Recognition of mandelate stereoisomers by chiral porphyrin hosts: Prediction of stereopreference in guest binding a priori using a simple binding model?

Article abstract of DOI:10.1016/j.tetlet.2013.12.046

Rigid porphyrin hosts that mimic the spatial arrangement of mandelate recognition motifs lead to stereoselective receptors and illustrate how subtle structural differences in host design have significant impact on guest recognition. The porphyrin hosts are obtained with minimal synthetic effort from readily available chiral amine precursors and are modular in design. The chiral recognition properties of the porphyrin-based hosts with chiral carboxylate-containing guests and chiral amines are described. UV/vis and ¹H NMR spectroscopic results indicate some of these porphyrin hosts undergo an induced fit conformational change upon guest binding.

Full text of DOI:10.1016/j.tetlet.2013.12.046

Tetrahedron Letters 55 (2014) 985–991
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Recognition of mandelate stereoisomers by chiral porphyrin hosts:
prediction of stereopreference in guest binding a priori using
a simple binding model?
⇑
Vijay Nandipati, Karthik Akinapelli, Lakshmi Koya, Stephen D. Starnes
Department of Chemistry, Texas A&M University-Commerce, Commerce, TX 75429, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Rigid porphyrin hosts that mimic the spatial arrangement of mandelate recognition motifs lead to ster-
eoselective receptors and illustrate how subtle structural differences in host design have significant
impact on guest recognition. The porphyrin hosts are obtained with minimal synthetic effort from readily
available chiral amine precursors and are modular in design. The chiral recognition properties of the por-
phyrin-based hosts with chiral carboxylate-containing guests and chiral amines are described. UV/vis and
1H NMR spectroscopic results indicate some of these porphyrin hosts undergo an induced fit conforma-
tional change upon guest binding.
Received 26 November 2013
Accepted 11 December 2013
Available online 27 December 2013
Keywords:
Porphyrins
Chiral recognition
Amine recognition
Anion recognition
Induced fit
Ó 2013 Elsevier Ltd. All rights reserved.
Introduction
its infancy however, with only a few hundred manuscripts pub-
lished in the field.5 There are a few examples of porphyrins that
To design selective synthetic hosts for chiral guests one must
battle another level of complexity in molecular recognition; in
addition to considering a guest’s size, geometry, recognition motifs,
charge density, ionization state, hydration, etc. one must, of course,
take into consideration spatial orientation of the guest features. On
one hand, it seems this might make selectively binding chiral spe-
cies easier than non-chiral species—if the guest does not fit the
pocket of a host spatially it might not bind at all. Thus, one devel-
oping host for chiral guests has an extra tool at their disposal! On
the other hand, having to consider a guest’s spatial orientation can
present challenges (synthetically and financially) in host design.
Additionally, for a guest to be chiral inherently means it will have
to be fairly large in size, which means a host’s binding cavity will
likely have to be as well, which presents additional synthetic
challenges.
show good selectivities in guest binding, for both chiral and non-
chiral guests.6
We recently reported an example of a chiral porphyrin (1, Fig. 1)
which showed modest selectivity in the binding of mandelate ster-
eoisomers.7 Host 1 is characterized by an introverted functional
group (an amide) which projects over the porphyrin surface. The
amide N–H group was shown to aid in guest binding presumably
through a hydrogen bonding interaction with the mandelate hy-
droxy group. The amide group is positioned to work cooperatively
with the zinc metal center in guest binding. Host 1 was shown to
bind S-mandelate preferentially to R-mandelate (selectivity ꢀ2).
Figure 1 also depicts the structure for the proposed complex with
S-mandelate.
Chiral recognition is a field in supramolecular chemistry with
new applications continually emerging. Recent applications have
been illustrated in the kinetic resolution of chiral amines,1 enantio-
meric excess determination,2 chiral supramolecular assemblies,3
and catalysis for example a large part of the field of chiral organoc-
atalysis is based on principles of molecular recognition.4 Investiga-
tors have been working in the field of chiral host–guest chemistry
for over 20 years; this area of supramolecular chemistry is still in
H
N
H
N
N
O
H
O
N
O
O
O
O
N
Ph
Ph
H
H
Ph
O
H N
H N
N
N
N
N
N
N
Zn
Zn
N
1
1: S-Mandelate
⇑
Corresponding author. Tel.: +1 903 886 5389; fax: +1 903 468 6020.
