Active template rotaxane synthesis through the Ni-catalyzed cross-coupling of alkylzinc reagents with redox-active esters

The synthesis of unsymmetrical axle [2]rotaxanes through a recently developed Ni-catalyzed C(sp 3 )-C(sp 3 ) cross-coupling of redox-active esters (formed directly from carboxylic acids) and organozinc reagents (derived from alkyl bromides) is reported. The method also furnishes, as a minor product, the symmetrical axle [2]rotaxanes resulting from the homo-coupling of the organozinc half-thread. The rotaxanes are formed in up to 56% yield with the ratio of unsymmetrical rotaxane increasing with the cavity size of the macrocycle. In the absence of the redox-active ester neither rotaxane is formed, even though the homo-coupling rotaxane product does not incorporate the redox-active ester building block. A Ni( iii ) intermediate is consistent with these observations, providing support for the previously postulated mechanism of the Ni-catalyzed cross-coupling reaction. The Ni-catalyzed C(sp 3 )-C(sp 3 ) cross-coupling of redox-active esters and organozinc reagents is used for the active template synthesis of 'impossible' rotaxanes.