Eva Svobodová

Photooxidation of Sulfides to Sulfoxides Mediated by Tetra-O-Acetylriboflavin and Visible Light

Chemoselective photooxidation: Tetra-O-acetylriboflavin and blue light mediate the selective oxidation of sulfides to sulfoxides without overoxidation to sulfones. Reaction is very effective in ethanol/water (95:5,v/v) mixture reaching a quantum yield of Φ=0.60. The photooxidation reaction scope includes aliphatic and aromatic sulfides including sterically hindered substrates.

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

New photocatalysts from the flavin family were found to mediate the [2+2] photocycloaddition reaction. 3‐Butyl‐10‐methyl‐5‐deazaflavin (3 a) and 1‐butyl‐7,8‐dimethoxy‐3‐methylalloxazine (2 e), if irradiated by visible light, were shown to allow an efficient (Φ≈3–10 %) intramolecular cyclisation 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 demonstrated by the cyclisation of photosensitive cinnamyl (E)‐3‐iodoallyl ether. Structure–activity studies found alloxazine 2 e 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 3 a. 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 cyclisation.

Copper(II) and Zinc(II) Complexes of Conformationally Constrained Polyazamacrocycles as Efficient Catalysts for RNA Model Substrate Cleavage in Aqueous Solution at Physiological pH

As part of a quest for efficient artificial catalysts of RNA phosphodiester bond cleavage, conformationally constrained mono- and bis-polyazamacrocycles in which tri- or tetraazaalkane chains link the ortho positions of a benzene ring were synthesized. The catalytic activities of mono- and dinuclear copper(II) and zinc(II) complexes of these polyazamacrocycles towards cleavage of the P-O bond in 2-hydroxypropyl-4-nitrophenylphosphate (HPNP) in aqueous solution at pH 7 have been determined. Only the complexes of the ligands incorporating three nitrogen atoms in a macrocycle proved to be capable of efficiently catalyzing HPNP transesterification. The dinuclear complexes were found to be approximately twice as efficient as their mononuclear counterparts, and exhibited Michaelis-Menten saturation kinetics with calculated rate constants of kcat ≈10(-4)  s(-1) . By means of quantum chemical calculations (DFT/COSMO-RS), several plausible reaction coordinates were described. By correlating the calculated barriers with the experimental kinetic data, two possible reaction scenarios were revealed, with activation free energies of 20-25 kcal mol(-1) .

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 derivatives immobilized on mesoporous silica: a versatile tool in visible-light photooxidation reactions

Anchoring riboflavin tetraacetate to mesoporous silica (MCM-41) provides a heterogenized flavin photocatalyst, which was proven to be an effective catalyst for chemoselective sulfoxidation, benzyl alcohol oxidation, and photoesterification reactions. Analogously heterogenized 1-butyl-7,8-dimethoxyalloxazine was shown to mediate sulfoxidation and benzylic oxidation reactions. This is the first application of an alloxazine photocatalyst in a redox reaction. The use of these heterogeneous catalysts allows simple operation and work-up of the reaction mixtures.Graphical abstract