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
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
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 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).