Iminium salt-mediated catalytic asymmetric

P.C.B. Page, B.R. Buckley, D. Barros, H. Heaney, A.J. Blacker, B.A. Marples, Non-aqueous iminium salt mediated catalytic asymmetric epoxidation, Tetrahedran 62 (2006) 6607. 

Yang and co-workers have suggested that the inherent difficulties in the preparation/isolation of unstable exocyclic iminium salts can be overcome by in situ formation of the catalytic species from chiral pyrrolidines and aldehydes. The catalytic asymmetric epoxidation of phenylcyclohexene and dihydronaphthalene mediated by the iminium salt derived from pyrrolidine 67 and aldehyde 68 has been examined (Scheme 27) <20010L2587>. 

SCHEME 5.21 The standard conditions applied for catalytic asymmetric epoxidation mediated by Oxone and iminium salts. 

V.K. Aggarwal, M.F. Wang, Catalytic asymmetric synthesis of epoxides mediated by chiral iminium salts, Chem. Commun. (1996) 191. 

P.C.B. Page, G. A. Rassias, D. Barros, A. Ardakani, D. Bethell, E. Merifield, New organocatalysis for the asymmetric catalytic epoxidation of alkenes mediated by chiral iminium salts, Synlett 4 (2002) 580. 

Two C-C Bond-Forming Events In 2008, Frechet and coworkers described an impressive asymmetric cascade reaction promoted by soluble star polymers with core-confined catalytic entities [10]. The encapsulation of catalysts into soluble star polymers allowed the use of incompatible catalysts and prevented undesired interactions between these catalytic systems. The organocascade corresponded to a nucleophilic addition of Af-methylindole to a,p-unsaturated aldehydes followed by a Michael addition of the adduct to methylvinylketone (MVK) in the presence of H-bonding additive (Scheme 12.5). Each catalyst - imidazolidinone 8 for the nucleophilic addition and diphenylprolinol methyl ether 9 for the Michael addition - or their combination cannot mediate both reaction steps. In particular, p-toluenesulfonic acid (p-TSA) diminished the ability of the chiral pyrrolidine 9 to effect enamine activation. Therefore, p-TSA and 9 were encapsulated in the core of star polymers, which cannot penetrate each other. Imidazolidone 8 was added to the acid star polymer and diffused to the core to form the salt, which allowed the iminium activation and catalyzed the first step. The second step was catalyzed by the pyrrolidine star polymer in presence of the H-bonding additive 10, which... 

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