Path selection for conformational interconversions in [2]catenanes

Article abstract of DOI:10.1021/ol060550n

The conformational interconversions of several [2]catenanes containing a dibenzo-34-crown-10 ether (BPP34C10) interlocked with rings containing two 4,4′-dipyridyls tethered by different aryl spacers have been studied. Blocking groups on the tethers enabled the two pathways for circumrotation of the BPP34C10 to be open or blocked. The activation barrier for migration along the open tethers varied from 11 to 13 kcal/mol. This study demonstrates an ability to select the pathway for conformational interconversions in [2]catenanes.

Full text of DOI:10.1021/ol060550n

ORGANIC
LETTERS
2006
Vol. 8, No. 10
2119-2121
Path Selection for Conformational
Interconversions in [2]Catenanes
Ronald L. Halterman,* David E. Martyn, Xingang Pan, Diana B. Ha,†
Michael Frow,† and Kathryn Haessig†
Department of Chemistry and Biochemistry, UniVersity of Oklahoma,
620 Parrington OVal, Norman, Oklahoma 73019
rlhalterman@ou.edu
Received March 6, 2006
ABSTRACT
The conformational interconversions of several [2]catenanes containing a dibenzo-34-crown-10 ether (BPP34C10) interlocked with rings containing
two 4,4 -dipyridyls tethered by different aryl spacers have been studied. Blocking groups on the tethers enabled the two pathways for
′
circumrotation of the BPP34C10 to be open or blocked. The activation barrier for migration along the open tethers varied from 11 to 13
kcal/mol. This study demonstrates an ability to select the pathway for conformational interconversions in [2]catenanes.
Noncovalent interactions are of central importance in deter-
mining the properties of novel materials1 and biological
systems.2 To better detail the role of nonbonding interactions
during conformational changes or reactions, we have un-
dertaken a study of conformational interconversions in
bistable [2]catenane systems. The low-energy conformations
of [2]catenanes comprised of π-electron-rich dibenzo-34-
crown-10 ether (BPP34C10) mechanically interlocked with
rings containing two π-electron-deficient 4,4′-dipyridyls are
well-established and are based on electrostatic and π,π-
stacking interactions.
Such catenanes form bistable complexes where the crown
ether prefers to π-stack over either of the dipyridyl groups.3
Stoddart has reported activation barriers for conformational
interconversions of several [2]catenanes where the two tethers
between the dipyridyl groups are identical, and thus the
energy barrier for migration of the crown ether along either
pathway would be the same.3,4 As part of our developing
program to control the energetic pathway for changes in
noncovalent interactions, we report herein our initial findings
(3) Amabilino, D. B.; Anelli, P. L.; Ashton, P. R.; Brown, G. R.; Cordova,
E.; Godinez, L. A.; Hayes, W.; Kaifer, A. E.; Philip, D.; Slawin, A. M. Z.;
Spencer, N.; Stoddart, J. F.; Tolley, M. S.; Williams, D. J. J. Am. Chem.
Soc. 1995, 117, 11142-11170.
(4) Anelli, P. L.; Ashton, P. R.; Balardini, R.; Delgado, M.; Gandolfi,
M. T.; Goodnow, T. T.; Kaifer, A. E.; Philip, D.; Pietraszkiewicz, M.; Prodi,
L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.;
Christina, V.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193-218.
† Undergraduate research participants.
(1) (a) Lehn, J.-M. Polym. Int. 2002, 51, 825-839. (b) Brunsveld, L.;
Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. ReV. 2001, 101,
4071-4097.
(2) (a) Guntas, G.; Ostermeier, M. J. Mol. Biol. 2004, 336, 263-273.
(b) Keizer, H. M.; Sijbesma, R. P. Chem. Soc. ReV. 2005, 34, 226-234.
10.1021/ol060550n CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/20/2006

