Magic ring rotaxanes by olefin metathesis

Article abstract of DOI:10.1002/anie.200351167

Trick or treat? Ruthenium alkylidene catalyzed ring-closing metathesis of crown ether like diene substrates around a dumbbell-shaped secondary ammonium ion affords [2]rotaxanes. The reversible nature of this process has been demonstrated through a magic ring synthesis, wherein the preformed olefinic macrocycle and dumbbell-shaped component equilibrate to form the hydrogen-bond-stabilized [2]rotaxane in the presence of a metathesis catalyst (see scheme).

Full text of DOI:10.1002/anie.200351167

Angewandte
Chemie
Reversible Rotaxane Synthesis
Magic Ring Rotaxanes by Olefin Metathesis**
Andreas F. M. Kilbinger, Stuart J. Cantrill,
Andrew W. Waltman, Michael W. Day, and
Robert H. Grubbs*
Rotaxanes[1] are mechanically interlocked molecules com-
posed of a dumbbell-shaped component around which one or
more macrocycles are trapped. In some cases, the ability to
move—in a controlled fashion—these intimate noncovalently
linked components with respect to one another, has led to the
creation of molecular switches[2] which have had a significant
impact on the emerging field of molecular computing.[3]
Although the majority of rotaxane syntheses to date have
relied upon kinetic control,[4] that is, employing an irreversible
final reaction step, a resurgence in the popularity of dynamic
covalent chemistry[5] has spurred an interest in pursuing
rotaxane formation under thermodynamic control.[6] Rever-
sible reactions[7] utilized for this purpose include the forma-
tion (and subsequent associated cleavage) of functional
groups such as imines[8] and disulfides.[9] Olefin metathesis,
mediated by functional-group-tolerant ruthenium alkylidene
catalysts,[10] has been used in the thermodynamically con-
trolled synthesis of a [2]catenane,[8d,11] and, more recently, has
been applied[12] to the anion-templated synthesis of a
[2]rotaxane. One of the most established[13] supramolecular
synthons employed in the construction of rotaxanes exploits
the mutual recognition exhibited by secondary dialkylammo-
nium (R2NH2 ) ions and suitably sized crown ethers,[14] most
+
notably dibenzo[24]crown-8 (DB24C8). Here, we combine
[*] Prof. R. H. Grubbs, Dr. A. F. M. Kilbinger, Dr. S. J. Cantrill,
A. W. Waltman
Arnold and Mabel Beckman Laboratory of Chemical Synthesis
Division of Chemistry and Chemical Engineering
Pasadena, CA 91125 (USA)
Fax: (+1)626–564–9297
E-mail: rhg@caltech.edu
Dr. M. W. Day
Beckman Institute
X-Ray Crystallography Laboratory
California Institute of Technology
Pasadena, CA 91125 (USA)
[**] A.F.M.K. thanks the Alexander von Humboldt Foundation for a
Feodor–Lynen Research Fellowship. We thank Dr. Mona Shahgholi
for performing the mass spectrometric analyses, Dr. Jennifer Love
for assisting with initial NMR spectroscopic measurements, and Dr.
Steven Goldberg for valuable comments regarding this manuscript.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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DOI: 10.1002/anie.200351167
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1
the reliability of this supramolecular synthon with the
above. Indeed, subsequent H NMR spectroscopic investiga-
versatile reversible ring-closing-metathesis (RCM) reaction
to form rotaxanes under thermodynamic control.
tions revealed that 8 does interact with dibenzylammonium
hexafluorophosphate (DBA·PF6) to form a 1:1 complex with
a threaded geometry, that is, a pseudorotaxane. The associ-
ation constant (Ka)[16] for this process is 100mꢀ1 (CD3CN,
295 K), which is slightly lower than the value of 320mꢀ1
reported[17] for DB24C8 under similar conditions. The major
contributing factor[18] to this reduced binding affinity arises
presumably from the “loss” of two ether oxygen atoms—on
going from DB24C8 to 8—thereby decreasing the hydrogen-
bonding potential between the two components.
+
In an effort to mimic the significant R2NH2 ion binding
capability of DB24C8, our first target macrocycle was
designed such that it retained many of the desirable features
of the original crown ether. Pentaethylene glycol (3) was
bisalkylated with 5-bromo-1-pentene (5) to afford (Scheme 1)
Encouraged by these findings, the RCM reaction of the
terminal diolefin 6 was repeated (Scheme 1) in the presence
of a dumbbell-shaped template containing an ammonium ion,
namely bis(3,5-dimethoxybenzyl)ammonium hexafluoro-
phosphate (10·PF6),[19] to afford, in 73% yield, the corre-
sponding [2]rotaxane 11·PF6 as a mixture of E and Z isomers.
