Highly Enantioselective Biphasic Iminium‐Catalyzed Epoxidation of Alkenes. On the Importance of the Counterion and of N(sp²)C(sp³) Rotamers

Abstract

Diastereomeric biaryliminium cations made of an (Ra)-5,5 ',6,6',7,7',8,8'-octahydrobinaphthyl core and exocyclic appendages derived from (S)-or (R)-3,3-dimethylbutan-2-amine are effective asymmetric epoxidation catalysts for unfunctionalized alkenes. Herein, we report that the negative counterion of the iminium salts has to be chosen wisely. While the hexafluoroantimonate anion [SbF 6À ] is optimal for reliable results, one has to be careful about other anions and tetraphenylborate [BPh 4 À ] in particular. We also detail that the so far unexplained "lack" of stereochemical control from the chiral exocyclic appendage in this type of catalysts is due to

Asymmetric Epoxidation Using Iminium Salt Organocatalysts Featuring Dynamically Controlled Atropoisomerism

“…Although the barrier to rotation about the exocyclic C–N bond of the catalysts is similar, at about 11–12 kcal mol –1 in both the methylated and parent catalyst structures, as shown by VT-NMR experiments, the introduction of the new substituents is likely to alter the relative population of rotamers, and this could in turn affect the enantiocontrol in the catalyzed epoxidation process . NOESY experiments performed at −80 °C suggest that in the parent systems conformations close to that shown in 34 , with the two indicated C–H bonds having a syn -periplanar relationship, are favored.…”

“…We chose for initial experimentation the ( S,S )-acetonamine-derived biphenyl iminium salt catalyst 5 , which imparts good to high levels of enantiocontrol in asymmetric epoxidation. At −80 °C, four isomers can be seen in the NMR spectrum of 5 , corresponding to atropoisomerism about the biphenyl axis and to rotation around the exocyclic C–N bond . We believe on the basis of NOESY experiments that the two major isomers correspond to a pair of isomers about the C–N bond, each having the R axial conformation; the ratio of the R ax pair to the S ax pair is then 89:11, and the ratio of the two pairs of observed C–N rotamers 83:17.…”

“…24 Although the barrier to rotation about the exocyclic C−N bond of the catalysts is similar, at about 11−12 kcal mol −1 in both the methylated and parent catalyst structures, as shown by VT-NMR experiments, the introduction of the new substituents is likely to alter the relative population of rotamers, and this could in turn affect the enantiocontrol in the catalyzed epoxidation process. 14 4). This arrangement would be expected to provide a less reactive catalyst, as there is greater steric bulk around the site of oxygen transfer (presumed to be on the opposite face from the new methyl substituents).…”

“…At −80 °C, four isomers can be seen in the NMR spectrum of 5, corresponding to atropoisomerism about the biphenyl axis and to rotation around the exocyclic C−N bond. 14 We believe on the basis of NOESY experiments that the two major isomers correspond to a pair of isomers about the C−N bond, each having the R axial conformation; the ratio of the R ax pair to the S ax pair is then 89:11, and the ratio of the two pairs of observed C−N rotamers 83:17. The analogous binaphthyl system (see below) possesses a similar ratio of the two observed C−N rotamers of 88:12.…”

Synthetic methods : Part (ii) Oxidation and reduction methods

“…values of the products. 27 Several reports concern iminium ion catalysis in a slightly different context: epoxidation of enals and enones by means of nucleophilic peroxide addition. A significant disclosure from Gilmour pertains to the exploitation of the fluorine-iminium ion gauche effect to obtain good enantioselectivities.…”