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Flavinium salts are frequently used in organocatalysis but their application in photoredox catalysis has not been systematically investigated to date. We synthesized a series of 5‐ethyl‐1,3‐dimethylalloxazinium salts with different substituents in the positions 7 and 8 and investigated their application in light‐dependent oxidative cycloelimination of cyclobutanes. Detailed mechanistic investigations with a coumarin dimer as a model substrate reveal that the reaction preferentially occurs via the triplet‐born radical pair after electron transfer from the substrate to the triplet state of an alloxazinium salt. The very photostable 7,8‐dimethoxy derivative is a superior catalyst with a sufficiently high oxidation power (E*=2.26 V) allowing the conversion of various cyclobutanes (with Eox up to 2.05 V) in high yields. Even compounds such as all‐trans dimethyl 3,4‐bis(4‐methoxyphenyl)cyclobutane‐1,2‐dicarboxylate can be converted, whose opening requires a high activation energy due to a missing pre‐activation caused by bulky adjacent substituents in cis‐position. [ABSTRACT FROM AUTHOR]

An artificial flavin system has been firstly proved to employ an N(5)‐adduct for a catalytic transformation. This mode of catalysis occurs in some flavoenzymes but it is unknown in chemocatalysis, still exclusively using only C(4a)‐adducts. In our report, an ethylene‐bridged biomimetic flavin has been shown to participate in the Mitsunobu esterification reaction as an alternative to dialkyl azodicarboxylate. The reaction occurs via a flavin N(5)‐triphenylphosphane adduct and is catalytic from the point of view of the flavin, which is regenerated by oxygen. This approach distinguishes from other catalytic Mitsunobu reaction procedures which require an extra catalytic system. [ABSTRACT FROM AUTHOR]

Flavinium salts, both isoalloxazinium and alloxazinium derivatives, are useful organocatalysts of redox reactions, and particularly in recent years, the number of transformations catalysed by flavinium salts has increased. This review outlines the synthetic applications of flavinium catalysts in oxygenation reactions with oxygen and hydrogen peroxide as terminal oxidising agents; these oxidations involve flavin hydroperoxide as an intermediate, thus mimicking flavin monooxygenases. Special attention is paid to mechanistic studies and to the design of catalytic systems, especially the effects of the structures of flavinium catalysts, and reaction conditions on the efficiency, chemoselectivity and stereoselectivity of oxygenations. [ABSTRACT FROM AUTHOR]

The amphiphilic flavin derivatives 1,10-ethylene-bridged 3-alkylalloxazinium salts (alkyl=butyl, octyl, dodecyl and hexadecyl) were shown to catalyse the chemoselective oxidation of sulfides to sulfoxides with hydrogen peroxide in chloroform/water or dichloromethane/water systems. The efficiency of the catalytic oxidation depended substantially on the pH of the aqueous phase to reach maximum values at pH 7. Under such conditions, the complete conversion of aliphatic and aromatic sulfides occurred within a maximum of 24 h, whereas the non-catalysed reaction gave only traces of products. This new concept of two-phase oxidation utilises the activation and the increase in the lipophilicity of hydrogen peroxide by its covalent bonding to the alloxazinium catalyst. A practical application in the synthesis of modafinil was demonstrated. [ABSTRACT FROM AUTHOR]

A series of substituted planar chiral flavinium salts with a phenyl 'cap' have been prepared as potential catalysts for enantioselective sulfoxidation reactions with hydrogen peroxide by using an approach based on the synthesis of (arylamino)uracils and their condensation with substituted nitrosobenzenes. The effect of substituents at various positions on the ability of the catalyst to promote enantioselective sulfoxidation recations was investigated. Introduction of the tyrosine group into the side-chain of the flavinium species or substitution of the nitrogen N-3 of the flavin unit by o-isopropylphenyl has a remarkably positive effect on the enantioselectivity of the sulfoxidation reactions of aromatic and aliphatic substrates, respectively. On the other hand, substitution of the phenyl 'cap' led to a substantial decrease in the efficiency of the catalyst. In summary, optimisation of the structures of the planar chiral flavinium catalysts led to enantioselectivities of up to 61 % ee for aromatic sulfides and of up to 65 % ee for tert-butyl methyl sulfide. [ABSTRACT FROM AUTHOR]