The significant templating effect of the R2NH2+ ion can be
appreciated by comparing this reaction with the untemplated
macrocyclization of 6 to form 8. Whereas the untemplated
reaction is carried out at about 5 mm (to avoid oligo/polymer-
ization) and yields only 48% of the desired macrocycle, the
templated reacton can be performed at much higher concen-
trations (ca. 100 mm in this case) to give 73% of the ring-
closed product, namely the rotaxane. Subsequent character-
ization by 1H and 13C NMR spectroscopy, mass spectrometry
(electrospray), and single-crystal X-ray analysis[20] (Figure 1)
Scheme 1. Synthesis of the olefin macrocycles 8 and 9, via their corre-
sponding diolefin precursors 6 and 7, respectively. Rotaxane synthesis
can be achieved through either a ring-closing-metathesis approach, by
utilizing 6 and 7 as starting materials (lower left pathway), or by a
magic ring synthesis in which the preformed macrocycles 8 and 9 are
employed (lower right pathway).
Figure 1. Ball-and-stick representation of the solid-state structure of
the [2]rotaxane 11·PF6. Hydrogen-bonding geometries [N+···O],
the terminal diolefin 6 in moderate yield. Treatment of 6 with
+
ꢀ
[H···O] [Š], [N H···O] [8]: a) 2.86, 2.04, 156; b) 2.87, 2.08, 142; c) 3.03,
[15]
¼
(PCy3)2(Cl)2Ru CHPh (1), under dilute conditions, gave
ꢀ
2.27, 137; [C···O], [H···O] [Š], [C H···O] [8]: d) 3.35, 2.89, 109; e) 3.41,
the 24-membered olefinic crown ether analogue 8 as a
mixture of E and Z isomers. The constitution of 8 is strikingly
similar in nature to the macrocyclic skeleton of DB24C8,
thereby satisfying our primary design criterion outlined
2.48, 161; f) 3.34, 2.88, 110; g) 3.53, 2.66, 152.
confirmed unambiguously the interlocked nature of 11·PF6.
The cluster of peaks in the carbon–carbon double bond region
of the final difference map, as well as displacement ellipsoids,
some anomalous bond lengths, and the high R and GOF val-
ues indicate a degree of conformational disorder of this
hydrocarbon loop and suggest that a mixture of E and
Z isomers are present. No attempt was made to model this
disorder. Subsequent hydrogenation (H2/Pd/C) of 11·PF6 gave
the saturated rotaxane 13·PF6 (see Supporting Information),
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in which the two olefin isomers collapsed into a single species,
thereby facilitating the spectroscopic analysis.
solution[25] containing equimolar quantities of crown ether 8
and the dumbbell-shaped compound 10·PF6 was prepared.
1
Derivatization of the crown ether, to allow for the
construction of extended supramolecular arrays,[21] is most
easily achieved through a benzo ring that is fused to the
parent macrocycle. The benzo analogues of both 6 and 8 were
prepared (Scheme 1) in a similar fashion. Alkylation of the
known[22] diol 4 afforded the diolefin 7, which was subse-
quently converted into the desired benzocrown ether ana-
logue 9 through a RCM reaction mediated by 1. The Ka value
for the interaction between 9 and DBA·PF6 was shown[23] to
be about 10mꢀ1 (CD3CN, 295 K), approximately one order of
magnitude lower that that observed for 8. This decreased
affinity for ammonium ion binding can be attributed to the
reduced hydrogen-bonding ability of the benzo-substituted
macrocycle, namely, two of the oxygen atom donors are now
phenolic in nature and are consequently less basic. Nonethe-
less, the benzo-substituted rotaxane 12·PF6 (E/Z mixture) was
obtained (Scheme 1) upon the RCM reaction of 7 in the
presence of 10·PF6, albeit in a reduced yield (30% instead of
73%). In addition to NMR spectroscopic and mass spectro-
metric characterization, X-ray quality single crystals[24] of this
compound (Figure 2) were obtained upon slow evaporation
of an EtOAc solution. The crystal structure of 12·PF6 contains
both E and Z isomers, which are present at the same site in an
approximately 1:1 ratio. Reasonable geometries in this
disordered region were only obtained with a number of
restraints. Once again, the E/Z mixture contributes to the
high R and GOF values obtained.
Prior to the addition of any catalyst, H NMR spectroscopic
analysis revealed that, as expected, the terminally bulky-
substituted secondary ammonium cation 10+ cannot pass
through the cavity of the 24-membered macrocyclic ring.
Enabling the metathesis pathway, however, upon the addition
[26]
¼
of (IMesH2)(PCy3)(Cl)2Ru CHPh (2) (10 mol%), allows
the system to equilibrate to a thermodynamic minimum (a
+
ꢀ
ꢀ
process driven by the creation of N H···O and C H···O
hydrogen bonds), and results in the formation of the
[2]rotaxane 11·PF6. This transformation was monitored
(Figure 3) by 1H NMR spectroscopy, and equilibrium
Figure 3. Partial 1H NMR spectra showing the change over time in the
environment of Ha and Hb as the dumbbell (D) component converts
into the rotaxane (R) during the magic ring synthesis in which macro-
cycle 8 and 10·PF6 were employed as starting materials.
