet nitrene minimum which is formed in the first reaction step of the photolysis, that is, N₂ elimination from AMMS. Full assignment of the observed infrared spectra of AMMS has been carried out based on comparison with density functional theory and second-order perturbation Møller-Plesset methods.

Interactions in flavanone and chalcone derivatives: Hirshfeld surface analysis, energy frameworks and global reactivity descriptors

The present study examines a series of flavanone and chalcone derivatives substituted with electron-withdrawing groups (Cl or Br) and electron-donating groups (OH, CH₃ and OCH₃), namely, 7-methoxy-2-phenyl-3,4-dihydro-2H-1-benzopyran-4-one, C₁₆H₁₄O₃, 2-(4-methoxyphenyl)-3,4-dihydro-2H-1-benzopyran-4-one, C₁₆H₁₄O₃, 2-(4-methoxyphenyl)-6-methyl-3,4-dihydro-2H-1-benzopyran-4-one, C₁₇H₁₆O₃, 2-(4-chlorophenyl)-3,4-dihydro-2H-1-benzopyran-4-one, C₁₅H₁₁ClO₂, 8-bromo-6-methyl-2-phenyl-3,4-dihydro-2H-1-benzopyran-4-one, C₁₆H₁₃BrO₂, (2E)-1-(2-hydroxyphenyl)-3-(4-methoxyphenyl)prop-2-en-1-one, C₁₆H₁₄O₃, and (2E)-1-(2-hydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one, C₁₅H₁₂O₃. It compares the two groups of derivatives with regard to their intermolecular interactions in the crystal lattice and lattice energy calculations, together with energy framework visualization and global reactivity descriptors (chemical hardness, chemical potential and electrophilicity index). It also discusses the relationships between different noncovalent interactions derived from Hirshfeld surface analysis, crystal lattice energy and global reactivity descriptors of the compounds.