Dihydroisoquinolinium asymmetric epoxidation

TABLE 5.3 Catalytic asymmetric epoxidation mediated by the new dibenzo[c,c]azepinium salts (30 and 31) a comparison to the corresponding dihydroisoquinolinium salts (17 and 24) ... 

Proceeding via a similar reaction mechanism to the ketone-mediated epoxidation reactions, iminium salts offer an alternative source of catalysts for asymmetric organocatalytic epoxidations. The first example of the appHcation of iminium salts to asymmetric epoxidations used dihydroisoquinolinium-based catalyst 20 (Figure 19.9), affording a 33% ee for the epoxidation of ( )-stilbene [81]. 

Closely related to the ketone/Oxone epoxidation system is the use of iminium salts as promoters. As isolated oxaziridinium salts are known to effect alkene epoxidation [38], these are presumed to be the reactive intermediates in this catalytic system (see Scheme 12.1 X = NR.2+). The first asymmetric example used the dihydroisoquinolinium-based system 15 (Fig. 12.7), which afforded 33% ee for the epoxidation of F-stilbene [39]. 

Key Words Iminium, Oxaziridinium, Oxaziridine, Ketiminium, Oxone, Tetra-phenylphosphonium monoperoxysulphate, Isopinocampheylamine, Alkene, Epoxide, Enantiomeric excess. Asymmetric synthesis, Organocatalysis, 2-(2-Bromoethyl)benzaldehyde, Levcromakalim, Dihydroisoquinolinium, Spiro, Azepinium, Benzopyran, Dielectric constant, Binol. 2008 Elsevier B.v. 

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