Catalysis in the Self-Assembly of [2]Rotaxanes and [2]Pseudorotaxanes. Effect of the length of Polyethereal Side Arms and Terminal Stoppers

The template effects exerted by compounds 3, 5, 6, and 8 in the ring closure reaction of the trication 1(3+) to yield the [2]pseudorotaxanes 15(4+) and 17(4+) and the [2]-rotaxanes 18(4+) and 20(4+) have been quantitatively evaluated in acetonitrile at 62 degreesC by UV/vis spectrophotometry. The rate of ring closure of the trication 1(3+) largely increases in the presence of the templates up to a maximum of ca. 200 times in the case of 5 at a concentration of ca. 0.08 M. The results indicate that, in the self-assembly of a rotaxane, the template effect of the guest is lowered by the presence of two large stoppers (triisopropylsilyl) at the end of the linear chains; this effect is more important the shorter the chain and can be mainly ascribed to steric effects. Previously hypothesized entropic effects appear to be of lesser importance.