on the conformational interconversions of asymmetrically
tethered [2]catenanes.
To determine the extent to which translation could be
restricted to a predetermined path of our choosing, we
designed a series of [2]catenanes in which sterically bulky
groups could be appended to either of the two different
phenyl ring spacers connecting the two dipyridyl groups.
With a blocking group attached to tether B in Figure 1,
Figure 1. Cartoon representation of [2]catenanes.
translation occurs along pathway A. With A blocked,
interconversion occurs via pathway B. With neither blocking
group present, free translocation would be observed. Extend-
ing the work of Stoddart,5 we have prepared catenanes 1-6
having either the resorcinol-based tether blocked or not
blocked and the xylene-based tether blocked or not blocked
(Figure 2).
The preparation of the two blocking groups is shown in
Scheme 1. Grignard addition of 4-methylphenylmagnesium
bromide to the available ester 76 gave the tertiary alcohol 8
which was reduced7 and then deprotected8 to give substituted
resorcinol 9. Following a known sequence,9 diester 10 was
prepared by a Suzuki coupling, then reduced, and brominated
to give substituted 1,3-bis(bromomethyl)-5-(4-tert-butylphen-
yl)benzene (11).
Figure 2. [2]Catenanes in study.
15a or 15b, 1.2 equiv of 1,3-xylenes or 1,4-xylenes, or 11
and 3 equiv of BPP34C10 in acetonitrile at room temperature
under 1 atm of nitrogen for 4 days. Catenanes 5 and 6 were
similarly prepared using 1,4-di(bromomethyl)benzene. After
solvent removal, the catenanes were isolated by preparative
TLC on silica, initially at 50 °C with 1:1 methanol-ethyl
Scheme 1. Synthesis of Blocking Groups
The preparation of the previously unreported unsubstituted
[2]catenane 1 was accomplished as depicted in Scheme 2.
Resorcinol (12a) or 5-substituted resorcinol 9 was converted
to 1,3-bis(2-hydroxylethyl)benzenes 13a,b which were bro-
minated to the 1,3-bis(2-bromoethyl)benzenes 14a,b whose
treatment with excess 4,4′-dipyridyl6 gave bis(pyridiniums)
15a and 15b. Catenanes 1-6 were formed through three
component reactions between 1 equiv of bis(pyridiniums)
(5) Asakawa, M.; Dehaen, W.; L’abbe, G.; Menzer, S.; Nouwen, J.;
Raymo, F. M.; Stoddart, J. F.; Williams, D. J. J. Org. Chem. 1996, 61,
9591-9595.
(6) Yang, Y.; Escobedo, J. O.; Wong, A.; Schowalter, C. M.; Touchy,
M. C.; Jiao, L.; Crowe, W. E.; Fronczek, F. R.; Strongin, R. M. J. Org.
Chem. 2005, 70, 6907-6912.
(7) Gribble, G. W.; Leese, R. M.; Evans, B. E. Synthesis 1977, 61, 172-
176.
(8) Punna, S.; Meunier, S.; Finn, M. G. Org. Lett. 2004, 6, 2777-2779.
(9) Mu¨ller, W.; Lowe, D. A.; Neijt, H.; Urwyler, S.; Herrling, P. L.;
Blaser, D.; Seebach, D. HelV. Chim. Acta 1992, 75, 855-864.
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Org. Lett., Vol. 8, No. 10, 2006

