Guest-induced unidirectional dual rotary and twisting motions of a spiroborate-based double-stranded helicate containing a bisporphyrin unit

Article abstract of DOI:10.1002/anie.201302560

Influential guests: The intercalation of an electron-deficient aromatic guest (shown in red) between the two porphyrin rings of an optically active, porphyrin-linked double-stranded spiroborate helicate triggered rotary motion of the porphyrin rings in one direction in conjunction with a unidirectional twisting motion of the spiroborate helix. This system has potential for the development of chirality-responsive molecular machines. Copyright

Full text of DOI:10.1002/anie.201302560

Angewandte
Chemie
Communications
Host–Guest Systems
Influential guests: The intercalation of an
electron-deficient aromatic guest (shown
in red) between the two porphyrin rings of
an optically active, porphyrin-linked
double-stranded spiroborate helicate
triggered rotary motion of the porphyrin
rings in one direction in conjunction with
a unidirectional twisting motion of the
spiroborate helix. This system has
S. Yamamoto, H. Iida,
E. Yashima*
&&&& — &&&&
Guest-Induced Unidirectional Dual
Rotary and Twisting Motions of
a Spiroborate-Based Double-Stranded
Helicate Containing a Bisporphyrin Unit
potential for the development of chirality-
responsive molecular machines.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201302560
Host–Guest Systems
Guest-Induced Unidirectional Dual Rotary and Twisting Motions of
a Spiroborate-Based Double-Stranded Helicate Containing
a Bisporphyrin Unit**
Shinya Yamamoto, Hiroki Iida, and Eiji Yashima*
The development of molecular systems that enable the
control of molecular movements, in particular, rotary and
elastic (extension–contraction) motions triggered by external
stimuli, is an urgent and expanding research area in supra-
molecular chemistry and nanotechnology owing to their
potential application to molecular machines, such as molec-
ular motors and springlike devices.[1] Although sophisticated
biological molecular motors and springs exist in every muscle
cell and function cooperatively to generate force and move-
ment for muscle contraction by converting chemical energy
into mechanical functions,[2] the control of unidirectional
motions that further amplify to macroscopic movements with
artificial molecular and helical systems remains a chal-
lenge.[3,4]
Thus, no ion-triggered springlike motion was observed for 1b
and 2.
With the aim of the further development of molecular
springs that twist unidirectionally and perform intelligent
functions, we designed and synthesized the porphyrin-linked,
double-stranded spiroborate helicate 3aNa (Figure 1c). We
2
anticipated that the linker porphyrins introduced in the
middle of the strands could stack face-to-face in a helical
orientation and thus provide a chiral cavity for the further
inclusion of electron-deficient aromatic guests, such as G1
and G2, by an induced-fit mechanism.[9] We hypothesized that
the expansion of the bisporphyrin cavity upon the encapsu-
lation of guests would be accompanied by a rotation of the
porphyrin rings in one direction and a corresponding unidir-
ectional twisting of the spiroborate helix. Thus, the helicate
should be able to perform dual unidirectional motions
induced by guest encapsulation (Figure 1c). To the best of
our knowledge, such stimuli-responsive unidirectional dual
motions (rotation and twisting) that take place simultane-
ously have not been reported previously.
Recently, we reported a unique optically active double-
stranded helicate 1a in which two ortho-linked tetraphenol
strands with a biphenylene unit in the middle were bridged by
two spiroborates that sandwiched a Na+ ion between them
(Figure 1a).[5] The optically active helicate underwent
sodium-ion-triggered,
reversible
extension–contraction
motion coupled with a twisting motion in one direction
upon the release and binding of the Na+ ion (Figure 1b).[5a]
During this unidirectional springlike motion, the helicate
maintained its helical handedness.[5a] Such ion-induced molec-
ular motions are key processes in a number of biological
events and reminiscent of biological machines that operate in
muscle cells.[6] Biphenol (in 1b)[7] and bipyridine groups (in
2)[8] could also be used as linker units to connect the two outer
biphenol units and produce optically active double-stranded
helicates similar to 1a after conventional optical resolution;
however, Na+ ions coordinated in the center of helicates 1b
and 2 could not be released in the presence of [2.2.1]cryptand,
which had been used to remove the Na+ ion from 1a, because
the Na+ ions were strongly bound by the biphenol oxygen and
bipyridine nitrogen atoms as well as the spiroborate anions.
