Chemistry Reference
In-Depth Information
Fig. 23 (a) Structural formula of catenane 20.(b) Processes that enable unidirectional ring
rotation [
92
].
A
,
B
recognition sites;
X
1
,X
2
blocking groups
stimulus S
1
restores the recognition ability of site A. The system has now to
reach the starting co-conformation wherein the moving ring surrounds site A. The
presence of the hindering group K again makes anticlockwise rotation faster than
clockwise rotation. The original catenane structure is then obtained with a reset
stimulus S
2
, by which the blocking group X is put back in place. Unidirectional
rotation in such a catenane occurs by a “flashing ratchet” mechanism [
90
,
91
], which
is based on a periodic change of the potential energy surface viewed by the moving
part (Fig.
22
) by orthogonal (i.e., independent) reactions. It is worth noting that the
direction of rotation can be inverted by reversing the order of the two input stimuli.
This concept was cleverly realized using light as a stimulus with catenane 20
shown in Fig.
23a
[
92
]. Its larger ring contains two recognition sites for the smaller
ring - namely, a photoisomerizable fumaramide unit (A) and a succinamide unit (B) -
and two bulky substituents that can be selectively detached and reattached - namely,
a silyl (X
1
) and a triphenylmethyl (
X
2
) group. In the starting isomer (I in Fig.
23b
), the
smaller ring surrounds the fumaramide site. On
E
Z
photoisomerization of such a
unit with 254 nm light and subsequent desilylation (II in Fig.
23b
), the smaller ring
moves in the clockwise direction to surround the succinamide site. At this point, a
silyl group is reattached at the original position (III in Fig.
23b
). Piperidine-assisted
back-isomerization of the maleamide unit to the fumaramide unit, followed by
removal of the triphenylmethyl group (IV in Fig.
23b
), causes another half-turn of
the smaller ring in the clockwise direction to surround the fumaramide unit. Reat-
tachment of a triphenylmethyl substituent regenerates the starting isomer (I in
Fig.
23b
). The overall result is a net clockwise rotation of the smaller ring about
the larger one. Exchanging the order in which the two blocking groups are
manipulated produces an equivalent anticlockwise rotation of the smaller ring.
The structures of the compounds obtained after each of the above reaction steps,
and particularly the position of the smaller ring, were determined by
1
HNMR
!