(> 95% interlocked species) was achieved in 45 minutes.
Spectral assignments were made by comparison to an
authentic sample of the rotaxane isolated previously in the
RCM approach, and mass spectrometric analysis also con-
firmed that the major component in solution was, indeed, the
[2]rotaxane 11·PF6.
In an analogous fashion, the magic ring experiment was
repeated with the benzo-derivative 9 and resulted (Figure 4)
in a similar outcome. In this case, the equilibrium position is
reached in less time (ca. 20 min), and affords a smaller
proportion of the interlocked species (ca. 85% of the
[2]rotaxane 12·PF6), which presumably reflects the reduced
affinity of this particular macrocycle for secondary dibenzyl-
ammonium ions. No decrease of catalyst activity was
observed over the duration of the magic ring syntheses
described above, as indicated by the undiminished intensity of
the carbene (benzylidene) singlet of the catalytic species (d =
Figure 2. Ball-and-stick representation of the solid-state structure of
the benzo-substituted [2]rotaxane 12·PF6. Hydrogen-bonding geome-
+
+
ꢀ
tries [N ···O], [H···O] [Š], [N H···O] [8]: a) 3.10, 2.55, 119; b) 3.03,
2.14, 162; c) 3.08, 2.55, 117; d) 2.88, 1.98, 167; [C···O], [H···O] [Š],
[C-H···O] [8]: e) 3.14, 2.53, 120; f) 3.41, 2.56, 143; g) 3.48, 2.53, 161.
To demonstrate the inherent reversibility of rotaxane
formation by olefin metathesis, a “magic ring”[11a] experiment
was performed in which the rotaxane was assembled
(Scheme 1) directly from its preformed components, namely,
the dumbbell and the macrocycle. A CD2Cl2/CD3NO2 (80:20)
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systems to almost quantitative conversion allows for the
pursuit of high-molecular-weight polymers with novel inter-
locked architectures.
Experimental Section
General procedure for “magic ring” experiments: Catalyst 2 (0.8 mg,
10ꢀ3 mmol) was added under a dry N2 atmosphere to a solution of
unsaturated crown ether 8 or 9 (10 mmol) and 10·PF6 (4.6 mg,
10ꢀ2 mmol) in CD2Cl2/CD3NO2 (80:20, 1.0 mL). The reaction was
followed by 1H NMR spectroscopy under ambient conditions. For all
other experimental procedures and characterization data, see the
Supporting Information.
Received: February 13, 2003 [Z51167]
Published online: July 3, 2003
Keywords: macrocycles · metathesis · rotaxanes · ruthenium ·
.
template synthesis
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Figure 4. Partial 1H NMR spectra showing the change over time during
the magic ring synthesis in which macrocycle 9 and 10·PF6 were
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d=5.45 ppm, corresponding to the olefinic protons of the free crown
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metathesis catalyst 2, and, under this particular set of
conditions, the transformations occur with very high yields
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tative numbers.
[6] For the earliest reported examples of a rotaxane synthesis that
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In conclusion, diolefin macrocyclic precursors have been
shown to cyclize around an appropriately substituted diben-
zylammonium salt to produce the corresponding [2]rotaxanes.
Exploiting ruthenium carbene mediated olefin metathesis in
this “clipping” procedure allows for the assembly process to
occur in a reversible fashion, namely, it can operate under
thermodynamic control. This dynamic feature can be utilized
to synthesize mechanically interlocked molecules under
magic ring conditions, whereby preformed components can
be exposed to the necessary catalytic agent (1 or 2 in this case)
causing the macrocycle to open and close repeatedly while the
system strives to reach a thermodynamic minimum. The
successful application of this methodology to the synthesis of
simple [2]rotaxanes, has enabled the creation of other, more
intricate, interlocked molecular architectures (for example,
catenanes,[1] daisy chains,[21] and other interlocked polymeric
systems) to be explored. Moreover, the ability to drive these
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Sauvage, R. H. Grubbs, J. Org. Chem. 1999, 64, 5463 – 5471,
respectively.
(distance, planarity, and anisotropic displacement parameter
restaints in the macrocylic double bond region which was
disordered ca. 1:1 E:Z), and 582parameters; R(6838 reflections
> 2s(I), all data) = 0.063, 0.090; Rw(6838 reflections > 2s(I), all
data) = 0.105, 0.107; S = 3.48; all hydrogen atoms were riding on
attached atoms; greatest final electron density difference
[12] J. A. Wisner, P. D. Beer, M. G. B. Drew, M. R. Sambrook, J. Am.