We first synthesized two porphyrin-containing tetraphe-
nols 6a,b linked by m- and p-phenylene units by Suzuki–
Miyaura coupling of the corresponding m- and p-meso-
di(iodophenyl)porphyrins 4a,b with a boronic acid ester 5
(Scheme 1; see also Schemes S1 and S2 in the Supporting
Information). The tetraphenols 6a and 6b were then treated
with an equimolar amount of NaBH4 in [D3]acetonitrile at
808C for 24 h under similar conditions to those used for the
synthesis of 1 and 2.[5,7,8] The boron helicate 3aNa was
2
obtained almost quantitatively (99% yield), whereas the
reaction of 6b gave none of the desired helicate, but produced
a complex mixture of products. We attempted to obtain 3bNa
2
from 6b by using NaBH4 in various solvents at different
temperatures, without success (see Table S1 in the Supporting
Information): the p-phenylene linkage of 6b probably ham-
pers spiroborate formation considerably owing to steric
hindrance. In contrast, the m-phenylene unit was favorable
as a linker for the formation of a porphyrin-containing
intertwined double helicate owing to its bent structure. The
[*] S. Yamamoto, Dr. H. Iida, Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
helicate 3aNa was fully characterized by 1H, 13C, and 2D NMR
2
spectroscopy and electrospray ionization (ESI) mass spec-
trometry (Figure 2; see also Figures S1a–S6 in the Supporting
Information).
E-mail: yashima@apchem.nagoya-u.ac.jp
Homepage: http://helix.mol.nagoya-u.ac.jp/
X-ray crystallographic analysis unambiguously revealed
[**] This research was supported in part by a Grant-in-Aid for Scientific
Research (S) (E.Y.) and a Grant-in-Aid for Young Scientists (B) (H.I.)
from the Japan Society for the Promotion of Science (JSPS).
that 3aNa adopted a double-stranded helical structure with
2
pseudo-D2 symmetry in which the two tetraphenol strands
were intertwined with one another through two spiroborate
bridges, and the two porphyrin units were stacked face-to-face
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302560.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
at a distance of 4.1 ꢀ and with
a torsional angle of 718 with respect
to the two meso–meso porphyrin
axes (Figure 3a). The two terminal
benzene rings of each tetraphenol
strand were twisted by 3688 with
respect to one another, and the
double-helical structure around the
spiroborate bridges was similar to
that of the contracted form of the
biphenylene-linked spiroborate hel-
icate 1a (Figure 1a).[5a] Because of
the large porphyrin linkers, the two
negatively charged spiroborate resi-
dues were located far away from
each other with a B–B distance of
15.4 ꢀ, which is much larger than the
B–B distance in the contracted
(6.0 ꢀ)
and
extended
forms
(13.0 ꢀ) of 1a.[5a] As a result, the
two Na+ ions were not included in
the helicate cavity, but were coordi-
nated to the two oxygen atoms of the
spiroborate moieties on the outside
of the structure (Figure 3a). This key
structural feature of 3aNa suggested
2
that the ion-triggered springlike
extension and contraction motion
observed in 1a may not be antici-
pated upon sodium-ion release and
binding (see below).
1
The H NMR spectrum of 3aNa
2
in [D3]acetonitrile agrees well with
the pseudo-D2-symmetric structure
determined by X-ray crystallo-
graphic analysis in the solid state.
The signals for the aromatic hydro-
gen atoms labeled l and k on the
terminal benzene ring Awere shifted
upfield to a considerable extent from
Figure 1. a) Structures of Na+-included helicates 1a, 1b, and 2. b) Schematic illustration of the
unidirectional twisting of 1a upon Na+-ion release and binding. c) Structures of porphyrin-
containing helicates 3a and 3b and the unidirectional dual rotary and twisting motions of 3a upon
guest inclusion.
