Evaluating brominated thioxanthones as organo‐photocatalysts

Abstract

Bromination of the widely used triplet sensitizer thioxanthone extends the absorption spectrum into the visible range with only minor loss of lowest triplet state energy (3 kcal/mol for di-bromination). Because of bromine substitution, a slight increase in triplet quantum yield was observed. The di-brominated derivative was effective as organo-photocatalyst in performing [2 + 2] cross-photocycloaddition of acrylimide-based compounds under visible light irradiation. | INTRODUCTIONThioxanthone 1 is a popular triplet sensitizer in photoinitiated reactions because of its high molar absorptivity in the near-UV spectral region (300-400 nm), high triplet quantum yield and relatively high triplet energy. [1,2] Thioxanthone and its derivatives are frequently used in organo-photocatalytic reactions [3,4] and as initiator in photopolymerization formulations. [5] Although 1 absorbs strongly in the near-UV spectral region (λ max = 381 nm), its absorption shows only a minor tailing in the visible region above 400 nm, which makes it only poorly suited as a sensitizer/catalyst for visible light illumination. We envisioned a simple structural modification of the thioxanthone scaffold that would result in a bathochromic shift without altering the triplet energy significantly. [6] A wide variety of thioxanthone derivatives have been reported where substituents not only shift the absorption bathochromically but also reduce the triplet energy significantly. [7][8][9][10][11] This prompted us to investigate the effect of brominating thioxanthone on its photochemical and photophysical properties, as we expected the heavy atom to facilitate a more-efficient intersystem crossing. [12][13][14][15][16][17] Unlike other substitutions (eg, amino substitution) that alters the triplet excited-state characteristics of thioxanthone, we believed that substituting bromine will be advantageous because of its dual nature of both electron donation (by resonance) and electron withdrawal (by induction) that will minimally alter the triplet state energy, while providing us an avenue to excite the molecule effectively with visible light. | RESULTS AND DISCUSSIONBrominated thioxanthones were synthesized by electrophilic bromination of parent thioxanthone 1. We found that the use of bromine in glacial acetic acid with catalytic amounts of iodine (Scheme 1) resulted in a mixture of mono-brominated thioxanthone 2 and di-brominated thioxanthone 3. The brominated derivatives were chromatographically separated and characterized by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry. The regioselectivity of bromination was ascertained by single crystal X-ray powder diffraction (XRD) analysis.[18] † This article is published as part of a special issue to celebrate the 80 th birthday of Professor Waldemar Adam

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

“…Product 6b thus obtained gave crystals that allowed for the determination of its absolute configuration by X-ray diffraction ( Figure 2). [17] Asecond observation that supported the hypothesis of an enantiodifferentiation after initial cyclopropane formation relates to the development of the enantioselectivity over time. [8][9][10] Consequently,C ÀCb ond formation was expected to lead to the opposite enantiomer ent-6a but not to 6a.T wo key observations confirmed the notion that the enantioselectivity was not determined during the initial cyclopropane formation.…”

“…The latter catalyst has been reported to have at riplet energy of E T = 272 kJ mol À1 . [17] Asecond observation that supported the hypothesis of an enantiodifferentiation after initial cyclopropane formation relates to the development of the enantioselectivity over time. Upon irradiation of substrate 5a (l = 420 nm) in the presence of catalytic quantities (10 mol %) of thioxanthone 4 ( Table 1, entry 5), the enantioselectivity increased from very low values to the recorded value (55 % ee)w ithin 1h and remained constant thereafter ( Figure 3A).…”

A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light‐Mediated Cycloadditions

“…Luminescencem easurements in dichloromethane ( Figure 3) revealed thatt he triplet energy (E T )o fc ompound 4 is surprisingly low.F rom the phosphorescence emission (77 K, dichloromethane) at the long wavelength shoulder the energy was calculated as E T = 235 AE 2kJmol À1 which is much lower than the reported triplet energy of thioxanthen-9-one (E T = 272 kJ mol À1 ). [18] Luminescence spectra of the two carboxylic acids showedt he typical signature of a,b-unsaturated enones [19] with ar eadily detectable 0-0 transition. The triplet energy calculated from this transition was E T = 288 AE 2kJmol À1 for compound 9 and E T = 287 AE 2kJmol À1 for compound 12 (77 K, pentane/isopentane).…”

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

“…The enantiomerically enriched material obtained by chiral HPLC purification (99 % ee ) underwent rapid racemization (Scheme ) upon irradiation at λ =420 nm in the presence of achiral 9‐thioxanthenone ( 7 ). The latter catalyst has been reported to have a triplet energy of E T =272 kJ mol −1 …”