Chem. Soc. 2002, 124, 12469 – 12476. During the course of our
investigations, this report of rotaxane formation, by using ring-
closing-olefin metathesis, appeared in the literature. Although
the authors note that the metathesis reaction is under thermo-
dynamic control, this aspect of the reaction is not investigated
further.
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3
excursions of 0.78 e·Š and ꢀ0.82e·Š. [28]
[25] CD3NO2 is required to ensure complete dissolution of 10·PF6.
[26] IMesH2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene, see M.
Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1,
953 – 956.
[27] G. M. Sheldrick, SHELXL-97, Program for Structure Refine-
ment, University of Göttingen, Göttingen (Germany), 1997.
[28] CCDC 188147 (11·PF6) and CCDC 188995 (12·PF6) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12Union Road, Cambridge CB12EZ, UK; fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk). Structure fac-
tors are available from the authors via email:xray@caltech.edu.
[16] The association constant for this slowly equilibrating system was
determined using the single-point method, see J. C. Adrian, C. S.
Wilcox, J. Am. Chem. Soc. 1991, 113, 678 – 680. It should be
noted that, in this case, the Ka value reflects a total association
constant, which combines the contributions from both the cis
and trans isomers of the crown ether present in solution.
[17] P. R. Ashton, R. A. Bartsch, S. J. Cantrill, R. E. Hanes, Jr., S. K.
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[18] Hydrogenation of 8 affords the saturated crown ether 15, which
1
was shown (by H NMR spectroscopy) to bind dibenzylammo-
nium hexafluorophosphate with a Ka value of 90mꢀ1. This value
differs only slightly from that observed for 8, thus suggesting that
the carbon–carbon double bond has little effect upon the ability
to bind secondary ammonium ions.
[19] S. J. Cantrill, M. C. T. Fyfe, A. M. Heiss, J. F. Stoddart, A. J. P.
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[20] Crystal data for 11·PF6: [C36H58NO10][PF6]; Mr = 809.80; 0.19 ”
0.20 ” 0.24 mm; colorless; monoclinic; spave group P21/n
(no. 14),
Z = 4;
a = 13.3814(7),
b = 19.3278(10),
c =
16.1791(8) Š, b = 109.105(1)8; V= 3594.0(4) Š3; 1calcd = 1.360
gcmꢀ3; MoKa radiation (l = 0.71073 Š); 2qmax = 56.88; w-scans
on a SMART 1000 ccd; T= 98 K; 67646 reflections measured
(all 9326 unique used); ꢀ17 ꢁ h ꢁ 17, ꢀ25 ꢁ k ꢁ 25, ꢀ21 ꢁ L ꢁ
20; Lorentz factor but no absorption correction (m = 0.15 mmꢀ1
)
applied; structure solved and refined with full-matrix least-
squares on F2 with SHELXL-97[27]; 9326 data, 0 restraints, and
583 parameters; R(6046 reflections > 2s(I), all data) = 0.070,
0.107; Rw(6046 reflections > 2s(I), all data) = 0.109, 0.113; S =
2.62; hydrogen atoms on the macrocyle were riding, while those
on the cation were unrestrained; greatest final electron density
3
difference excursions of 1.41 e·Š at 1.02Š from H(11A) and
3
[28]
ꢀ1.36 e·Š at 0.42Š from C(12).
[21] S. J. Cantrill, G. J. Youn, J. F. Stoddart, D. J. Williams, J. Org.
Chem. 2001, 66, 6857 – 6872.
[22] B. Colonna, L. Echegoyen, Chem. Commun. 2001, 1104 – 1105.
[23] Hydrogenation of olefin macrocycle 9 affords the saturated
analogue 16, which shows little change in the binding affinity for
DBA·PF6 (Ka ~ 10mꢀ1).
[24] Crystal data for 12·PF6: [C40H58NO10][PF6]; Mr = 857.84; 0.24 ”
0.31 ” 0.33 mm; colorless; monoclinic; space group P21/n
(no. 14), Z = 4; a = 14.3407(6), b = 19.2063(8), c = 15.8720(7) Š,
b = 107.353(1)8; V= 4172.7(3) Š3; 1calcd = 1.366 gcmꢀ3; MoKa
radiation (l = 0.71073 Š); 2qmax = 55.48; w-scans on
a
SMART 1000 ccd; T= 98 K; 83324 reflections measured (all
9644 unique used); ꢀ18 ꢁ h ꢁ 18, ꢀ25 ꢁ k ꢁ 24, ꢀ21 ꢁ L ꢁ 21;
Lorentz factor but no absorption correction (m = 0.15 mmꢀ1
)
applied; structure solved and refined with full-matrix least-
squares on F2 with SHELXL-97[27]; 9644 data, 235 restraints
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