1
their positions in the H NMR spec-
trum of 6a as a result of the ring-
current effect of the benzene rings
in the other strand (Figure 2a,b).[5a]
Moreover, significant upfield shifts
of the signals due to the porphyrin
hydrogen atoms in the 1H NMR
spectrum (Figure 2a,b) and the
characteristic blue shift of the
Soret band in the absorption spec-
trum (see Figure S7a) suggested the
close proximity of the two linker
[10,11]
porphyrin rings of 3aNa
.
Fur-
2
thermore, 2D COSY and ROESY
experiments showed clear cross-
peaks between the hydrogen atoms
on phenol ring A, those on the
meso-phenylene ring C, and the
porphyrin hydrogen atoms (see Fig-
Scheme 1. Synthesis of the boron helicate 3aNa and its optical resolution by the formation of
2
a diastereomeric salt with optically pure (R,R)- or (S,S)-7+·BrÀ. The chiral ammonium (7) was then
replaced with the achiral ammonium salt TBA+·BrÀ.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
spectrum of 3aNa upon the sequential addition of [2.2.1]crypt-
2
and and Na+PF6
.
À
The optical resolution of 3aNa was then performed by
2
diastereomeric salt formation as reported previously.[5a,7]
Thus, the Na+ cations were exchanged for enantiomerically
pure (R,R)- and (S,S)-7+·BrÀ to yield the diastereomeric salts
(+)-3a[(R,R)-7] and (À)-3a[(S,S)-7] , respectively, as crystals
2
2
(Scheme 1; the prefixes (+) and (À) denote the signs of the
Cotton effect at 418 nm). The obtained diastereomers were
further converted into the corresponding enantiomers
through cation exchange with achiral tetrabutylammonium
bromide (TBA+ BrÀ) to afford a pair of enantiomers (+)- and
(À)-3aTBA . The ee values of (+)- and (À)-3aTBA were
2
2
1
determined to be above 99% on the basis of the H NMR
spectra of the enantiomers in the presence of (1R,2S)-N-
benzyl-N-methylephedrinium bromide as
a chiral shift
reagent (see Figure S8).
The enantiomers (+)- and (À)-3aTBA showed perfectly
2
mirror imaged CD spectra in the m-phenylene (200–330 nm)
and characteristic porphyrin chromophore regions (the Soret
band and Q bands centered at 418 and 503, 539, 576, and
631 nm, respectively) in CH3CN (Figure 4). Interestingly, (+)-
Figure 2. Partial 1H NMR spectra (500 MHz, RT, CD3CN, 0.5 mm) of
a) 6a, b) 3aNa , c) 3aNa +G1 (0.5 equiv), d) 3aNa +G1 (1 equiv), and
2
2
2
e) G1. For the full assignment of signals, see Figures S2–S6 in the
Supporting Information.
Figure 4. CD and absorption spectra (CH3CN, 0.02 mm, 258C) of
(+)-3aTBA , (À)-3aTBA , (+)-3aTBA ·G1, and (À)-3aTBA ·G1. The inset CD
2
2
2
2
spectra were measured at a concentration of 0.5 mm.
and (À)-3aTBA exhibited positive and negative bisignate
2
Cotton effects, respectively, centered close to the maximum of
the porphyrin Soret band. These Cotton effects enabled us to
determine the twist sense of the bisporphyrins on the basis of
the exciton-coupled CD method:[12,13] the two linker porphy-
rin rings of (+)- and (À)-3aTBA are probably twisted with
2
clockwise and counterclockwise orientations, respectively,
with respect to the two meso–meso porphyrin axes (Fig-
ure 3a). If this conclusion is correct, the helical sense of
(+)-3aTBA and (À)-3aTBA can be further rationally assigned
Figure 3. X-ray crystal structures of a) 3aNa and b) 3aNa ·G1. A right-
handed double-helical structure is depicted; all hydrogen atoms and
solvent molecules are omitted for clarity.
2
2
2
2
as left- and right-handed, respectively, on the basis of the
molecular structure determined by X-ray crystallography for
the racemate of 3aNa (Figure 3a).
