Electroepoxidation of natural and synthetic alkenes mediated by sodium bromide

Abstract

Electroepoxidation of synthetic alkenes (styrene, trans-stilbene and trans-β-methylstyrene) and of some natural terpenes (limonene, terpinolene, geraniol, α-terpinene, γ-terpinene and   αterpineol) mediated by sodium bromide was performed in MeCN:H 2 O (4:1) at platinum electrodes. The indirect electrochemical oxidation of the olefins led to the corresponding epoxides in yields ranging from 23% to 79%. Résumé L'électroépoxydation d'alcènes synthétiques (styrène, trans-stilbène et trans-βméthylstyrène) et d'alcènes naturels terpéniques (limonène, terpinolène, géraniol, α-terpinène, γ-terpinène et α-terpinéol) à l'aide de bromure de sodium a été réalisée dans un mélange MeCN:H 2 O (4:1) avec des électrodes de platine. L'oxydation électrochimique indirecte d'oléfines conduit aux époxydes correspondants avec des rendements allant de 23% à 79%.

Br−/BrO−-mediated highly efficient photoelectrochemical epoxidation of alkenes on α-Fe2O3

“…The epoxidation of styrene substrates (Table 1, Supplementary Figs. [27][28][29][30][31][32][33][34][35][36] with different para-groups such as halogen (15)(16)(17), -CN (18), alkyl (19)(20) and phenyl (21) on α-Fe 2 O 3 were also compatible and effective. The selectivity for the substrate with strong electron-deficient substitute (-CF 3 22) showed a slight decrease (81 ± 6%).…”

“…3d). Under these conditions, besides Br 2, Br radical is also formed 19,20 . We propose that the dibromo product is attributed to Br 2 and Br radical.…”

“…As shown in Table 1 (Supplementary Figs. [15][16][17][18][19][20][21][22][23][24][25][26], trans-stilbene substrates with various electron-rich and electrondeficient substituents, including methyl (2-6), cyano (7), methoxy (8,9) and halogen (10)(11)(12), could be efficiently oxygenated to the corresponding epoxides. It is observed that the para-methyl-substitution of trans-stilbene (compare the precursors of 2, 3, 4, 5, 6) significantly increased the FE value for their Br-mediated epoxidation.…”

“…The small negative potential shift on Pt and TiO 2 anodes suggests that the oxidation of Br − on these electrodes may proceed dominantly through an electron transfer pathway to Br 2 . Actually, it has been reported that the Br-mediated reaction on Pt anodes occurs through a Br − /Br 2 pathway, in which Br − is oxidized to bromine radical by a single-electron process and then form Br 2 [18][19][20]22,34 . TiO 2 have a high valence band potential (E 0 vb = 3.1 V vs. NHE) 35,36 .…”

“…However, the direct electrochemical and photoelectrochemical epoxidation of alkenes behaved badly 3,14 , which are limited by poor faradaic efficiency (FE) and narrow substrate applicability. Instead of poor direct electrochemical epoxidation behaviors, the indirect electrochemical epoxidations by using redox mediators (such as Cl − /Cl 2 , Br − /Br 2 ) have been reported to promote the performance of the electrochemical epoxidation, by using precious metals (such as Pt) as anodes 4,[15][16][17][18][19][20][21][22] . For example, by coupling the heterogeneous chlorine evolution reaction and homogeneous alkene oxidation reaction with an interface, Leow et al, recently reported the chloride-mediated electrochemical epoxidation of ethylene and propylene can reach 97% selectivity and 71% FE 4 .…”