E-mail address: stephen.starnes@tamuc.edu (S.D. Starnes).
Figure 1. Host 1 and binding model for recognition of S-mandelate.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.12.046

986
V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
Host 1 and the hosts described here (Fig. 2) are fairly rigid in
Since host 1 binds S-mandelate preferentially, will hosts with
similar spatial arrangement of a hydrogen bond donating substitu-
ent (in the Pro-S position, Fig. 3, such as 4, 6, and 8) also preferen-
tially bind S-mandelate? Will hosts 3, 5, and 7, which have the
opposite spatial arrangement of a hydrogen bond donating group
(in the Pro-R position), bind R-mandelate preferentially? One
would predict so using the binding model in Figures 1 and 3.
Hosts 2–8 were prepared by reacting amines 9–15 (Fig. 4) with
porphyrin isocyanate 178 (Scheme 1 illustrates a representative
eg.,—the synthesis of 3). Amines 9, 14, and 15 are commercially
available. Amines 10–13 were synthesized as illustrated in
Scheme 1. The synthesis of each compound was straightforward
and proceeded smoothly, but with a couple of interesting observa-
tions. First, compounds 19–22 (Scheme 1) appear to exist as a mix-
ture of conformational isomers in slow exchange on the NMR
timescale. As revealed by 13C NMR (Supporting information), the
four pyrrolidine ring carbons appear as two signals each at room
temperature which collapse to broad signals at higher tempera-
tures. Compounds 19–22 may exist as a mixture of cis and trans
carbamate derivatives similar to proline amide derivatives.9 Sec-
ond, during the metallation final step in the synthesis of hosts 3
structure—they are dynamic due to free rotation around urea
O@C–N bonds, but they do not have much conformational space
available to themselves due to the preferred planar nature of ureas.
With a rigid framework imparted from the porphyrin and urea
platform, guest recognition sites are fairly well spatially pre-orga-
nized and thus positioned to complement the orientation of a chi-
ral guest’s molecular recognition motifs. Due to the rigid nature of
these types of hosts, we envisioned that carboxylate-containing
guests would in general bind as illustrated in Figure 3, where the
guest carboxylate would coordinate the metallo center and hydro-
gen bond to the porphyrin urea hydrogen, a polar substituent on
the guest
or piperidine substituent, and a bulky or aryl group on the guest
-position could interact favorably or unfavorably with the por-
phyrin -surface. Thus, there should be at least three points of spa-
a-position could hydrogen bond to a polar pyrrolidine
a
p
tially different interactions between host and guest, which is
necessary for chiral recognition. If this binding model is accurate
and general for these types of guests, we might be able to predict
a priori which enantiomer of a guest will preferentially bind to
these hosts.
O
O
N
H
N
N
H
N
H
H
N
N
N
O
O
O
Ph
Ph
Ph
H
N
H
N
N
H
N
N
N
N
N
N
Zn
O
Zn
O
Zn
N
N
N
N
N
N
2
3
4
O
O
O
S
H
S
CH3
C
N
N
N
H
H
N
N
N
O
O
O
Ph
Ph
Ph
N
H
N
N
H
H
N
N
N
N
N
N
Zn
Zn
Zn
N
N
N
N
N
N
5
6
7 (R isomer)
8 (S isomer)
Figure 2. Porphyrin hosts.
Hydrogen bond donor or acceptor in the
illustrated Pro-S or Pro-R position.
