Azepinium salts

By Production of Azepinium Salts, Protonation, Complex Formation and Alkylation... 

H- and 3//-Azepines are generally unstable in aqueous acid solution and the few examples of simple azepinium salts, namely perchlorates,77 bromides,105 picrates35201 and a solitary iodide,105 have been prepared under nonaqueous conditions. The fractional crystallization of oxalate salts has been used for the separation of mixtures of 4- and 6-substituted 3f/-azepines,66 and 3,6-di-tm-butyl- and 2,5-di-tert-butyl-3//-azepine, on treatment with tetrafluoroboric acid in acetonitrile, are converted quantitatively into their crystalline tetrafluoroboratc salts.70... 

The binaphthyl azepinium salt 59 (TT= tris(tetrachlorobenzenediolato)phosphate(V)) and corresponding azepine 60 were developed as effective catalysts for the enantioselective epoxidation of unfunctionalised alkenes, with enantiomeric excesses up to 87% <06TA2334>. 

In our ongoing efforts to develop new and more selective catalysts based on iminium salts, a new family of catalyst was produced, in which the dihydroisoquinolinium moiety has been replaced by a biphenyl structure fused to a sevenmembered azepinium salt [34]. A similar system was developed some years ago by Aggarwal but with axial chirality, achiral at the nitrogen [11] the system gave some good results, although the enantioselectivity of the catalyst was dependent upon the substitution pattern of the alkene. 

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) ... 

Several new azepinium salt catalysts, derived from (- -)- and (—)-5-amino-2,2-dimethyl-4-phenyl-l,3-dioxane 23 [39] and (—)-isopinocamphenylamine 17 moieties, and fused to R or S binaphthalene units [40,41], were directly prepared, in good yields, from the bromomethyl carbaldehyde intermediate 32, which we prepared in turn from commercial R or S (l,l )-binaphthalenyl-2,2 -diol (Binol) (Scheme 5.20, Table 5.4). 

As previously obserx ed under the aqueous Oxone conditions, the enantioselectivity of the dibenzo[c,e[azepinium salt catalysts is markedly substrate dependent. 

TABLE 5.13 Catalytic asymmetric epoxidation of 1-Phenylcyclohexene using the dibenzo[c,e] azepinium salts and YPPP"... 

Again, we observed the switch in major enantiomer formed upon changing the reaction solvent, and again catalyst 36 appeared to be less substrate dependent than did the corresponding dibenzo[c,e]azepinium salts. 

Dimethyl-2,3-dihydro-l( 0 lienz[c]azepinium salts may be obtained by the palladium-catalyzed cyclization of 3-acetoxy-3-(2-dimethylaminomethylphenyl)-prop-l-enes <94TL2877>. However, if the alkene chain is extended there is an increasing tendency towards the formation of dihydro-isoindolinium salts (Scheme 21). 

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