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Relevant academic research and scientific papers

Flavin Catalysis Employing an N(5)-Adduct: an Application in the Aerobic Organocatalytic Mitsunobu Reaction

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.

Azodicarboxylate-free esterification with triphenylphosphine mediated by flavin and visible light: method development and stereoselectivity control

Triphenylphosphine (Ph3P) activated by various electrophiles (e.g., alkyl diazocarboxylates) represents an effective mediator of esterification and other nucleophilic substitution reactions. We report herein an aza-reagent-free procedure using flavin catalyst (3-methyl riboflavin tetraacetate), triphenylphosphine, and visible light (448 nm), which allows effective esterification of aromatic and aliphatic carboxylic acids with alcohols. Mechanistic study confirmed that photoinduced electron transfer from triphenylphosphine to excited flavin with the formation of Ph3P+ is a crucial step in the catalytic cycle. This allows reactive alkoxyphosphonium species to be generated by reaction of an alcohol with Ph3P+ followed by single-electron oxidation. Unexpected stereoselectivity control by the solvent was observed, allowing switching from inversion to retention of configuration during esterification of (S)- or (R)-1-phenylethanol; for example with phenylacetic acid, the ratio shifting from 10?:?90 (retention?:?inversion) in trifluoromethylbenzene to 99.9?:?0.1 in acetonitrile. Our method uses nitrobenzene to regenerate the flavin photocatalyst. This new approach to flavin re-oxidation has also been successfully proved in benzyl alcohol oxidation, which is a “standard” process among flavin-mediated photooxidations.

Photocatalytic esterification under Mitsunobu reaction conditions mediated by flavin and visible light

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.

Transition-metal-free aerobic oxidative cleavage of C-C bonds in α-hydroxy ketones and mechanistic insight to the reaction pathway

Clear cut: For the title reaction, O2, the ideal oxidant, was used as the only oxidizing reagent. The dimer intermediate (see scheme) and isotopic labeling control experiments with 18O2 partially disclosed the reaction mec