Comparison of riboflavin-derived flavinium salts applied to catalytic H₂O₂oxidations

Abstract

A series of flavinium salts, 5-ethylisoalloxazinium, 5-ethylalloxazinium, and 1,10-ethylene-bridged alloxazinium triflates, were prepared from commercially available riboflavin. This study presents a comparison between their optical and redox properties, and their catalytic activity in H2O2 oxidations of sulfide, tertiary amine, and cyclobutanone. Reflecting the difference between the π-conjugated ring structures, the flavinium salts displayed very different redox properties, with reduction potentials in the order of: 5-ethylisoalloxazinium > 5-ethylalloxazinium > 1,10-ethylene-bridged alloxazinium. A comparison of their catalytic activity revealed that 5-ethylisoalloxazinium triflate specifically oxidises sulfide and cyclobutanone, and 5-ethylalloxazinium triflate smoothly oxidises tertiary amine. 1,10-Bridged alloxazinium triflate, which can be readily obtained from riboflavin in large quantities, showed moderate catalytic activity for the H2O2 oxidation of sulfide and cyclobutanone.

Photocatalytic Oxidative [2+2] Cycloelimination Reactions with Flavinium Salts: Mechanistic Study and Influence of the Catalyst Structure

“…Alloxazinium salts 2 were prepared from the corresponding alloxazines 5 via reductive alkylation into position 5 using acetaldehyde followed by oxidation of the flavin skeleton with sodium nitrite (Scheme 3a). In contrast to literature reports, [18a,20a, 23] the reductive alkylation step was performed in dichloromethane providing good yields of the salts 2 (38–74 %). Two approaches were applied to synthesize alloxazines 5 .…”

Flavinium and Alkali‐Metal Assembly on Sulfated Chitin: A Heterogeneous Supramolecular Catalyst for H2O2‐Mediated Oxidation

“…The catalytic cycle of 1 under the optimized reaction conditions is shown in Scheme A ,. The flavinium cation 1 is transformed into the oxidatively active hydroperoxyflavin ( 1 OOH ) species by the nucleophilic addition of H 2 O 2 , which participates in the monooxygenation of the substrate (S) to give the oxidized product (SO) and hydroxyflavin ( 1 OH ).…”

“…Therefore, the closely situated three components of sc‐1⋅Na display a unique cooperative catalytic effect that enables an efficient BV oxidation of ketones. It is anticipated that the rate‐limiting step in the catalytic cycle of 1 is the addition of H 2 O 2 to 1 (Scheme A) because the electrophilicity of the 1,10‐bridged flavinium cations such as 1 is not high . Accordingly, it is the cationic form 1 that predominantly exists under the present reaction conditions, whereas the neutral catalytic intermediates such as 1 OOH and 1 OH that may lead to the dissociation of the flavin catalyst from the anionic polymeric scaffold are scarcely present.…”

Green Aerobic Oxidation of Thiols to Disulfides by Flavin–Iodine Coupled Organocatalysis

“…First, we investigated the effects of various flavin catalysts, iodine sources, and solvents on the oxidation of 1-octanethiol (2a) under air (1 atm, balloon) in MeOH at 26 °C for 4 h (Tables 1, S1, and S2). Among the various neutral flavins (1 and 4, Figure 1, 5 mol%) 16 and cationic flavinium salts (5-7) 17,18 tested in the presence of I2 (5 mol%), 1a, which was easily prepared by the acetylation of riboflavin (1c), 16 successfully promoted the oxidation of 2a to afford the corresponding disulfide 3a in 86% yield (entry 1, Table 1). Commercially available riboflavin tetrabutyrate (1b) showed comparable catalytic activity, whereas the poor solubility of the non-protected riboflavin (1c) resulted in only a modest yield (entries 2 and 3).…”

“…First, we investigated the effects of various flavin catalysts, iodine sources, and solvents on the oxidation of octane-1-thiol (2a) under air (1 atm, balloon) in MeOH at 26°C for four hours (Table 1 and Supporting Information, Tables S1 and S2). Among the various neutral flavins 1 and 4 (Figure 1, 5 mol%) 16 and cationic flavinium salts 5-7 17,18 that we tested in the presence of I 2 (5 mol%), 1a, which is B M. Oka et al…”