2
ures S4–S6). These cross-peaks indicated that the helicate
retains the intertwined double-stranded helical structure in
solution. Owing to the absence of Na+ ions in the helicate
cavity, reversible changes due to an ion-triggered extension–
contraction motion[5a] were not observed in the 1H NMR
Next, we investigated the ability of the bisporphyrin-
linked helicate to include aromatic guests and explored the
possibility of further unidirectional dual motions triggered by
guest intercalation between the porphyrin units by the use of
NMR, absorption, CD, and fluorescence spectroscopy and X-
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
ray analysis. Upon the addition of an electron-deficient
intertwined, double-stranded helical structure (see Figur-
aromatic guest G1 (0.5 equiv) to a solution of 3aNa , new
es S13–S18), as evidenced by the clear ROE cross-peaks
between the proton signal for the guest (x) and the proton
signals for hydrogen atom g on meso-phenylene ring C and
the porphyrin hydrogen atoms b’ and c’.
2
signals attributed to the formation of 3aNa ·G1 appeared in the
2
1H NMR spectrum (Figure 2c). The further addition of G1
(1.0 equiv) quantitatively produced the 1:1 inclusion complex
3aNa ·G1 (Figure 2d). The signal for G1 was significantly
We anticipated that guest encapsulation and the subse-
quent unidirectional dual rotary and twisting motions would
induce chiroptical changes in the helicate on the basis of the
crystal structures of 3aNa and 3aNa ·G1. In fact, the CD
2
shifted upfield from 8.8 to 3.9 ppm after encapsulation in the
bisporphyrin cavity owing to the ring-current effect of the
bisporphyrin (Figure 2d,e).[9a–c,e,f,h] The formation of the 1:1
2
2
complex 3aNa ·G1 was also confirmed by ESI mass spectrom-
spectra of left-handed helical (+)- and right-handed helical
2
etry (see Figure S1b) and a continuous variation plot (Job
plot; see Figure S9a). The fluorescence titration data com-
bined with the Job plot provided a remarkably high associ-
(À)-3aTBA ·G1 were different from those of (+)- and (À)-
2
3aTBA , respectively. In particular, the Cotton effect intensities
2
around the Soret band and the m-phenylene chromophore
regions were significantly and cooperatively enhanced upon
sandwich formation with G1, since the rotary motion of the
porphyrin rings in one direction led to exciton coupling and
ation constant (Ka = (2.2 Æ 0.3) ꢁ 109 mÀ1) of 3aNa with G1 in
2
CH3CN at 258C (see Figure S9).[14] The analogous electron-
deficient aromatic guest G2 was also included between the
porphyrins of 3aNa by intercalation with a smaller Ka value
a unidirectional twisting of the spiroborate helicate
(Figure 4). Similar guest-induced chiroptical changes were
2
((2.5 Æ 0.2) ꢁ 105 mÀ1; see Figure S10 and S11). The absorption
and fluorescence spectra of the complexes of G1 and G2 with
observed for (+)- and (À)-3aTBA ·G2 (see Figure S10d),
2
3aNa in CH3CN also provided evidence for the formation of
although the CD spectral changes were not significant.
In summary, we found unprecedented unidirectional dual
rotary and twisting motions induced by guest encapsulation in
a porphyrin-linked, double-stranded spiroborate helicate.
Detailed structural analysis of the helicate before and after
guest inclusion by X-ray crystallography and 2D NMR
spectroscopy unraveled the molecular and mechanistic details
of the unique dual motions: the linker porphyrins sandwich an
electron-deficient aromatic guest, which triggers the rotary
motion of the porphyrin rings in one direction; this rotary
motion is coupled with a unidirectional twisting motion of the
spiroborate helix. The helicate is fluorescent and can be
readily resolved into optically pure enantiomers. Therefore, it
offers significant potential for the development of chirality-
responsive molecular machines with functions such as asym-
metric catalysis and chiral sensing:[15] its unique unidirectional
rotary and twisting motions would be triggered by the
inclusion of the chiral guests.
2
sandwich complexes, which showed a characteristic hypo-
chromic effect (see Figures S7a and S10a) and efficient
quenching of the fluorescence of the emissive aromatic guests
(see Figures S7b,c and S10b,c), as anticipated. These charac-
teristics enable the naked-eye sensing of these aromatic
compounds.[9e,f,h] The helicate complexed with ammonium
cations, 3aTBA , also formed a similar sandwich complex with
2
G1, as evidenced by its hypochromic effect and fluorescence
quenching (see Figure S12).
