New Organocatalysts for the Asymmetric Catalytic Epoxidation of Alkenes Mediated by Chiral Iminium Salts

Abstract

New iminium salt organocatalysts for catalytic asymmetric epoxidation based upon dibenzazepinium salts are reported, providing ee of up to ca 60%.

Enantioselective Organocatalytic Epoxidation Driven by Electrochemically Generated Percarbonate and Persulfate

“…[13] This provides evidence that the active oxidant in this instance is indeed monoperoxysulfate. As described above, the percarbonate-driven system provides lower ees.…”

“…This is puzzling when it is considered that under both our original Oxone-and TPPP-mediated conditions this conversion proceeds with ees of between 17 and 49% for these substrates. [13] As a further test, catalyst 2 was also evaluated. In this instance, low to moderate ees were observed.…”

“…[13] As percarbonate is a novel oxidant for this catalyst system, the possibility exists that significant background epoxidation is occurring, which would be expected to reduce the observed ees significantly. Entry i shows that this is unlikely: a control experiment run in the absence of iminium salt showed only a 1.9% conversion into epoxide in the same time period, demonstrating that the background epoxidation process does not make a significant contribution to the observed conversions, and ruling out the significant involvement of peroxyimidic acid derived from the acetonitrile solvent.…”

Applications of Chiral Three-membered Rings for Total Synthesis: A Review

“…Page and co-authors reported the application of another chiral iminium salt-catalyzed epoxidation as key step in the first highly enantioselective synthesis of (+)-scuteflorin A, one of the numerous and pharmacological active compounds recently isolated from Scutellaria species (Scheme 47) [102]. A previously reported chiral iminium salt 115 [103] served as organocatalyst under non aqueous conditions, to epoxidize the starting Z-alkene 116 into the corresponding key enantiopure epoxide 117 in 97% yield. Epoxide hydrolysis/oxidation and esterification completed the synthesis.…”

Iminium Salt‐Catalysed Asymmetric Epoxidation using Hydrogen Peroxide as Stoichiometric Oxidant

“…[5] We were the first to report iminium salt catalysts based on tetrahydroisoquinolines bearing an exocyclic chiral substituent on the nitrogen atom, [6] and a dioxane-based substituent has proved to be optimum for both yield and induction of enantioselectivity. [7][8][9] Three of the most successful catalysts are the dihydroisoquinolinium salt 3a, [9,10] biphenylazepinium salt 4, [8,9,11] and binaphthalene-azepinium salt 5, which can provide enantiomeric excesses of up to 97%. [9,12] The standard iminium salt epoxidation conditions employ the triple salt Oxone (2 KHSO 5 ·KHSO 4 ·K 2 SO 4 ) as a stoichiometric oxidant, sodium carbonate, and acetonitrile:water as the solvent mixture (Scheme 1).…”