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.

[reaction: see text] Efficient 1,4-asymmetric induction with an enantiopure aminoalkyl group as the chiral auxiliary has been achieved in the first example of diastereoselective alkylation (up to 98:2) of a sulfenate anion, readily prepared by oxidation of the corresponding thiolate. The stereochemistry of the sulfoxide produced is the opposite of that obtained by the conventional route based on oxidation of the sulfide precursor.

No abstract