Catalytic effect of alloxazinium and isoalloxazinium salts on oxidation of sulfides with hydrogen peroxide in micellar media

Abstract

Three novel amphiphilic alloxazinium salts were prepared: 3-dodecyl-5-ethyl-7,8,10-trimethylisoalloxazinium perchlorate (1c), 1-dodecyl-5-ethyl-3-methylalloxazinium perchlorate (2b), and 3-dodecyl-5-ethyl-1-methylalloxazinium perchlorate (2c). Their catalytic activity in thioanisole (3) oxidation with hydrogen peroxide was investigated in micelles of sodium dodecylsulfate (SDS), hexadecyltrimethylammonium nitrate (CTANO3) and Brij 35. Reaction rates were strongly dependent on the catalyst structure, on the type of micelles, and on pH value. Alloxazinium salts 2 were more effective catalysts than isoalloxazinium salts 1. Due to the contribution of micellar catalysis, the vcat/v0 ratio of the catalyzed and non-catalyzed reaction rates was almost 80 with salt 2b solubilized in CTANO3 micelles. Nevertheless, the highest acceleration was observed with non-amphiphilic 5-ethyl-1,3-dimethylalloxazinium perchlorate (2a) in CTANO3 micelles (vcat/v0 = 134). In this case, salt 2a presumably acts as a phase-transfer catalyst bringing hydrogen peroxide from the aqueous phase into the micelle interior. Synthetic applicability of the investigated catalytic systems was verified on semi-preparative scale.

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

“…The hydroxyalloxazine (All-OH), formed simultaneously, eliminates water to give the alloxazinium salt (All + ), which then acts as catalyst in the following cycles. Only recently, alloxazinium salts themselves were used for the acceleration of sulfoxidations in buffered micellar [17] and in homogeneous aqueous solutions [28,29].…”

“…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.…”

“…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].…”

Synthesis of Flavin–Calix[4]arene Conjugate Derivatives

“…At the same time, flavins that are obtained by condensation between a-phenylenediamine and alloxane [14] are involved in several biological processes [15]. These are generally redox cofactors involved in electron redox processes [16] of enzymatic reactions [17] [18]. To this end, we considered that compounds containing both flavin and calixarene units, i.e., 9 and 10, respectively, might have interesting biological activities.…”