Pyrazinium Salts as Efficient Organocatalysts of Mild Oxidations with Hydrogen Peroxide

Abstract

A series of 3-substituted pyrazinium tetrafluoroborates was prepared as simple analogues of flavinium salts which are efficient organocatalysts for oxidations with hydrogen peroxide. It was shown that pyrazinium derivatives with an electronwithdrawing substituent catalyze mild oxidations of sulfides to sulfoxides and Baeyer-Villiger oxidations in a similar way to flavinium catalysts. The most reactive catalyst, 3-cyanopyrazinium tetrafluoroborate, was efficiently employed in preparative sulfoxidations of aromatic and aliphatic sulfides as well as in Baeyer-Villiger oxidations of cyclobutanones. A proposed mechanism for the catalysis is based on the formation of pyrazine hydroperoxide which is the agent oxidizing the substrate.

Electron-Deficient Heteroarenium Salts: An Organocatalytic Tool for Activation of Hydrogen Peroxide in Oxidations

“…The order of the efficiency of substituted 1-methylpyrazinium triflates 3a-3f corresponds to that found for analogous 1-ethylpyrazinium tetrafluoroborates (Table 3), 20 nevertheless, an effect of the counterion and 1-alkyl group was still observed. The change of a methyl for an ethyl group slightly increases the reactivity of the pyrazinium catalyst due to a steric effect (cf.…”

“…In our preliminary investigation, 20 pyrazinium tetrafluoroborates 3 (X = BF 4 in Scheme 2) with an electron-withdrawing substituent were shown to catalyze sulfoxidations with hydrogen peroxide in a similar way as flavinium salts; i.e., hydrogen peroxide adds to salt 3 to form heterocyclic hydroperoxide 3-OOH, which oxidizes a substrate. This important finding led us to consider if the ability to catalyze H 2 O 2 oxygenations is limited to pyrazinium salts, representing simple models of alloxazinium catalysts 2 with the same diazadiene N=C-C=N grouping (Scheme 2), or if this useful property is general among nitrogen containing heteroarenium compounds.…”

Electron‐Deficient Alloxazinium Salts: Efficient Organocatalysts of Mild and Chemoselective Sulfoxidations with Hydrogen Peroxide

“…The oxidation of thioanisole catalyzed by 2g is 160 times faster comparing with those in the presence of 2a taking in account the initial reaction rate. The analogical activation (by introducing a cyano group) of 1‐ethylpyrazinium catalyst leads only to 30‐fold increase in reaction rate of H 2 O 2 sulfoxidation 16…”

Insight into the catalytic activity of alloxazinium and isoalloxazinium salts in the oxidations of sulfides and amines with hydrogen peroxide

“…This is evident from the values of pK R+ and K H 2 O , describing the equilibrium between the flavinium salt and its corresponding pseudobase [47][48][49][50] (hydroxy adduct) ( Table 2). The apparent pK R+ values for the alloxazinium salts are higher than those obtained for the isoalloxazinium salts, by about six units.…”