Macrocycle size matters: "Small" functionalized rotaxanes in excellent yield using the CuAAC active template approach

Article abstract of DOI:10.1002/anie.201100415

By shrinking the macrocycle in the CuAAC active template reaction not only is it demonstrated to be possible to use smaller macrocycles, but, surprisingly, that smaller macrocycles lead to higher yields of rotaxane product (see scheme). The synthesis of "

Full text of DOI:10.1002/anie.201100415

DOI: 10.1002/anie.201100415
Rotaxane Synthesis
Macrocycle Size Matters: “Small” Functionalized Rotaxanes in
Excellent Yield Using the CuAAC Active Template Approach**
Hicham Lahlali, Kajally Jobe, Michael Watkinson, and Stephen M. Goldup*
The conceptually novel active template (AT) approach to
mechanically interlocked molecules recently introduced by
Leigh and co-workers relies on a catalytically active metal
center, bound within the cavity of macrocycle, which mediates
the formation of a new covalent bond through the ring.[1]
Much as the development of passive template methods over
the last three decades has allowed the realization of increas-
ingly complex interlocked molecular architectures[2] as well as
prototypical molecular machines[3] such as switches,[4]
motors,[5] and ratchets,[6] the AT approach has the potential
to significantly increase the diversity of synthetically acces-
sible interlocked structures. The AT approach is particularly
synthetically powerful as, in principle, any metal-mediated
bond formation can be adapted for use in the key bond
forming step. Indeed in the six years since the first ATreaction
was reported,[1a] based on the Sharpless–Huisgen–Meldal–
Thus, we set out to ascertain the effect of macrocycle size
in the active template synthesis of rotaxanes using the
CuAAC-AT reaction as a model. The results obtained
indicate that, although size matters, bigger macrocycles are
not always better. Indeed, we demonstrate that this approach
allows ready access to “small” functionalized rotaxanes based
on commercially available materials in consistently excellent
yields.
The size of the macrocyclic cavity was varied by modifying
the length of the alkyl linker between the phenolic oxygens in
bipyridine macrocycles 3, and their efficacy in the CuAAC-
AT reaction was assessed (Scheme 1, Table 1). In the case of
Fokin
copper-catalyzed
azide–alkyne
cycloaddition
(CuAAC) reaction,[7] the concept has been extended to nine
other metal-mediated bond forming reactions, and applied to
the synthesis of molecular shuttles,[1d,e,g,l] catenanes,[1h,i] and
molecules with multiple mechanical bonds.[1d,k]
However, although a number of different combinations of
metal and ligand motif have been investigated and shown to
effectively mediate rotaxane synthesis, all AT reactions
disclosed have employed macrocycles with relatively large
cavities, mandating the use of half-threads bearing extremely
bulky stoppering units (typically substituted trityl moieties).[1]
This prohibits the use of simple derivatives of commercially
available materials and places limitations on the structure and
properties of the rotaxane products, undermining the syn-
thetic utility of the approach. Further, in some applications,
including those where the macrocycle mechanically protects
the thread from the local environment (prodrugs,[8] insulated
molecular wires,[9] and mechanically protected dyes[10]) and
situations in which information transfer between the thread
and macrocycle is desirable (sensing[11] and mechanically
interlocked chiral catalysts[12]), a “tight” fit between macro-
cycle and thread is required.
Scheme 1. Variation of macrocycle size in the bipyridine-mediated
CuAAC-AT reaction.
Table 1: The effect of macrocycle size in the bipyridine-mediated CuAAC-
AT reaction[a]
Entry
Macrocycle
T
[8C]
Consumption
Yield of
isolated 4 [%]
[*] H. Lahlali, K. Jobe, M. Watkinson, Dr. S. M. Goldup
School of Biological and Chemical Sciences
Queen Mary University of London
of 1 [%][b]
1
2
3
4
5
6
3a
3a
3b
3b
3c
3d
25
80
25
80
80
80
>95
>95
<10
>95
>95
>95
85
48
Joseph Priestley Building, Mile End Road, London, E1 4NS (UK)
E-mail: s.m.goldup@qmul.ac.uk
<5[b]
34
[**] We thank Majid Motevalli for assistance with the X-ray crystal
structure of rotaxane 6. This work was supported by the Leverhulme
Trust (Early Career Fellowship to S.M.G.), the Royal Society, and
Q.M.U.L.. S.M.G. is a Royal Society Research Fellow. CuAAC=
copper-catalyzed azide–alkyne cycloaddition.
76
100
[a] Reactions were carried out as outlined in Scheme 1 employing 1 equiv
each of 1, 2, and 3 (0.01m in CH2Cl2) in a sealed CEM microwave vial. See
Supporting Information for further details. [b] Determined by H NMR
analysis of the crude reaction mixture.
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100415.
