7,8‐Dimethoxy‐3‐methyl‐1,10‐ethylenealloxazinium chloride (1a) was found to be a superior photooxidation catalyst among substituted ethylene‐bridged flavinium salts (R=7,8‐diMeO, 7,8‐OCH2O‐, 7,8‐diMe, H, 7,8‐diCl, 7‐CF3 and 8‐CF3). Selection was carried out based on structure vs catalytic activity and properties relationship investigations. Flavinium salt 1a proved to be robust enough for practical applications in benzylic oxidations/oxygenations, which was demonstrated using a series of substrates with high oxidation potential, i. e., 1‐phenylethanol, ethylbenzene, diphenylmethane and diphenylmethanol derivatives substituted with electron‐withdrawing groups (Cl or CF3). The unique capabilities of 1a can be attributed to its high photostability and participation via a relatively long‐lived singlet excited state, which was confirmed using spectroscopic studies, electrochemical measurements and TD‐DFT calculations. This allows the maximum use of the oxidation power of 1a, which is given by its singlet excited state reduction potential of +2.4 V. 7,8‐Dichloro‐3‐methyl‐1,10‐ethylenealloxazinium chloride (1 h) can be used as an alternative photocatalyst for even more difficult substrates.
Nitrosobenzene has been demonstrated
to participate in the Mitsunobu
reaction in an analogous manner to dialkyl azodicarboxylates. The
protocol using nitrosobenzene and triphenylphosphine (1:1) under mild
conditions (0 °C) provides the ester derivatives of aliphatic
and aromatic acids using various alcohols in moderate yield and with
good enantioselectivity, giving the desired products predominantly
with an inversion of configuration. The proposed mechanism, which
is analogous to that observed using dialkyl azodicarboxylates, involves
a nitrosobenzene–triphenylphosphine adduct and an alkoxytriphenylphosphonium
ion and was supported by density functional theory calculations,
31
P NMR spectroscopy, and experiments conducted with isotopically
labeled substrates.
Nitroalkane oxidases (NAO) are flavoenzymes that catalyse the oxidation of nitroalkanes to their corresponding carbonyl compounds while producing nitrite anion. Herein, we present an artificial catalytic system using flavins or...
Palladium-catalyzed Suzuki reactions of brominated flavin derivatives (5-deazaflavins, alloxazines, and isoalloxazines) with boronic acids or boronic acid esters that occur readily under mild conditions were shown to be an effective tool for the synthesis of a broad range of 7/8-arylflavins. In general, the introduction of an aryl/heteroaryl group by means of a direct C−C bond has been shown to be a promising approach to tuning the photophysical properties of flavin derivatives. The aryl substituents caused a bathochromic shift in the absorption spectra of up to 52 nm and prolonged the fluorescence lifetime by up to 1 order of magnitude. Moreover, arylation of flavin derivatives decreased their ability to generate singlet oxygen.
The front cover picture, prepared by Dr D. Prukala, illustrates ethylene‐bridged 7,8‐dimethoxyflavinium salt excited with blue light. This salt is characterised by relatively long‐lived excited state and high photostability thus making it superior among flavin photocatalysts for aerobic oxidations/oxygenations. The flavinium salt was demonstrated to provide photooxidation of various benzylic substrates, even those with high oxidation potential (X=Cl, CF3). Details can be found in the Research Article by Sikorski, Cibulka and co‐workers. (A. Pokluda, Z. Anwar, V. Boguschová, I. Anusiewicz, P. Skurski, M. Sikorski, R. Cibulka, Adv. Synth. Catal. 2021, 363, 4371–4379; DOI: 10.1002/adsc.202100024)
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