Scheme 2. [2]Catenane Synthesis
Table 1. Summary of Data and Calculation of Activation
Energy
coalescence
temp (K)a
frequency
difference (Hz)
energy of activation
(kcal/mol)b
catenane
1
2
3
4
5
6
240
255
255
>335
220
240
15.5
22.2
39.3
25.5
19.5
64.8
12.5
13
12.5
>18c
11
12
a Approximated to the nearest 5 °C. b An error of 5 °C in determining
the coalescence temperature corresponds to an error of 0.2 kcal/mol in the
activation energy. c No exchange observed by 2D 1H NMR exchange
experiments.
Catenane 1 having unsubstituted resorcinol and 1,3-xylyl
linkers gave an activation barrier of 12.5 kcal/mol. When
both the resorcinol and xylene groups were substituted in 4,
both pathways were blocked. Because no line broadening
1
was observed in the H NMR spectra up to 60 °C nor was
any evidence for exchange seen in 2D EXSY spectra,11 the
activation barrier for passing over either of these blocking
groups should be significantly higher than 18 kcal/mol. When
just the resorcinol ring was blocked in 3, passage over the
1,3-xylyl ring required 12.5 kcal/mol, whereas a 13 kcal/
mol barrier was measured for passage over the 1,3-bis-
(ethyloxy)benzene tether in 2. Passage along the 1,4-xylyl
linker was significantly more facile than the 1,3-xylyl group
as indicated by the 12 kcal/mol barrier in 6. Apparently,
BPP34C10 has a more difficult time passing over the tighter,
more constricted turn in the 1,3-xylyl tether in 3 than over
the longer, narrower 1,4-xylyl tether in 6. Passage over the
long 3-bis(ethyloxy)benzene in 2 demanded energy require-
ments similar to that of the 1,3-xylyl tether, and both are
significantly more difficult than turning over the 1,4-xylyl
tether.
In summary, through the appropriate incorporation of
blocking groups on one or both of the phenyl linkers, it was
possible to block one or both of the two pathways for
circumrotation in bistable catenanes 1-6. The energy barrier
for passage along a 1,3-bis(ethyloxyl)benzene tether was 13
kcal/mol, more than that for the 1,4-xylyl tether which was
12 kcal/mol and for the 1,3-xylyl tether which was 12.5 kcal/
mol. This study points out an ability to choose different
pathways for conformational changes in noncovalently linked
systems.
acetate to promote dethreading of any pseudorotaxanes and
rapid elution of the crown ether to the top of the plate. A
second elution at room temperature with 7:2:1 methanol-
10% aqueous ammonium chloride-nitromethane4 moved the
catenanes to about 0.3-0.4 Rf and left uncoordinated pyridyls
near the origin of the plate. The silica gel with the catenanes
was removed and extracted with the 7:2:1 solvent system.
The filtrate was concentrated, and aqueous NH4PF6 was
added to precipitate the catenanes as orange to red solids in
19-32% yield.
Two rotational temperature-dependent isomerizations of
related catenanes have been established by Stoddart.3 Typi-
cally, the rotation of a BPP34C10 about a single dipyridyl
group has an energy barrier of approximately 16 kcal/mol
determined using established NMR techniques.10 The energy
barrier for the translocation or circumrotation of the crown
ether from one dipyridyl group to the other typically requires
several kilocalories per mole less energy. Consistent with
1
these prior findings, catenanes 1-6 exhibited H NMR
spectra at room temperature that were in the fast exchange
region for translocation of the crown ether around the second
ring but in an intermediate exchange region for the rotation
of the crown ether about its center. At higher temperatures,
both processes were fast and a single set of averaged signals
was observed for the protons at the 2 and 2′ dipyridyl
positions. Below -40 °C, the 1:1 set of dipyridyl signals
indicated that both exchange processes were in the slow
exchange region. The coalescence temperature and frequency
difference of exchanging sets of signals were used to
calculate the activation barrier for the translocation of the
crown ether between the bistable states.10 The data are
summarized in Table 1.
Acknowledgment. The support for R.L.H. as a DAAD
guest professor at TU-Berlin and the University of Oklahoma
Research Council is appreciated.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL060550N
(11) (a) Heise, J. D.; Raferty, D.; Breedlove, B. K.; Washington, J.;
Kubiak, C. P. Organometallics 1998, 17, 4461-4468. (b) Cobas,
J. C.; Martin-Pastor, M.; EXSYCalc, version 1.0; http://www.
mestrec.comproducto.php?id+9.
(10) Friebolin, H. P. Basic 1D and 2D NMR Spectroscopy; VCH
Publishers: New York, 1991; Chapter 11, pp 271-272.
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