View from front with three-point-guest interactions
Pro-S Positioned
H-Bonding Group
H
Pro-R position
n = 0, 1
n = 0, 1
H
N
N
Pro-S position
RL
O
O
O
O
C
N
Ph
H N
O-
H
N
N
RS
Zn
N
N
Rs here
Metal
coordination
minimizes
steric
interaction
Zn
Guest π−interaction
or steric interaction
with porphyrin ring
RL = large alkyl or aryl group, Rs = small alkyl or aryl group
Figure 3. Proposed binding model of hosts with -hydroxy guests.
a

V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
987
O
S
N
O
S
N
S
S
O
N
N
N
O
O
O
H
H
N
N
N
H
H
H
H
H
H
N
H
N
H
N
H
N
H
N
H
N
H
N
H
11
12
13
14
9
10
15
Figure 4. Amines utilized in the synthesis of 2–8.
S
S
Ph
Ph
N
N
NH2
N
H
N
H
H
H
Ph-N=C=S
DCM
TFA, CH2Cl2
N
N
N
H
19, R, 98%
20, S, 88%
10, R, 95% crude
11, S, 76% crude
Boc
Boc
(R) or (S) isomer
NH2
Ts
N
Ts
N
H
H
TFA, CH2Cl2
TsCl
N
H
N
N
21, R, 56%
22, S, 53%
12, R, 86% crude
13, S, 33% crude
DCM
Boc
Boc
(R) or (S) isomer
O
C
N
Ph
N
H2N
1b. amine 9 - 15
N
Hosts
2 - 8
1a. triphosgene
N
H
N
N
H
H
H
N
N
2. Zn(OAc)2
1:1 CHCl3:CH3OH
30 minutes - 1 hour
N
CH2Cl2
triethylamine
17
16
O
S
Ph
Ph
S
Ph
N
N
N
N
H
N
H
H
H
N
H
H
H
N
N
O
O
N
H
Ph
Ph
Zn(OAc)2
10
H N
H N
N
N
N
N
1:1 CHCl3:CH3OH
17
N
N
H
Zn
H
N
N
** thiourea
converted to urea
3, 7%
18, 43%
Scheme 1. General scheme for the synthesis of hosts 2–8.
and 4, the thiourea was converted to a urea. This became apparent
from high-definition mass spectral analysis of chromatographic
fractions. The only fraction with mass near the correct mass had
a mass 16 units off (see Supporting information). We believe zin-
c(II) acetate with trace water catalyzed the conversion of the thio-
urea to a urea—a hydrogen sulfide smell was apparent during the
reaction. More work is needed to confirm if zinc(II) acetate was
the culprit leading to the conversion. To the best of our knowledge,
this would be the first example of zinc(II) catalyzed conversion of a
thiourea to a urea. Zinc sulfide nanoparticles have been prepared
from the hydrolysis of thiourea10 and bismuth(III) nitrate has re-
cently been demonstrated to catalyze the conversion of thioureas
and thioamides to their oxygen analogs.11 The normally high yield-
ing metallation step used to prepare 3 and 4 proceeded in only 7%
and 5% yield, respectively,—the low yield is probably due to side
product formation from the thiourea reaction.
tive UV/vis titration spectra. Anion guests were prepared as their
tetrabutylammonium salts for solubility in dichloromethane and
CDCl3. Table 1 lists the binding constants of hosts 2–8 with a vari-
ety of anion guests as well as two chiral amines (L-nicotine and
(R,S)-ephedrine).
Host 2 generally showed binding constants with carboxylate-
containing guests of ꢀ1000–5000 MÀ1. Hosts 3–6 binding con-
stants are 2–3 orders of magnitude larger, which we attribute to
the strong hydrogen bonding ability of ureas and sulfonamides,
which are commonly utilized in the design of anion hosts.12 To
our surprise, hosts 5 and 6 did not show selectivity for any pair
of enantiomer guests. Hosts 3 and 4, however, did show modest
selectivity for mandelate stereoisomers (ratio of binding constants
ꢀ3). Host
4
binds S-mandelate with
a
a
binding constant of
binding constant of
470,000 MÀ1 and R-mandelate with
150,000 MÀ1. The enantiomer of 4, host 3, showed opposite enanti-
oselectivity. Host 3 preferentially binds R-mandelate over S-man-
delate with a selectivity ꢀ3. If the binding model that we
proposed for host 1 (Fig. 1) and the general binding model for these
types of hosts (Fig. 3) is reasonable, then the observed selectivities
The chiral recognition properties of hosts 2–8 were examined
by UV/vis and 1H NMR titration experiments. UV/vis titrations
were conducted in dichloromethane and 1H NMR titration
experiments were conducted in CDCl3. Figure 5 shows representa-

988
V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
Figure 5. UV/Vis titration spectra of 3 with mandelate isomers.
