Deslipping of ester rotaxanes: A cooperative interplay of hydrogen bonding with rotational barriers

A series of rotaxanes has been synthesized which contain two ester groups in their axles. All rotaxanes bear the same tetralactam wheel. The kinetics of the de-slipping reaction of these rotaxanes were monitored in tetrachloroethane (TCE) and dimethyl sulfoxide (DMSO) resulting in the observation of a significant solvent effect. In TCE, two isomeric rotaxanes that differ merely with respect to the orientation of the ester groups show a remarkable difference in their deslipping behavior. When the ester carbonyl group is directly attached to the axle center piece, the rotaxane decomposes with a half life of ca. 10 h at 100 degreesC. The reverse orientation with the carbonyl group attached to the stopper blocks deslipping almost completely and a lower limit for the half life at 100 degreesC of 25,000 h was obtained. These results can be interpreted by inferring a cooperative action of hydrogen bonding between wheel and axle and differences in rotational barriers. Molecular modeling and AM1 calculations support this interpretation. The implications of these results for the determination of steric size and the optimization of molecular machines are discussed. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.