Thioxanthone

Abstract: Thermally and photochemically initiated thiol-ene click reactions using thiol- and allyl- end functionalized linear polystyrenes with various enes (allyl bromide, methyl acrylate, and methyl methacrylate) and thiol (3-mercaptopropionic acid) have been investigated. Allyl- and thiol-end-capped polystyrenes with controlled molecular weight and low polydispersity were prepared by atom transfer radical polymerization (ATRP) of styrene using functional initiator and end group modification approaches, respectively. Thiol-ene reactions can be initiated by both cleavage type photoinitiators such as (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and H-abstraction type photoinitiators such as benzophenone (BP), thioxanthone (TX), camphorquinone (CO), and classical thermal initiator, 2,2'-azobis(isobutyronitrile) (AIBN) at 80 °C. The kinetics of the reactions was monitored online with a real time ATR-FTIR monitoring system and the conversions were determined by 1 H NMR spectroscopy. A comparison of click efficiencies of the studied initiator systems was performed. Compare to the thermal initiators and H-abstraction type photoinitiators, cleavage type photoinitiators were found to induce thiol-ene click reactions with higher efficiency.

Abstract: In the presence of a chiral thioxanthone catalyst (10 mol %) the title compounds underwent a clean intermolecular [2 + 2] photocycloaddition with electron-deficient olefins at λ = 419 nm. The reactions not only proceeded with excellent regio- and diastereoselectivity but also delivered the respective cyclobutane products with significant enantiomeric excess (up to 95% ee). Key to the success of the reactions is a two-point hydrogen bonding between quinolone and catalyst enabling efficient energy transfer and high enantioface differentiation. Preliminary work indicated that solar irradiation can be used for this process and that the substrate scope can be further expanded to isoquinolones.

Abstract: The compounds 2-thioxanthone-thioacetic acid and 2-(carboxymethoxy)thioxanthone, bimolecular photoinitiators for free radical polymerization, are synthesized and characterized. Their capability to act as initiators for the polymerization of methyl methacrylate was examined. The postulated mechanism is based on the intermolecular electron-transfer reaction of the excited photoinitiator with the sulfur or oxygen atom of the ground state of the respective photoinitiator followed by decarboxylation. The resulting alkyl radicals initiate the polymerization. Structures of the photoinitiators.