Table 1
Association constants (K, MÀ1) for receptors 2–8 with guests and selectivity of receptors 3 and 4 with guestsa
Guest
2
3
4
5
6
7
8
Selectivity ratio of K (3/4)
Acetate
2000
4100
2000
1900
1400
1600
2700
2900
5100
5000
4200
3200
18,000
100,000
292,000
460,000
152,000
331,000
299,000
299,000
304,000
171,000
184,000
312,000
173,000
6500
97,000
123,000
300,000
58,000
133,000
245,000
54,000
—
—
19,000
20,000
—
—
—
—
—
—
—
—
—
—
—
14,000
16,000
—
—
—
—
—
—
—
—
—
1.0
1.0
3.0
0.32
1.0
0.91
1.1
1.0
0.97
1.2
1.0
1.2
1.3
N-Ac-glycine
R-Mandelate
S-Mandelate
296,000
150,000
470,000
328,000
326,000
273,000
291,000
176,000
157,000
302,000
144,000
5000
66,000
63,000
N-Ac-
N-Ac-
N-Ac-
N-Ac-
N-Ac-
N-Ac-
D
-alanine
-alanine
-Ph-alanine
-Ph-alanine
-Ph-glycine
-Ph-glycine
180,000
163,000
137,000
177,000
131,000
123,000
154,000
104,000
34,000
200,000
175,000
175,000
130,000
140,000
128,000
149,000
105,000
21,000
L
D
L
D
L
S-Ibuprofen
Naproxen
L-Nicotine
RS-Ephedrine
1100
1060
1070
2700
2200
—
—
1.0
a
Anions as their tetrabutylammonium salts for solubility in dichloromethane. Error 10%.
large group has more
space if here
than if here
H
H
O
H
Ph
H
N
(a)
N
N
H
N
N
N
H
O
H
H
O
O
O
O
H
O
N
Bu4N
O
Ph
H
O
Ph
O
H N
H N
N
N
N
N
tetrabutylammonium
R-Mandelate
N
Zn
Zn
N
N
induced fit binding
3: R-Mandelate
3
(c)
(d)
methyl group is too
small to exhibit a
spatial preference
in this system
Ph
HN
Ph
O
O
CH2 spacer allows
phenyl to avoid a
steric interaction
(b)
HN
H
HN
HN
H3C
Ph
H
AcHN
N
H
O
N
N
N
N
O
O
H
O
O
O
O
N
Ph
H
O
O
N
AcHN
H
Ph
H
O
Ph
O
Ph
H N
N
N
H
H
N
N
N
N
N
N
N
Zn
Zn
Zn
N
N
N
4: S-Mandelate
3 or 4 with
L- or D-Ac-Alanine
3 or 4 with L- or
D-PhenylAlanine
Figure 6. Proposed host/guest complexes of 3 and 4.

V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
989
of hosts 3 and 4 are exactly what one would predict a priori. Host 3,
with a hydrogen bond donating group in the Pro-R position of the
pyrrolidine ring binds (R)-mandelate preferentially. Host 4, with a
hydrogen bond donating group in the Pro-S position of the pyrrol-
idine ring binds (S)-mandelate preferentially, similar to the molec-
ular recognition stereopreferences of 1, which has a hydrogen
bonding group in the Pro-S position and also showed a preference
for S-mandelate.
Figure 6 depicts the proposed binding model for 3 and 4 with
mandelate isomers. Similar to host 1 and other porphyrins that
we have recently reported on,13 we believe hosts 3, 4, 7, and 8 exist
in a conformation which has the zinc internally coordinated by the
Figure 7. (a) R-mandelate NBu4 in CDCl3. (b) 2 Titration with S-mandelate, (c) 3 titration with S-mandelate, (d) 3 titration with R-mandelate.

