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Flavin Photocatalysts for Visible Light [2+2] Cycloadditions: Structure, Reactivity and Reaction Mechanism



TOC (002) (výška 215px)New photocatalysts from the flavin family were found to mediate the [2+2] photocycloaddition reaction. 3-Butyl-10-methyl-5-deazaflavin (3a) and 1-butyl-7,8-dimethoxy-3-methylalloxazine (2e), when irradiated by visible light, were shown to allow an efficient (Φ ~ 3-10%) intramolecular cyclization of various types of substrates including substituted styrene dienes and bis(aryl enones), considered as electron-rich and electron-poor substrates, respectively, without any additional reagent. The versatility of the procedure was also demonstrated by the cyclization of photosensitive cinnamyl (E)-3-iodoallyl ether. Structure vs. activity studies found alloxazine 2e was more active than 7-monosubstituted (R = Cl, Br and MeO) alloxazines. The introduction of chlorine and bromine atom on the deazaflavin skeleton did not enhance the catalytic efficiency of 3a. A detailed electrochemical and spectroscopic study explains the reaction mechanism proceeding through energy transfer from the flavin excited triplet state to the diene followed by its cyclization.

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


Graphical abstract: 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.

Visible Light [2+2] Photocycloaddition Mediated by Flavin Derivative Immobilized on Mesoporous Silica


Link to full-size graphical abstractAbstract

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



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 EZ isomerisation of electron-rich cinnamyl derivatives to produce mixtures enriched with the Z isomer (with Z/E ratios of up to 77:23).

Link to full-size graphical abstract

Aktualizováno: 12.10.2017 15:25, Autor: Martina Kovandová

A BUDOVA A Ústav organické chemie se nachází ve 2. patře budovy A na straně ke Studentské ulici, sekretariát ústavu – místnost A278
B BUDOVA B V 1. patře se nachází Laboratoř forenzní analýzy biologicky aktivních látek
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