We report a new class of synthetic molecular pumps that
use a stepwise
information ratchet mechanism to achieve the kinetic gating required
to sequester their macrocyclic substrates from bulk solution. Threading
occurs as a result of active template reactions between the pump terminus
amine and an acyl electrophile, whereby the bond-forming reaction
is accelerated through the cavity of a crown ether. Carboxylation
of the resulting amide results in displacement of the ring to the
collection region of the thread. Conversion of the carbamate to a
phenolic ester provides an intermediate rotaxane suitable for further
pumping cycles. In this way rings can be ratcheted onto a thread from
one or both ends of appropriately designed molecular pumps. Each pumping
cycle results in one additional ring being added to the thread per
terminus acyl group. The absence of pseudorotaxane states ensures
that no dethreading of intermediates occurs during the pump operation.
This facilitates the loading of different macrocycles in any chosen
sequence, illustrated by the pump-mediated synthesis of a [4]rotaxane
containing three different macrocycles as a single sequence isomer.
A [5]rotaxane synthesized using a dual-opening transamidation pump
was structurally characterized by single-crystal X-ray diffraction,
revealing a series of stabilizing CH···O interactions
between the crown ethers and the polyethylene glycol catchment region
of the thread.
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