990
V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
carbonyl of the urea or amide pyrrolidine substituent. kmax for
hosts 2, 5, and 6 is 420 nm, whereas that of 3, 4, 7, and 8 is
425 nm (host 1 kmax is 424 nm). Thus, we believe the red shift in
kmax for hosts 3, 4, 7, and 8 (and 1) compared to 2, 5, and 6 is attrib-
uted to this intramolecular zinc coordination which is not exhib-
ited in hosts 2, 5, and 6.
Upon guest binding, the kmax for host’s 2–8 complexes are typ-
ically 429–431 nm. Thus, there seems to be a general trend where-
by these types of porphyrin hosts which have introverted
functional groups exist in a conformation with the metallo center
intramolecularly complexed if this interaction is not sterically pre-
cluded (which it may be in 5 and 6, or the sulfonamide group may
be a poor ligand for zinc). Other porphyrin examples with this fea-
ture have been reported14 (this is a general phenomena in porphy-
rin-based metallo proteins as well). This represents another
example of conformationally induced guest binding.15
We believe hosts 3 and 4 bind mandelate through carboxylate
coordination to zinc and a hydrogen bond interaction between
the urea and the hydroxy group. Host 3 may bind R-mandelate
with greater affinity than S-mandelate because the R-isomer fits
the porphyrin binding cavity better. If R-mandelate binds in the
manner depicted in Figure 6, the phenyl group would not experi-
ence a steric interaction. If S-mandelate binds in a similar fashion
however, the phenyl group could experience a steric interaction
with the porphyrin surface. To avoid a steric interaction, we believe
S-mandelate binds to 3 in a different, un-defined conformation. A
1H NMR titration of 3 with R- and S-mandelate supports this
(Fig. 7).
Host 1 did not show any selectivity for other chiral guests that
we studied, just as is observed here for 3 and 4. This begs the ques-
tions: why is there no selectivity for other guests such as N-acetyl-
alanine and N-acetylphenylalanine stereoisomers? And why do not
hosts 5 and 6 show any stereoselectivity in guest binding, even
with mandelate stereoisomers? Since 5 and 6 did not show selec-
tivity in binding mandelate isomers, we predicted hosts 7 and 8
would not either. As Table 1 shows, hosts 7 and 8 show no selectiv-
ity in binding mandelate isomers; we expect no selectivity for 7
and 8 with other guests of Table 1 and thus they have not been
studied with other guests at this point. We believe the other chiral
guests examined show no stereopreference in guest binding be-
positioned to hydrogen bond with a guest than the N–H of the
amide or sulfonamide substituents of hosts 5–8. Small changes in
structure can have huge effects on molecular recognition, particu-
larly in rigid systems—one atom’s diameter, the direction of an
amide bond, etc. can make all the difference in the stability of a
host–guest complex.16
Figure 7 shows the 1H NMR titration of 2 with S-mandelate. Pro-
ton Hb shifts downfield with increasing concentration of S-mandel-
ate, but the shift is only apparent when large amounts of guest are
added (ꢀ5 equiv addition at a time), which implies weak binding of
S-mandelate to 2, which is in line with the results of Table 1. Thus,
the urea proton of 2 seems to contribute little to binding of guests
by itself.
For the titration of 3 with R-mandelate, the signals for the urea
protons (labeled b and g) rapidly move downfield with small incre-
ments of guest addition, which suggest the binding constant is
large compared to that of 2 with mandelate. The rapid change in
chemical shift of Hb suggests a cooperative interaction between
Hb, Hg and the metallo center in guest binding. The urea phenyl
proton signals (labeled h–j) are initially ꢀ7 ppm but move upfield
and two sets of protons become non-chemical shift equivalent in
the process. When 3 is titrated with S-mandelate, the signal for
urea Hb moves upfield! The signal for the urea phenyl protons also
moves upfield and splits into three sets of non-chemical shift
equivalent protons. This suggests that 3 forms conformationally
different complexes with R- and S-mandelate.
