copper template [8] and those operating via re-donor-re-acceptor interactions between electron rich axles and electron-poor wheels (or vice versa) [9]. Some utilize hydrogen bonding between ammonium ions and crown ethers [9] or between amide groups in the axle and wheel [10]. Even the hydrophobic effect has been used to introduce an axle into a cyclodextrin [11].

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). In non-competitive solvents, the tetralactam macrocycle strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as ''wheeled'' nucleophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocycle not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel.

0 0

Post a comment