Quaternary cinchona-derived dimeric

Scheme 4.2 General synthetic scheme for cinchona-derived dimeric quaternary ammonium salts, (a) bis(Bromomethyl)-linkers (0.5 equiv.), EtOH-DMF-CHCI3 (5 6 2), r.t. or reflux, (b) Allyl bromide or benzyl bromide (6.0 equiv.), 50% KOH (10.0 equiv.), CH2CI2, r.t.

Figure 12.7 Dimeric and trimeric cinchona-derived quaternary ammonium salt phase-transfer catalysts.

Phase-transfer catalysis with cinchona alkaloid derivatives is a very active area within the field of organocatalysis, as indicated in quite recent reviews [98c, 112]. In terms of structure, the quaternary ammonium salts can be varied in a straightforward manner by changing the structure of the benzylic compound used for the alkylation of the nitrogen atom of the quinuclidine moiety. In addition, dimeric and trimeric quaternary ammonium salts of cinchona alkaloids have been prepared [112] and applied successfully in catalysis, as exemplified in Scheme 6.55 for the epoxidation of 2,4-diarylenones [117]. 

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