In summary, we have developed a class of easy-to-synthesize
and modular chiral porphyrin-based hosts that are unique because
recognition motifs are directed over the porphyrin surface in a
well-defined fashion where they work in tune with the metal cen-
ter for cooperative guest binding. Preliminary results suggest that
we can predict a priori which stereoisomer of mandelate stereoiso-
mers will bind these hosts. Future work will be conducted to deter-
mine if these hosts might in general stereoselectively bind other
chiral
a-hydroxy carboxylate compounds. We are working to im-
prove on the chiral selectivity. Others working in this field have
noted the importance of geometrical confinement of the guest to
achieve good enantioselectivity in chiral recognition.17 Variations
of these types of porphyrins with optimally positioned recognition
motifs are in the works.
cause either the alkyl group on the a-position is too small to have
a spatial preference in these complexes (such as in N-acetylala-
Acknowledgments
nine) or as in the case of N-acetylphenylalanine isomers, the CH2
spacer of the
a
-side chain allows the phenyl ring the ability to
We gratefully acknowledge support from the Robert A. Welch
Foundation (T-0014), NSF-REU (Grant No. 0851966), the Chemistry
Department at Texas A&M-Commerce and the Office of Graduate
Studies & Research at Texas A&M-Commerce for funding this work.
avoid a steric interaction with the porphyrin surface (Fig. 6 illus-
trates this tentative explanation for the lack of stereoselectivity
with other guests.). Thus, if the amino acid derivatives form com-
plexes similar to mandelate with hosts 3 and 4 such that the car-
boxylate coordinates zinc and the N-acyl group hydrogen bonds
to the host, it is not surprising that N-acetylalanine stereoisomers
show no selectivity in binding since the methyl group does not
have much greater steric demands than a hydrogen. For phenylal-
anine, although the benzyl side chain is relatively large, the guest
could adopt a conformation in which the CH2 group is directed at
the porphyrin surface rather than the phenyl group, which would
minimize steric interactions and hence eliminate selectivity in
binding.
Supplementary data
Supplementary data (HD mass spectral data for hosts 2–8. 1H
NMR and 13C NMR spectra of compounds (room temperature and
45 °C)) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2013.12.046.
References and notes
1. Klauber, E. G.; Mittal, N.; Shah, T. K.; Seidel, D. Org. Lett. 2011, 13, 2464–2467.
2. (a) Leung, D.; Anslyn, E. V. Org. Lett. 2011, 13, 2298–2301; b) Shabbir, S.; Joyce,
L. A.; da Cruz, G. M.; Lynch, V. M.; Sorey, S.; Anslyn, E. V. J. Am. Chem. Soc. 2009,
131, 13125–13131.
3. Král, V.; Schmidtchen, F. P.; Lang, K.; Berger, M. Org. Lett. 2002, 4, 51–54.
4. Shubina, T. E.; Freund, M.; Schenker, S.; Clark, T.; Tsogoeva, S. B. Beilstein J. Org.
Chem. 2012, 1485–1498.
5. For a few recent examples (a) Jintoku, H.; Takafuji, M.; Odac, R.; Ihara, H. Chem.
Commun. 2012, 48, 4881–4883; (b) Tashiro, S.; Ogura, Y.; Tsuboyama, S.;
Tsuboyama, K.; Shionoya, M. Inorg. Chem. 2011, 50, 4–6; (c) Kubik, S. Chem. Soc.
Rev. 2009, 585–605.
Of course, the host/amino acid complexes could exhibit an en-
tirely different conformation than those of
a-hydroxycarboxylate
guests (mandelate) which would be reasonable if the N-acyl group
of amino acids does not match the hydrogen bonding motifs of the
host. Lastly, hosts 5–8 may show no selectivity with these guests
because the sulfonamide or amide substituents may not be opti-
mally positioned to interact with a guest that simultaneously com-
plexes zinc—the urea substituent of hosts 3 and 4 has an N–H
group 2 atoms further from the pyrrolidine ring that may be better

V. Nandipati et al. / Tetrahedron Letters 55 (2014) 985–991
991
6. (a) Takeuchi, M.; Shioya, T.; Swager, T. M. Angew. Chem., Int. Ed. 2001, 40, 3372–
3376; (b) Konishi, K.; Yabara, K.; Toshishige, H.; Aida, T.; Inoue, S. J. Am. Chem.