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Communications
macrocycle 3a (n = 8) which has previously been reported to
Macrocycle 3d is, in principle, small enough to allow the
synthesis of rotaxanes with significantly smaller stoppers than
have previously been reported in the CuAAC-AT reaction
with 3a and we set out to explore this possibility using
representative “small” functionalized half-threads. It should
be noted that, despite the efficacy of macrocycle 3d in the
CuAAC-AT reaction with half-threads 1 and 2, it is not
immediately obvious that rotaxane formation should be
successful with half-threads bearing smaller stoppers: both
the size and shape of the half-thread components may
significantly affect the outcome of the CuAAC-AT reaction.
Indeed, the substituted trityl units may play a role by
shielding one face of the macrocycle during covalent bond
formation (Scheme 1).
Gratifyingly, [2]rotaxane formation was observed in all
cases investigated when smaller functionalized half-threads
were employed (Figure 2). In most cases (5 and 7–9),
reactions employing one equivalent each of alkyne and
azide half-threads resulted in ca. 40% yield of the corre-
sponding [2]rotaxane. Increasing the number of equivalents
of half-threads to five resulted in clean and quantitative
conversion of macrocycle 3d to [2]rotaxane (as judged by
1H NMR analysis of the crude reaction mixture) and excellent
participate efficiently in the CuAAC-AT reaction with half-
threads 1 and 2,[1d] completion is achieved after 72 h at room
temperature resulting in 85% yield of [2]rotaxane 4a
(Table 1, entry 1). The first obvious effect of employing a
macrocycle, 3b (n = 6), with a smaller cavity was to slow the
reaction considerably: after 72 h at room temperature < 10%
of the half-thread components had been converted to triazole
products, although [2]rotaxane 4b was formed despite the low
conversion (entry 3). Raising the temperature to 808C
allowed the reaction to proceed to completion over 72 h to
give [2]rotaxane 4b in 34% yield (entry 4). Repeating the
reaction with 3a at 808C revealed that some but not all of the
reduced yield of [2]rotaxane with macrocycle 3b can be
attributed to the rise in reaction temperature (entry 2).
Reducing the cavity size by a further two methylene units
(macrocycle 3c, n = 4, entry 5) raised the yield of the
interlocked product to 76%. When macrocycle 3d (n = 2)
with only four methylene units between the phenolic ethers
was employed the reaction proceeded in quantitative yield
(entry 6). To our knowledge this is the highest-yielding AT
reaction to date with all of the organic components used in the
reaction being converted with 100% efficiency into inter-
locked product. Unfortunately, attempts to reduce the cavity
size further in this series proved problematic as macrocycles
with shorter alkyl linkers were synthetically inaccessible by
Williamson ether synthesis.
Variation of macrocycle size also leads to large differences
1
between the H NMR spectra of rotaxanes 4a–d (Figure 1).
Most strikingly, the chemical shift of Hg appears > 1 ppm
downfield in 4d compared with 4a, which appears to indicate
[13]
ꢀ
significantly enhanced H-bonding of this polar C H moiety
to the bipyridine nitrogens as the fit between macrocycle and
thread becomes tighter. Secondly, the resonances correspond-
ing to macrocycle protons HD, HE, and HH split into pairs of
diastereotopic signals as the macrocyclic cavity is reduced in
size, indicating that the translational asymmetry of the thread
is more clearly expressed in these more sterically congested,
conformationally and co-conformationally restricted prod-
ucts.
Figure 2. “Small” rotaxanes synthesized using the CuAAC-AT reaction.
Reactions were carried out as outlined in Scheme 1 at 808C. [a] Deter-
mined by 1H NMR analysis of the crude reaction mixture. [b] Yield of
isolated product after column chromatography.
Figure 1. 1H NMR spectra (400 MHz, CDCl3, 300 K) of a) rotaxane 4a,
b) rotaxane 4b, c) rotaxane 4c, and d) rotaxane 4d. Labeling as shown
in Scheme 1.
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isolated yields in all cases. Surprisingly, in the case of the
reaction employing 1 equivalent each of 3,5-di-tert-butyl
phenylazide and 3,5-di-tert-butyl phenylacetylene (the most
sterically demanding of these substrates) a quantitative yield
1
of [2]rotaxane 6 was observed by H NMR spectroscopy and
an excellent isolated yield (99%) obtained.
Slow evaporation of a saturated solution of [2]rotaxane 6
in Et2O/hexane yielded crystals suitable for single-crystal X-
ray analysis (Figure 3).[14] In addition to the anticipated
CH···N hydrogen bonding interaction between the bipyridine
nitrogens and Hd a number of weak CH···p interactions are
observed between HD and HH and the triazole unit. The
crystal structure clearly shows the extremely sterically con-
gested environment around the triazole unit. Indeed, given
the lack of space within the macrocyclic cavity it seems
surprising that the CuAAC reaction occurs at all, let alone in
the excellent yields obtained.
Figure 4. 1H NMR spectra (400 MHz, CDCl3, 300 K) of rotaxanes a) 5,
b) 6, c) 10, and d) and e) expansions (600 MHz, CDCl3) of the
indicated regions of spectrum (c) demonstrating the rotational dis-
symmetry of the macrocycle. Labeling as shown in Figure 2.
resonances observed which can be assigned to the macrocyclic
component compared with 13 in macrocycle 3b.