Further detailed analysis of the inclusion complex
3aNa ·G1 by X-ray crystallography (Figure 3b) and 2D
2
COSY and ROESY spectroscopy (see Figure S13–S18)
revealed remarkable changes in the helical 3aNa structure
2
as a result of anisotropic rotary and twisting motions induced
by guest intercalation. X-ray crystallographic analysis showed
that the bisporphyrins sandwiched G1 in a parallel fashion
through face-to-face stacking interactions. As a result, the
distance between the porphyrin rings in the cavity expanded
from 4.1 to 6.8 ꢀ. This expansion was accompanied by
a rotation of the porphyrin rings in one direction to give
a smaller torsion angle of 548 with respect to the two meso–
meso porphyrin axes (Figure 3). At the same time, guest
intercalation caused steric strain in the spiroborate helix; this
strain could be alleviated by unidirectional unwinding of the
helix to reduce the twist angle from 368 to 3308. These
molecular motions, however, brought about hardly any
contraction and extension motion: the spiroborate helicate
contracted slightly in length to a B–B distance of 15.0 ꢀ (from
15.4 ꢀ; Figure 3). In other words, in the case of the right-
Received: March 27, 2013
Published online: && &&, &&&&
Keywords: helical structures · host–guest systems ·
.
molecular devices · porphyrins · unidirectional motion
[1] For reviews, see: a) V. Balzani, A. Credi, F. M. Raymo, J. F.
Stoddart, Angew. Chem. 2000, 112, 3484 – 3530; Angew. Chem.
Int. Ed. 2000, 39, 3348 – 3391; b) C. A. Schalley, K. Beizai, F.
Vogtle, Acc. Chem. Res. 2001, 34, 465 – 476; c) J.-P. Collin, C.
Dietrich-Buchecker, P. GaviÇa, M. C. Jimenez-Molero, J.-P.
Sauvage, Acc. Chem. Res. 2001, 34, 477 – 487; d) T. R. Kelly,
Acc. Chem. Res. 2001, 34, 514 – 522; e) G. S. Kottas, L. I. Clarke,
D. Horinek, J. Michl, Chem. Rev. 2005, 105, 1281 – 1376; f) K.
Kinbara, T. Aida, Chem. Rev. 2005, 105, 1377 – 1400; g) W. R.
Browne, B. L. Feringa, Nat. Nanotechnol. 2006, 1, 25 – 35;
h) B. L. Feringa, J. Org. Chem. 2007, 72, 6635 – 6652; i) E. R.
Kay, D. A. Leigh, F. Zerbetto, Angew. Chem. 2007, 119, 72 – 196;
Angew. Chem. Int. Ed. 2007, 46, 72 – 191; j) A. Coskun, M.
Banaszak, R. D. Astumian, J. F. Stoddart, B. A. Grzybowski,
Chem. Soc. Rev. 2012, 41, 19 – 30.
handed double-stranded helicate (À)-3aTBA or (À)-3aNa
,
2
2
guest insertion causes right-handed twisting of the spirobo-
rate helix by 388 along with counterclockwise rotation of the
bisporphyrin orientation by 178 (see Figures 1c and 3). Thus,
guest encapsulation triggers unique unidirectional dual rotary
and twisting motions along the mutually orthogonal axes of
the double-stranded helicate through expansion of the
bisporphyrin cavity.
The 2D COSY and ROESY experiments also supported
the sandwich structure of 3aNa ·G1, which retained the
2
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
[2] a) M. Schliwa, Molecular Motors, Wiley-VCH, Weinheim, 2003;
b) D. S. Goodsell, Bionanotechnology: Lessons from Nature,
Wiley, Hoboken, 2004.
[9] For examples of free-base bisporphyrins that sandwich aromatic
guests in organic media, see: a) H. Iwamoto, M. Yamaguchi, S.
Hiura, Y. Fukazawa, Heterocycles 2004, 63, 2005 – 2011; b) T.
Haino, T. Fujii, Y. Fukazawa, J. Org. Chem. 2006, 71, 2572 – 2580;
c) M. Tanaka, K. Ohkubo, C. P. Gros, R. Guilard, S. Fukuzumi, J.