Soc. 1994, 116, 1337–1344; (c) Kuroda, Y.; Kato, Y.; Higashioji, T.; Hasegawa, J.-
Y.; Kawanami, S.; Takahashi, M.; Shiraishi, N.; Tanabe, K.; Ogoshi, H. J. Am.
Chem. Soc. 1995, 117, 10950–10958.
7. Wu, X.; Starnes, S. D. Org. Lett. 2012, 14, 3652–3655.
8. Landrum, J. T.; Grimmett, D.; Haller, K. J.; Scheidt, R.; Reed, C. A. J. Am. Chem.
Soc. 1981, 103, 2641.
9. Cox, C.; Young, V., Jr.; Lectka, T. J. Am. Chem. Soc. 1997, 119, 2307–2308.
10. Jayalakshmi, M.; Rao, M. M. J. Power Sources 2006, 624–629.
11. Mohammadpoor-Baltork, I.; Khodaei, M. M.; Nikoofar, K. Tetrahedron Lett.
2003, 44, 591–594.
12. (a) Lin, W.-C.; Tseng, Y.-P.; Lin, C.-Y.; Yen, Y.-P. Org. Biomol. Chem. 2011, 9,
5547–5553; (b) Zhang, Z.; Schreiner, P. R. Coord. Chem. Rev. 2009, 38, 1187–
1198; (c) Kondo, S.-I.; Nagamine, M.; Karasawa, S.; Ishihara, M.; Unno, M.;
Yano, Y. Tetrahedron 2011, 67, 943–950; (d) Hu, C.; He, Y.; Chen, Z.; Huang, X.
Tetrahedron: Asymmetry 2009, 20, 104–110.
13. Wienkers, M.; Ramos, J.; Jemal, H.; Cardenas, C.; Wiget, P.; Nelson, A.; Free, S.;
Wu, J.; Roach, R.; Vulcan, M.; Waynant, K.; Fort, K.; Vladimirova, A.; Sun, J.;
Hunt, S. E.; Rudkevich, D. M.; Starnes, S. D. Org. Lett. 2012, 14, 1370–1373.
14. (a) Le Gac, S.; Najjari, B.; Fusaro, L.; Roisnel, T.; Dorcet, V.; Luhmer, M.; Furet, E.;
Halet, J.-F.; Boitrel, B. Chem. Commun. 2012, 3724–3726; (b) Halime, Z.;
Lachkar, M.; Furet, E.; Halet, J.-F.; Boitrel, B. Inorg. Chem. 2006, 10661–10669.
15. (a) Gong, W.-T.; Gao, B.; Bao, S.; Ye, J.-W.; Ning, G.-L. J. Inclusion Phenom.
Macrocyclic Chem. 2012, 72, 481–486; (b) Beeren, S. R.; Sanders, J. K. M. J. Am.
Chem. Soc. 2011, 133, 3804–3807; (c) Swinburne, A. N.; Paterson, M. J.; Beeby,
A.; Steed, J. W. Chem. Eur. J. 2010, 16, 2714–2718.
16. (a) Starnes, S. D.; Rudkevich, D. M.; Rebek, J., Jr. Org. Lett. 2000, 2, 1995–1998;
(b) Starnes, S. D.; Rudkevich, D. M.; Rebek, J., Jr. J. Am. Chem. Soc. 2001, 123,
4659–4669.
17. (a) Jadhav, V. D.; Schmidtchen, F. P. J. Org. Chem. 2008, 73, 1077–1087; (b)
Zhong, Z.; Li, X.; Zhao, Y. J. Am. Chem. Soc. 2011, 133, 8862–8865.