Iminium salts, biphenyl

Three different chiral biphenyl iminium salts have been prepared and tested as asymmetric catalysts for epoxidation of prochiral alkenes with oxone in MeCN-H20 in the presence of a base. The complexes (74) and (75) have electron-withdrawing 3,3 -substitutents on the terminal phenyl units but salt (75) lacks the t-butyl and methoxy groups at the central biphenyl unit. The salt (76) bears no t-butyl or methoxy groups or electron-withdrawing substitutents at a biphenyl unit. The catalytic reactivity of these complexes in terms of the yield and ee of the epoxidized product is in the following order (76) > (75) > (74). This suggests that substitution on the biphenyl unit introduces steric bulk which is damaging for catalytic activity and enantioselectivity. ... 

Scheme 1.40 Epoxidation of 1-phenylcyclohexene with the biphenyl iminium salt...

Homologous biphenyl and binaphthyl tertiary azepines (4) and quaternary iminium salts, prepared from (+)-(5,5 )-L-acetonamine, behave as effective catalysts for the enantioselective epoxidation of unfunctionalized alkenes with Oxone (ee up to 83%).113... 

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catalysts based on iminium salts, a new family of catalyst was produced, in which the dihydroisoquinolinium moiety has been replaced by a biphenyl structure fused to a sevenmembered azepinium salt [34].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. 

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