Chemoselective sulfide oxidation mediated by bridged flavinium organocatalysts

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

“…Among the reported organocatalytic sulfoxidation methods, the most promising utilizes the oxidising power of flavin-4a-hydroperoxides formed in situ from the corresponding flavinium salt and hydrogen peroxide. This method is highly chemoselective, producing sulfoxides with no over-oxidation to sulfones [2,[13][14][15][16][17][18][19][20][21][22][23][24]. If a chiral flavinium salt is employed, the sulfoxidation proceeds with high enantioselectivity [25][26][27][28][29].…”

“…Research into catalysis with flavinium salts was initiated by the investigation of the mode of action of flavin-4a-hydroperoxides, which were recognized as oxidizing agents in flavoenzymes [30][31][32][33][34][35][36]. So far, flavinium salts have been found to catalyze sulfoxidations [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], N-oxidations [24,37,38], and Baeyer-Villiger oxidations [39][40][41] in the similar way as flavindependent monooxygenases [13,14,[30][31][32][33][34][35][36]. Biomimetic systems utilizing a flavinium salt and oxygen as terminal oxidant have also been designed [42][43][44].…”

“…Isoalloxazinium salts were first shown to catalyze the H 2 O 2 -oxidation of sulfides to sulfoxides [24]. Sulfoxidations catalyzed by isoalloxazinium salts (Isoall + ) are usually performed in methanol with a small excess of aqueous hydrogen peroxide and with 1-2 mol% of the catalyst [13][14][15][16][17][18][19][20][21][22][23][24]. Under such conditions, the oxidation proceeds by a mechanism consisting of: (i) formation of isoalloxazine-4a-hydroperoxide (Isoall-OOH), (ii) oxidation of the substrate (S) producing hydroxyisoalloxazine (Isoall-OH), and (iii) elimination of water.…”

“…Although isoalloxazinium and alloxazinium salts are important organocatalysts for sulfoxidations with hydrogen peroxide and several studies of their catalytic activity have been published [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], there has been no direct comparison of their efficiency under the same reaction conditions. Only individual comparative studies for both groups of flavinium salts have been reported.…”

A recyclable fluorous thiourea organocatalyst for the chemoselective oxidation of sulfides

“…Later, all kinds of the improved oxidation methods, especially by the use of transition metal (V, Mo, Ti) ion [9], were developed so as to overcome these drawbacks. But organocatalytic oxidative system was seldom reported [13,14].…”