Tuning Flavin‐Based Photocatalytic Systems for Application in the Mild Chemoselective Aerobic Oxidation of Benzylic Substrates
Abstract
Chemoselective oxidation of substituted benzyl alcohols and toluene derivatives has been developed using a flavin photocatalyst, visible light and a suitable additive. Depending on the flavin and the additive, oxidation takes place selectively to the corresponding aldehyde or carboxylic acid. The procedure is also useful for oxidation of secondary alcohols or benzylic methylene‐containing compounds to ketones.
New flavin‐based photocatalytic systems used for chemoselective aerobic visible‐light oxidations have been developed by tuning the flavin structure and reaction conditions. 1,3‐Dimethyl‐7‐trifluoromethylalloxazine (2) and 10‐butyl‐3‐methyl‐7‐trifluoromethylisoalloxazine (3) were shown to mediate the selective oxidation of benzyl alcohols to form aldehydes in the presence of Cs2CO3. Flavin 3 was superior in the oxidation of toluene derivatives to form aldehydes in the presence of trifluoroacetic acid. On the other hand, photooxidations provided by ethylene‐bridged quaternary flavinium salt 1 gave the corresponding carboxylic acids. The usefulness of the developed catalytic systems using 1–3 was also demonstrated in the oxidation of secondary benzylic and aliphatic alcohols, and benzylic methylene groups to form the corresponding ketones. The systems have the advantage of a broad substrate scope and metal‐free conditions, which distinguish them from the previously reported flavin photooxidation reactions.
Flavinium catalyzed photooxidation: Detection and characterization of elusive peroxyflavinium intermediates
Flavin Catalysis Employing an N(5)-Adduct: An Application in the Aerobic Organocatalytic Mitsunobu Reaction
An N(5)‐flavin adduct was utilized in a catalytic Mitsunobu reaction with triphenylphosphane (triphenylphosphine), in which flavin acts as a Mitsunobu reagent instead of dialkyl azodicarboxylate. Flavin is used in a catalytic amount after regeneration by dioxygen.
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.
Nitrosobenzene: Reagent for the Mitsunobu Esterification Reaction
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, 31P NMR spectroscopy, and experiments conducted with isotopically labeled substrates.
Combining flavin photocatalysis and organocatalysis: metal-free aerobic oxidation of unactivated benzylic substrates
DOI:10.1021/acs.orglett.8b03547
We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential (Eox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b (E(2b*) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH.
Azodicarboxylate-free esterification with triphenylphosphine mediated by flavin and visible light: method development and stereoselectivity control
Enantioselective complexation of 1‐phenylethanol with chiral compounds bearing urea moiety
Flavin derivatives immobilized on mesoporous silica: a versatile tool in visible-light photooxidation reactions
Flavin Photocatalysts for Visible Light [2+2] Cycloadditions: Structure, Reactivity and Reaction Mechanism
Photocatalytic esterification under Mitsunobu reaction conditions mediated by flavin and visible light
Abstract
The usefulness of flavin-based aerial photooxidation in esterification under Mitsunobu reaction conditions was demonstrated, providing aerial dialkyl azodicarboxylate recycling/generation from the corresponding dialkyl hydrazine dicarboxylate. Simultaneously, activation of triphenylphosphine (Ph3P) by photoinduced electron transfer from flavin allows azo-reagent-free esterification. An optimized system with 3-methylriboflavin tetraacetate (10%), oxygen (terminal oxidant), visible light (450 nm), Ph3P, and dialkyl hydrazine dicarboxylate (10%) has been shown to provide efficient and stereoselective coupling of various alcohols and acids to esters with retention of configuration.
Visible Light [2+2] Photocycloaddition Mediated by Flavin Derivative Immobilized on Mesoporous Silica
Abstract
7,8-Dimethoxy-3-methylalloxazine was immobilized on mesoporous silica (MCM-41) to provide a heterogenized flavin photocatalyst. Thus, the prepared heterogeneous catalyst 2 was found to sensitize the visible light [2+2] cycloaddition of various types of dienes to produce corresponding cyclobutanes in high yields and diastereoselectivities. Use of 2 enables procedures which are advantageous owing to simple operation and workup, no additives required, and minimum waste generation.
Flavin-Mediated Visible-Light [2+2] Photocycloaddition of Nitrogen- and Sulfur-Containing Dienes
Abstract
The [2+2] photocycloaddition mediated by 1-butyl-3-methyl-7,8-dimethoxyalloxazine (1) has been found to be an effective tool for cyclising ω-phenyl- and ω,ω′-diphenyl-4-aza-1,6-heptadienes, in which the nitrogen atom is protected by acylation or quaternisation, towards the synthesis of a variety of phenyl- and diphenyl-3-azabicyclo[3.2.0]heptanes and their corresponding quaternary salts. Thia derivatives, with the sulfur atom in the form of a sulfone group, underwent an analogous cyclisation. Advantageously, visible light (400 nm) was used for the cycloadditions in the presence of 1, in contrast to the previously described procedures affording azabicyclo[3.2.0]heptanes by using UV irradiation. Practical applications are demonstrated through the synthesis of bicyclic quaternary ammonium salts, 6-phenyl-azabicyclo[3.2.0]heptanes known to exhibit biological activity or chiral spiro ammonium salts. Flavin 1 was also found to promote the efficient E→Z isomerisation of electron-rich cinnamyl derivatives to produce mixtures enriched with the Z isomer (with Z/E ratios of up to 77:23).