New Insights in the Mechanism of Amine Catalyzed Epoxidation:  Dual Role of Protonated Ammonium Salts as Both Phase Transfer Catalysts and Activators of Oxone

Abstract

Amines have previously been reported to catalyze the epoxidation of alkenes using Oxone (2KHSO(5)+KHSO(4)+K(2)SO(4)), and significant levels of asymmetric induction were observed. From screening a series of amines based on 2-substituted pyrrolidines, it has now been found that more consistent and reproducible results are achieved with the HCl salt of the amine compared to the amine itself. Up to 66% ee was achieved in epoxidation of 1-phenylcyclohexene. The chiral amine could be reisolated in >90% yield when reactions were conducted at -10 degrees C, indicating that the integrity of the amine was maintained during the oxidation process. At -10 degrees C, (S)-2-(diphenylmethyl)pyrrolidine 1 reacted with Oxone to give a mixture of ammonium salts containing the peroxymonosulfate salt 6b. The enantioselectivity obtained with this salt was compared to the amine.HCl salt catalyzed process and identical results were observed, indicating that the true oxidant was the peroxymonosulfate salt 6b. The relative rates of oxidation of cis- and trans-beta-methylstyrenes together with the rho value of a series of 1-arylcyclohexenes were determined. These studies indicated that the amine catalyzed process involved electrophilic oxidation. On the basis of these findings, a new mechanism is advanced in which the protonated amine not only acts as a PTC but also activates Oxone, through hydrogen bonding, toward electrophilic attack.

Konformative Fluoreffekte in der Organokatalyse: eine neuartige Strategie zum molekularen Design

“…[72,73] Die Epoxidierung von 1-Phenylcyclohexen verläuft mit 46 % ee in Gegenwart von Schema 11. Ein fluorierter Organokatalysator zur stereoselektiven Epoxidierung von a,b-ungesättigten Aldehyden.…”

Preparation of C 2-Symmetric Biaryl Bisiminium Salts and Their Use as Organocatalysts for Asymmetric Epoxidation

“…9 Related chiral secondary amines have also been shown to catalyse the asymmetric epoxidation of alkenes, giving up to 66% ee. 10 We have reported that amines corresponding to a number of our iminium salts, such as 4 and 6 (R= H), are indeed effective epoxidation catalysts under our standard conditions, giving yields and ees very similar to the corresponding iminium species. We have postulated that the amine catalysts are oxidized in situ to the corresponding iminium salts, which in turn epoxidize the alkenes via the oxaziridinium salts.…”

Activation of Hydrogen Peroxide by Diphenyl Diselenide for Highly Enantioselective Oxaziridinium Salt Mediated Catalytic Asymmetric Epoxidation

“…15 Very few reports have emerged of the amine-catalysed epoxidation of alkenes. Aggarwal in 2003 16 and Yang in 2005 17 independently reported employing oxone and a secondary amine, although both we and Lacour have shown that amines related to 7 14 catalyse the epoxidation of unfunctional alkenes. 18 In these reactions, unlike those reported by Aggarwal and Yang, which are believed to occur through hydrogen bonding to persulfate, oxone converts the amines firstly to the iminium and subsequently to the oxaziridinium ions, thus enabling epoxidation to proceed.…”