Am. Chem. Soc. 2006, 128, 14625 – 14633; d) T. Haino, T. Fujii,
A. Watanabe, U. Takayanagi, Proc. Natl. Acad. Sci. USA 2009,
106, 10477 – 10481; e) T. Ema, N. Ura, K. Eguchi, Y. Ise, T. Sakai,
Chem. Commun. 2011, 47, 6090 – 6092; f) T. Ema, N. Ura, K.
Eguchi, T. Sakai, Bull. Chem. Soc. Jpn. 2012, 85, 101 – 109; g) T.
Haino, A. Watanabe, T. Hirao, T. Ikeda, Angew. Chem. 2012,
124, 1502 – 1505; Angew. Chem. Int. Ed. 2012, 51, 1473 – 1476;
h) A. Chaudhary, S. P. Rath, Chem. Eur. J. 2012, 18, 7404 – 7417;
for a review, see: i) D. Margetic, Curr. Org. Chem. 2012, 16, 829 –
851.
[10] a) J. P. Collman, C. M. Elliott, T. R. Halbert, B. S. Tovrog, Proc.
Natl. Acad. Sci. USA 1977, 74, 18 – 22; b) C. K. Chang, M.-S.
Kuo, C.-B. Wang, J. Heterocycl. Chem. 1977, 14, 943 – 945; c) J. P.
Collman, P. S. Wagenknecht, J. E. Hutchison, Angew. Chem.
1994, 106, 1620 – 1639; Angew. Chem. Int. Ed. Engl. 1994, 33,
1537 – 1554.
[11] Each type of porphyrin hydrogen atom in ligand 6a showed one
set of resonances in the 1H NMR spectrum. In contrast, the
helicate exhibited a pair of resonances in each case owing to the
nonequivalent pyrrole hydrogen atoms; thus, the 1H NMR
spectrum indicated a twisted orientation of the face-to-face
porphyrin units of the helicate.
[12] a) N. Harada, K. Nakanishi, Circular Dichroic Spectroscopy:
Exciton Coupling in Organic Stereochemistry, University Science
Books, Mill Valley, CA, 1983; b) N. Berova, K. Nakanishi, R. W.
Woody, Circular Dichroism: Principles and Applications, Wiley,
New York, 2000; c) N. Berova, L. D. Bari, G. Pescitelli, Chem.
Soc. Rev. 2007, 36, 914 – 931; d) N. Berova, P. L. Polavarapu, K.
Nakanishi, R. W. Woody, Comprehensive Chiroptical Spectros-
copy, Wiley, Hoboken, 2012.
[13] For CD studies on bisporphyrins, see: a) X. Huang, K. Naka-
nishi, N. Berova, Chirality 2000, 12, 237 – 255; b) V. V. Borovkov,
G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449 – 459;
c) G. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008,
108, 1 – 73; d) N. Berova, G. Pescitelli, A. G. Petrovic, G. Proni,
Chem. Commun. 2009, 5958 – 5980; e) I. C. Pintre, S. Pierrefixe,
A. Hamilton, V. Valderrey, C. Bo, P. Ballester, Inorg. Chem.
2012, 51, 4620 – 4635.
[14] This association constant is significantly larger than those (Ka ꢀ
106) reported for bisporphyrin hosts and electron-deficient
aromatic flat guests, including G1 and G2, in organic media.[9]
[15] The present helicate has potential for use as a chiral shift
reagent, since chiral recognition of the helicate enantiomers by
various enantiomerically pure organic ammonium salts was
possible, as supported by 1H NMR spectra, which showed peaks
due to the corresponding diastereomers (see Figure S19). Thus,
the enantiomerically pure helicate should be capable of the
chiral discrimination of the enantiomers of racemic ammonium
salts on the basis of 1H NMR spectroscopy.
[3] For examples of unidirectional motion with artificial systems,
see: a) T. R. Kelly, H. De Silva, R. A. Silva, Nature 1999, 401,
150 – 152; b) N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N.
Harada, B. L. Feringa, Nature 1999, 401, 152 – 155; c) D. A.
Leigh, J. K. Y. Wong, F. Dehez, F. Zerbetto, Nature 2003, 424,
174 – 179; d) S. P. Fletcher, F. Dumur, M. M. Pollard, B. L.