The “small” rotaxanes synthesized (Figure 2) demon-
strate a number of important advantages of the CuAAC-AT
approach using smaller macrocycles: 1) operationally trivial
and general access to functionalized rotaxanes with half-
threads derived from commercially available materials such
as simple dialkylbenzene units (5 and 6), benzene rings
bearing single-atom-reactive functionalities (7), trialkyl silyl
units (8), simple flurophores (9), and chiral pool materials
(10) without the need to modify them to include bulky
substituted trityl groups with the concomitant increase in
molecular weight and limitations with respect to solubility
that entails; 2) the ability to use simple aryl azides and
alkynes to generate [2]rotaxanes in which the thread is fully
conjugated (6, 7, and 9) with the potential to extend this
methodology to the synthesis of oligomeric insulated molec-
ular wires;[9] 3) 1H NMR and X-ray crystallographic analysis
of the rotaxane products indicate significantly increased
interaction between macrocycle and thread. In the case of
rotaxane 10 this results in efficient transfer of chiral informa-
tion from the thread to the macrocycle, raising the possibility
of using a chiral-pool-derived, enantiopure thread as a
mechanical chiral auxiliary for achiral macrocyclic ligands in
asymmetric catalysis.[12]
In conclusion, we have demonstrated that it is possible to
vary the size of the macrocycle in the CuAAC-AT reaction
and, by reducing the size of the macrocycle, gain access to
functionalized products in excellent yield. Further, employing
the smallest macrocycle investigated, perhaps surprisingly,
leads to the most efficient active template reaction reported
to date in which an equimolar mixture of macrocycle and half-
thread components leads to a quantitative yield of [2]rotax-
ane product. Work is currently underway to apply these
findings to the synthesis of functional materials and devices.
We are also investigating the effect of macrocycle size on
other AT reactions not only in order to increase their
synthetic utility, but also to determine if the size dependency
of the reaction yield can shed any light on the underlying
mechanism of the process.
Figure 3. Single-crystal X-ray structure[14] of [2]rotaxane 6 viewed along
the thread axis a) in capped sticks representation with selected close
contacts indicated (majority of H atoms omitted for clarity) and b) in
partial space-fill representation. Selected bond lengths (in ꢀ): N1–H31
2.57, N2–H31 2.49, C31–H23 2.99, C31–H12 2.80, N4–H1 2.65,
N4–H4 2.48, N5–H1 2.84, N5–H4 2.65.
1
The H NMR spectra of rotaxanes 5–10 reveal a number
of interesting features (partial 1H NMR spectra for rotaxanes
ꢀ
5, 6, and 10 are shown in Figure 4). Firstly, the triazole C H,
which is already significantly shifted downfield relative to the
free thread in rotaxane 5 (Dd = 0.8 ppm), resonates even
further downfield in rotaxanes 6–10. In the case of rotaxane 6,
Hd appears at 10.5 ppm—a Dd on interlocking of 2.5 ppm
(Figure 2b). Secondly, the desymmetrization of the two faces
of the macrocycle due to the mechanical bond is clearly
visible in the 1H NMR spectra of rotaxanes 5–10, as had
previously been observed for rotaxane 4d. The chiral glucose-
based stopper in rotaxane 10 leads to an even more
complicated 1H NMR spectrum as the macrocycle is now
completely desymmetrized by the mechanical bond: while
macrocycle 3d displays 8 separate 1H resonances, rotaxane 10
exhibits 23 which can be assigned to the macrocyclic
component, indicating a high degree of chiral information
transfer between thread and macrocycle. The same effect is
evident in the 13C NMR spectrum of rotaxane 10 with 25
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Communications
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Experimental Section
Synthesis of [2]rotaxanes 4 and 5–10: Macrocycle (0.025 mmol),
CuPF6·(MeCN)4 (8.4 mg, 0.0225 mmol), and the required equivalents
of azide and acetylene half-threads were combined in CH2Cl2
(2.5 mL) in a sealed CEM microwave vial which was purged with
N2 and then heated at 808C for 72 h. The reaction mixture was diluted
with CH2Cl2 (50 mL) and washed with a saturated solution of
ethylenediaminetetraacetic acid (EDTA) in 17.5% aqueous NH3
(50 mL). The aqueous phase was extracted with CH2Cl2 (2 ꢀ
50 mL), the organic extracts dried (Na2SO4), and the solvent removed
in vacuo. Chromatography (gradient elution: 1:1 CH2Cl2/petrol and
0!10% MeCN) gave the [2]rotaxane product.
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Received: January 17, 2011
Published online: April 1, 2011
Keywords: click chemistry · macrocycles · rotaxanes ·
.
self-assembly · template synthesis
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[14] The crystal data and details of the structural refinement for
rotaxane are provided in the Supporting Information.
6
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