A series of 10-arylflavins (10-phenyl-, 10-(2′,6′-dimethylphenyl)-, 10-(2′,6′-diethylphenyl)-, 10-(2′,6′-diisopropylphenyl)-, 10-(2′- tert-butylphenyl)-, and 10-(2′,6′-dimethylphenyl)-3-methylisoalloxazine ( 2 a- f)) was prepared as potentially nonaggregating flavin photocatalysts. The investigation of their structures in the crystalline phase combined with 1H-DOSY NMR spectroscopic experiments in CD3CN, CD3CN/D2O (1:1), and D2O confirm the decreased ability of 10-arylflavins 2 to form aggregates relative to tetra- O-acetyl riboflavin ( 1). 10-Arylflavins 2 a- d do not interact by π-π interactions, which are restricted by the 10-phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3H⋅⋅⋅O hydrogen bonds were detected in their crystal structures. In the structure of 10-aryl-3-methylflavin ( 2 f) with a substituted N3 position, weak CH⋅⋅⋅O bonds and weak π-π interactions were found. 10-Arylflavins 2 were tested as photoredox catalysts for the aerial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (model reaction), thus showing higher efficiency relative to 1. The quantum yields of 4-methoxybenzyl alcohol oxidation reactions mediated by arylflavins 2 were higher by almost one order of magnitude relative to values in the presence of 1. [ABSTRACT FROM AUTHOR]

Abstract: In addition to the already known organocatalysts dihydro-5-ethylalloxazines 1-H 2 and 5-ethylisoalloxazinium salts 2, other derivatives of alloxazines and isoalloxazines (flavins) have been shown to catalyze the oxidation of sulfides to sulfoxides with hydrogen peroxide with high efficiency. In the alloxazine series, the C4a-methoxy adduct 1-OCH 3 or 5-ethylalloxazinium salts 1 can also be used, but the latter only in the presence of a base, as a co-catalyst. In the isoalloxazine series, the C4a-hydroxy adduct 2-OH or dihydroform 2-H 2 can also be utilized but only in the presence of the catalytic amount of an acid. On the other hand, only alloxazine derivatives, i.e. dihydroalloxazines 1-H 2 and their methoxy adducts 1-OCH 3 or alloxazinium salts 1 can be used as catalysts for the oxidation of tertiary amines, as the catalytic activity of isoalloxazinium salts 1 and their derivatives is inhibited under basic conditions. The different reactivity of the alloxazinium and isoalloxazinium catalysts is a consequence of the change of the rate determining step in the catalytic cycle, resulting from their significantly different ability to react with nucleophiles. [Copyright &y& Elsevier]

Abstract: Three chiral N 1,N 10-ethylene-bridged flavinium salts with a stereogenic centre derived from l-valinol are prepared and investigated as oxidation catalysts. These salts efficiently catalyse chemoselective H2O2 oxidation of sulfides to sulfoxides and the oxidation of 3-phenylcyclobutanone to the corresponding lactone at room temperature. The flavinium salts react with hydrogen peroxide to form flavin-10a-hydroperoxide, which is the agent responsible for oxidation of the substrate. [Copyright &y& Elsevier]

Abstract: Amphiphilic flavinium salt [10-dodecyl-5-ethyl-3,7,8-trimethylisoalloxazinium perchlorate (1b)], solubilized in micelles of sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium chloride (CTAC), hexadecyltrimethylammonium nitrate (CTANO3) or Brij 35, catalyzes the chemoselective oxidation of thioanisole (2) to its corresponding sulfoxide 4. The sulfoxidation rates observed in micelles in the presence of 1b exceeded, in most instances, those of the reactions catalyzed by its hydrophilic homologue [5-ethyl-3,7,8,10-tetramethylisoalloxazinium perchlorate (1a)] in homogeneous solutions. Reaction rates were strongly dependent on the type of micellar matrix and on the pH value. The highest acceleration rate was found in SDS micelles at pH 4.4 (TOF=3×103 h−1). In our experiments, micelles favoured the catalytic process rather than the non-catalyzed reaction, this effect being especially pronounced in cationic micelles, for which the non-catalyzed reaction was markedly suppressed. The ratio of the catalyzed and non-catalyzed reaction rates was almost 40 in CTANO3, while in homogeneous solution its value did not exceed 7. [Copyright &y& Elsevier]

Two new photocatalytic systems based on flavinium species formed in situ by protonation of riboflavin-tetraacetate or prepared in advance via alloxazine quaternization are used as tools for the title reaction. [ABSTRACT FROM AUTHOR]

Review: [using hydrogen peroxide and oxygen as stoichiometric oxidizing agents; 51 refs. [ABSTRACT FROM AUTHOR]

An alloxazine derivative allows the efficient cyclobutane ring formation via intramolecular [2 + 2] cycloaddition of both electron-rich styrene dienes and electron-poor bis(arylenones). [ABSTRACT FROM AUTHOR]

A broad spectrum of mono- and disubstituted heteroarenium salts, mostly as triflates, is prepared and tested in a sulfoxidation process. [ABSTRACT FROM AUTHOR]

The article presents chemical equations related to the synthesis of planar chiral Flavinium salts and the effect of substituent on catalytic efficiency in enantioselective sulfoxidation reactions" by R. Jurok and colleagues, which was published in "European Journal of Organic Chemistry" in 2013.

A series of substituted alloxazinium perchlorates are prepared and tested as catalysts for the oxidation of sulfides to sulfoxides with hydrogen peroxide. [ABSTRACT FROM AUTHOR]