Feringa, Science 2005, 310, 80 – 82; e) T. Muraoka, K. Kinbara,
T. Aida, Nature 2006, 440, 512 – 515; f) G. Haberhauer, Angew.
Chem. 2008, 120, 3691 – 3694; Angew. Chem. Int. Ed. 2008, 47,
3635 – 3638; g) A. Martinez, L. Guy, J.-P. Dutasta, J. Am. Chem.
Soc. 2010, 132, 16733 – 16734; h) N. Ruangsupapichat, M. M.
Pollard, S. R. Harutyunyan, B. L. Feringa, Nat. Chem. 2011, 3,
53 – 60.
[4] Several synthetic helical molecules and polymers exhibit exten-
sion–contraction motion triggered by external stimuli, but these
helical systems have rarely undergone a unidirectional twisting
motion. For examples of their extension–contraction motions,
see: a) O.-S. Jung, Y. J. Kim, Y.-A. Lee, J. K. Park, H. K. Chae, J.
Am. Chem. Soc. 2000, 122, 9921 – 9925; b) E. Yashima, K.
Maeda, O. Sato, J. Am. Chem. Soc. 2001, 123, 8159 – 8160; c) M.
Barboiu, J. M. Lehn, Proc. Natl. Acad. Sci. USA 2002, 99, 5201 –
5206; d) M. Barboiu, G. Vaughan, N. Kyritsakas, J. M. Lehn,
Chem. Eur. J. 2003, 9, 763 – 769; e) E. Berni, B. Kauffmann, C.
Bao, J. Lefeuvre, D. M. Bassani, I. Huc, Chem. Eur. J. 2007, 13,
8463 – 8469; f) H.-J. Kim, E. Lee, H.-s. Park, M. Lee, J. Am.
Chem. Soc. 2007, 129, 10994 – 10995; g) V. Percec, J. G. Rudick,
M. Peterca, P. A. Heiney, J. Am. Chem. Soc. 2008, 130, 7503 –
7508; h) T. Hashimoto, T. Nishimura, J. M. Lim, D. Kim, H.
Maeda, Chem. Eur. J. 2010, 16, 11653 – 11661; i) Y. Ferrand, Q.
Gan, B. Kauffmann, H. Jiang, I. Huc, Angew. Chem. 2011, 123,
7714 – 7717; Angew. Chem. Int. Ed. 2011, 50, 7572 – 7575; j) M.
Numata, D. Kinoshita, N. Hirose, T. Kozawa, H. Tamiaki, Y.
Kikkawa, M. Kanesato, Chem. Eur. J. 2013, 19, 1592 – 1598; see
also Ref. [5].
[5] a) K. Miwa, Y. Furusho, E. Yashima, Nat. Chem. 2010, 2, 444 –
449; b) K. Miwa, K. Shimizu, H. Min, Y. Furusho, E. Yashima,
Tetrahedron 2012, 68, 4470 – 4478.
[6] For example, titin, which is a giant sarcomeric protein in cardiac
and skeletal muscles, provides a springlike motion controlled by
calcium-responsive conformational changes: a) H. Granzier, S.
Labeit, J. Physiol. 2002, 541, 335 – 342; b) C. A. Opitz, M. Kulke,
M. C. Leake, C. Neagoe, H. Hinssen, R. J. Hajjar, W. A. Linke,
Proc. Natl. Acad. Sci. USA 2003, 100, 12688 – 12693; c) D.
Labeit, K. Watanabe, C. Witt, H. Fujita, Y. Wu, S. Lahmers, T.
Funck, S. Labeit, H. Granzier, Proc. Natl. Acad. Sci. USA 2003,
100, 13716 – 13721; d) M. M. LeWinter, Y. Wu, S. Labeit, H.
Granzier, Clin. Chim. Acta 2007, 375, 1 – 9.
[7] H. Katagiri, T. Miyagawa, Y. Furusho, E. Yashima, Angew.
Chem. 2006, 118, 1773 – 1776; Angew. Chem. Int. Ed. 2006, 45,
1741 – 1744.
[8] Y. Furusho, K. Miwa, R. Asai, E. Yashima, Chem. Eur. J. 2011,
17, 13954 – 13957.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!